Transcript Document

COS220 Concepts of PLs
AUBG, COS dept
Lecture 06
Components of Programming
Languages, Part III
Reference: R.Sebesta, Chapter 8
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Assoc. Prof. Stoyan Bonev
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Lecture Contents:
• StatementLevel Control Structures
– Compound Statements
– Selection Statements
– Iterative Statements
• Guarded Commands (after E.Dijkstra)
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Levels of expressing execution
sequence:
The flow of control, or execution sequence in a
program can be examined in several levels:
• Low: flow of control within Expressions (operands,
operators, precedence, associativity)
• High: flow of control among Program Units (the
routines concept and program modules)
• Intermediate: flow of control among statements
– Selection statements (one-way, two-way, n-way)
– Iterative statements (counter and logically controlled)
– Unconditional branch stmts (goto, break, continue)
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Control Statements
• Computations in PL are accomplished by evaluating
expressions followed by assigning the resulting values to
variables and function calls (linear algorithms).
• There are very few useful programs that consist entirely
only of assignment statements and function call
statements.
• At least two additional linguistic mechanisms are
necessary to make programs flexible & powerful
– The mechanism of selecting among alternative control flow
paths of statement execution (branched algorithms)
– The mechanism of causing the repeated execution of certain
collection of statements (looped algorithms)
• These mechanisms are provided by control statements.
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Control Statements – structure
and configuration
• A great deal of theoretical research & discussion was
done in 1960s-70s. A primary conclusion was formed:
Although a single control statement is obviously sufficient
(unconditional branch – selectable goto) a PL that is
designed to not include goto needs only a small number
of different control statements. It was shown that
algorithms that can be expressed by flowcharts can be
coded in a PL with two only control statements:
– one for choosing between two control flow paths and
– one for logically controlled iterations
• An important result of this is that unconditional branch
statements are superfluous – one of the Structured
Programming topics.
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Ref. to Structured Programming
• Boehm/Jacopini, 1965/66, Comm of the ACM,
“Flow Diagrams, Turing Machines, and
Languages with Only Two Formulation Rules”
• Edsger Dijkstra, 1965, IFAC congress,
“Programming Considered a Human Activity”
• Edsger Dijkstra, 1968, Comm of the ACM,
“Goto Statement Considered Harmful”
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Structured Programming Topics
• Sequence, Selection and Pretest Logical
Loops are absolutely required to express
computation.
• Each Control Structure should have a single
entry (no multiple entries).
• Each Control Structure should have a single
exit (after G.Myers, but nowadays
controversial).
• A control structure described – on next slide
• Structured Programming is known as a
gotoless programming.
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Control Structure
• Definition: A Control Structure is a control
statement and the collection of statements whose
execution it (the control structure) controls.
• Typical control structures:
–
–
–
–
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Compound statement
Selection statement
Iterative statement
Program unit (routine, function, procedure, method)
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From Theory Towards Practice
• Programmers don’t care about results of
theoretical research on control statements.
• Programmers/Developers/ take care about
readability/writability of PL.
• All popular PL provide more control
statements than the two that are minimally
required. So, writability is enhanced by a
larger number of control statements:
– Various different versions of
selection/iteration statements
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Towards Practice
• Question: Which is the best (or optimal)
collection of control statements of a PL to
provide the required capabilities and the
desired writability?
• Answer: The solution is a compromise of
how much should a PL be expanded to
increase its writability at the expense of its
simplicity, size, and readability.
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Compound statements
Compound Statements
• Method to form statement collections. CS allow a
collection of statements to abstract as a single statement.
Algol
begin
stmt_1;
. . .
stmt_n;
end
Pascal
BEGIN
stmt_1;
. . .
stmt_n
END
C-like
{
stmt_1;
. . .
stmt_n;
}
• If CS contains definition/declaration, it’s a block.
Algol
begin
integer index,count;
. . .
end
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C-like,Perl,JavaScript,PHP
{
int index,count;
. . .
}
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Compound Statements
• Syntax diagram illustrates compound
statement.
C-like
begin
stmt_1;
. . .
stmt_n;
End
<CS> ::= begin <stmt> { ; <stmt> }* end
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Compound Statements
• Early versions made definitions to appear
in the block beginning only. No such a
restriction now. The scope of the variables
is from definition point to the end of block.
C-like PL
{
. . .
int a;
. . .
float b;
. . .
}
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Selection statements
Selection Statements
• A selection statement provides the means
of choosing between two or more paths of
execution in a program.
• Classification:
– One-Way selection construct
– Two-Way selection construct
– N-Way (multiple) selection construct
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Selection
If/Then structure
(single selection)
(multiple selection)
T
F
If/Then/Else structure
(double selection)
F
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..
.
T
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One-Way &
Two-Way Selector
• General Form:
if control_expression | if control_expression
then_clause
|
then_clause
|
else_clause
• Design Issues:
– Form and type of the expression that controls the
selection
– How are then_ and else_ clauses specified?
– How should the meaning of nested selectors be
specified?
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VBasic selection statements include
If condition Then
statement sequence
End If
If condition Then
statement sequence1
Else
statement sequence2
End If
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C/C++ selection statements include
if (expression)
statement sequence
if (expression)
statement sequence1
else
statement sequence2
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One-Way & Two-Way Selector
• Syntax diagram illustrates selector stmt
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One-Way & Two-Way Selector
• The Control Condition/Expression
–
–
–
–
To be in parentheses if then reserved word is not used
C89: arithmetic expression
C99, C++: arithmetic or Boolean expression
Ada, Java, C#: Boolean expression only
• The Clause Form
– In most PL then_ and else_ clauses appear as either
single statements or compound statements
– Perl: always as compound statements
– Braces { … } used to form compound statements
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Nesting Selectors
• The problem: when two-way selection constructs can be
nested
if (sum == 0)
if (count == 0)
result = 0;
else
result = 1;
• This construct may be interpreted in two different ways,
depending on whether the else clause matches the first or
the second then clause
• In most PL (C, C++, C#, Java) static semantics specifies:
the else clause is always paired with the nearest most
recent unpaired then clause (above the else clause)
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Nesting Selectors
• Perl does not have the problem because it requires that all then and
else clauses should be compound
if (sum == 0)
if (count == 0)
result = 0;
else
result = 1;
• The corresponding Perl text in two versions with no ambiguity:
{
if (sum == 0)
if (count == 0) {
result = 0;
}
}
else
{
if (sum == 0)
if (count == 0) {
result = 0;
}
{
result = 1;
}
else
{
result = 1;
}
}
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Multiple Selectors
• The multiple selection construct allows the
selection of one of any number of statements or
statement groups
• Design Issues:
– Form and type of the expression that controls the
selection
– How are the selectable segments specified?
– Is execution flow restricted to include just a single
selectable segment?
– How should unrepresented selector expression values
be handled, if at all?
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Multiple Selectors - Examples
• Early versions (Fortran)
– Arithmetic IF
IF (<expression>) 10, 20, 30
– Computed GOTO
GOTO (<lab1>,<lab2>, … <labn>), <expression>
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Multiple Selectors - Examples
Visual Basic
Select Case JobClass
Case 1
Bonus = salary * 0.1
Case 2
Bonus = salary * 0.09
Case 3, 4
Bonus = salary * 0.075
Case 5 To 8
Bonus = salary * 0.05
Case Is > 8
Bonus = salary * 0.02
Case Else
Bonus = 0
End Select
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Multiple Selectors - Examples
• C, C++, Java
switch (<expression>)
{
case <const_expression_1>: <statement_1>;
case <const_expression_2>: <statement_2>;
...
case <const_expression_n>: <statement_n>;
[ default: <statement_n+1>; ]
};
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Multiple Selectors - Examples
• Syntax diagram illustrates selector stmt.
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Multiple Selectors - Examples
• C, C++, Java switch statement Comments:
– Expressions are integer type
– The default segment is for unrepresented values of the
control expression
– The switch construct does not provide implicit
branches at the end of its code segments. This allows
control flow through more than one selectable code
segment on a single execution
– To logically separate these segments, an explicit
branch must be included
– The break statement, which is actually a restricted
goto, is normally used for exiting switch constructs
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Multiple Selectors - Examples
• C# switch statement
• It differs from its C-like predecessors in that a
static semantic rule disallows the implicit
execution of more than one case segment. The
rule is that every selectable segment must end
with explicit unconditional branch statement:
either a break, which transfers control out of the
switch, or a goto, which can transfer control to
one of the selectable case segments.
• Example on next slide
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Multiple Selectors - Examples
• C# switch statement
switch (value)
{
case -1:
case 0:
case 1:
default:
negatives++;
break;
Zeros++;
goto case 1;
Positives++;
break;
Console.WriteLine(“Error in switch”);
}
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Multiple Selection using if
• In many situations, a switch construct is inadequate
for multiple selection. For example, when
selections must be made on the basis of a Boolean
expression rather than some ordinal type, nested
two-way selectors can be used to simulate multiple
selection.
• In order to alleviate the poor readability of deeply
nested two-way selectors, Ada, Perl have been
extended specifically. The else-if sequences are
replaced with a special keyword (elsif) and the
special closing word on the nested if is dropped
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• Example on next slide
Multiple Selection using if
if
Count < 10 then Bag1 := True;
elsif Count < 100 then Bag2 := True;
elsif Count < 1000 then Bag3 := True;
end if;
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if Count < 10 then
Bag1 := True;
else
if Count < 100 then
Bag2 := True;
else
if Count < 1000 then
Bag3 := True;
end if;
end if;
end if;
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Multiple Selection using if
if
Count < 10 then Bag1 := True;
elsif Count < 100 then Bag2 := True;
elsif Count < 1000 then Bag3 := True;
end if;
if Count < 10
then Bag1 := True; end if
if Count < 100 then Bag2 := True; end if
if Count < 1000 then Bag3 := True; end if;
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Iterative statements
Iterative Statements
• An iterative statement causes a statement or a
collection of statements to be executed 0, 1, or
more times. An iterative construct is called loop
• Basic Design Issues (questions):
– How is the iteration controlled
• Counter controlled loops
• Logically controlled loops
• Structure iterators
– Where should control mechanism appear in the loop
• Pretest condition
• Posttest condition
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Iterative Statements
• The loop body is the collection of statements
whose execution is controlled by the iteration
statement.
• The iteration statement and the associated loop
body together form an iteration construct.
• What does the term pretest mean?
• What does the term posttest mean?
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Counter-Controlled Loops
• All loops have a loop variable, in which the count
value is maintained. Three Loop parameters are:
– Initial and terminal value of the loop variable
– Difference btw sequential loop variable values, stepsize
• Specific Design Issues:
– Type and scope of the loop variable
– Value of the loop variable at loop termination
– Is it legal to change loop variable or loop parameters in
the loop body?
– Should loop parameters be evaluated only once, or once
for every iteration
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The Do stmt of Fortran 95
• Formal syntax:
Do label variable = initial, terminal [,stepsize]
• Examples:
Do 100 I = 1,20,1
...
100 Continue
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For … Next Loop Statement of VBasic:
For counter = initVal To endVal [Step incVal]
...
One or more VB statements
...
Next [counter]
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The for stmt of C-like PL
• Formal syntax:
for(<exp1> ; <exp2> ; <exp3>) <statemnetnt> ;
• Flow chart and syntax diagram
• Examples:
for (int j=1; j<=10 ; j++) b[j] = j+j;
for (sum=0, k=1 ; k<=10 ; sum+=k, k++) NULL;
for ( ; ; ) cout <<“\nAUBG”;
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Logically Controlled Loops
• Collections of statements are repeatedly
executed, but repetition control is based on
Boolean expression rather than a counter.
• Logically controlled loops are more
general than counter-controlled loops.
• Every counting loop can be built with a
logical loop, but the reverse is not true.
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For loop converted to while
for(<exp1> ; <exp2> ; <exp3>) <statement>;
<exp1> ;
while (<exp2>)
{
<statement>;
<exp3>;
}
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Logically Controlled Loops
• Design Issues:
– Should the control be pretest or posttest?
– Should the logically controlled loop be a
special modified form of a counting statement
or it should be a totally separate statement
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Logic Loops - Syntax
• Pretest loop
while ( <control_expression> )
<loop body>
• Flow chart and syntax diagram
• Posttest loop
do
<loop body>
while ( <control expression> )
• Flow chart
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Logic Loops – C# Examples
sum = 0;
indat = Int32.Parse(Console.ReadLine());
while (indat >= 0) {
sum += indat;
indat = Int32.Parse(Console.ReadLine());
}
value = Int32.Parse(Console.ReadLine());
do {
value /= 10;
digits++;
} while (value > 0);
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Do Loops in VBasic
Do While boolean expression
statements
Loop
-----------------------------------------Do Until boolean expression
statements
Loop
-----------------------------------------Do
statements
Loop While boolean expression
-----------------------------------------Do
statements
Loop Until boolean expression
-----------------------------------------Do
statements
Loop
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Do Loops in VBasic
Pretest loops
----------------------------------------Do While boolean expression
statements
Loop
----------------------------------------Do Until boolean expression
statements
Loop
----------------------------------------7/20/2015
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Do Loops in VBasic
Posttest loops
----------------------------------------Do
statements
Loop While boolean expression
----------------------------------------Do
statements
Loop Until boolean expression
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Do Loops in VBasic
Infinite /endless/ loops
----------------------------------------Do
statements
Loop
-----------------------------------------
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User-Located Loop Controls
• Basic idea: to create an option for the
programmer to choose a location for loop control
other than the top or bottom of the loop.
• It is easy to implement user-located loop control,
or exit of the loop.
• It is more complicated to consider whether a
single loop or several nested loops can be exited.
• More comments on next slide.
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User-Located Loop Controls
• Mechanism to unconditionally exit loop
– C/C++: unconditional unlabeled exit – break
– Java/Perl/C#: unconditional labeled exit – ( break in
Java, C# and last in Perl)
• Mechanism to interrupt current iteration and stay
within loop
– C/C++: unlabeled continue
– Java/Perl/C#: statements similar to continue, except
that they can include labels to indicate which loop is to
be continued
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Loops – Practical Application
• Iterative processing concept (intro for beginners)
if (there are any steps to repeat)
{
loop required;
}
else
{
no loop required
}
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Loops – Practical Application
if (know in advance how many times to repeat)
{ use Counter controlled loop; }
else
{ use Sentinel controlled loop;
or
use Flag controlled loop;
or
use End File controlled loop;
or
use Input Validation loop;
or
use General conditional loop;
}
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Loop Patterns
• Sentinel Controlled Loops
1. Read the first data item.
2. while the sentinel value has not been read
3. Process the data item.
4. Read the next data item.
• Flag Controlled Loops
1. Set the flag to false.
2. while the flag is false
3. Perform some action.
4. Reset the flag to true if the anticipated event occurred.
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Example of Sentinel
Controlled Loop
#define SENTINEL (-99)
...
int sum=0, var;
cout << "\nEnter value:"; cin >> var;
while (var != SENTINEL)
{
sum += var; cin >> var;
}
cout << sum;
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Example of End File (End of
Data) Controlled Loop
int var, sum=0;
cout << "\nEnter value:"; cin >> var;
while ( !(cin.eof()) )
{
sum+=var;
cout << Enter new value:";
cin >> var;
}
cout << "\n\nSum=" << sum ;
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Example of Input Validation
Loop
int numobs;
...
cout << “\nEnter number of observed values:”;
cin >> numobs;
while (numobs <=0 )
{
cout <<“\nNegative or Zero observations invalid.”;
cout << “ Try again:”;
cin >> numobs;
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Iteration Based on Data
Structures
• Instead of using a counter or a Boolean
expression to control the iterations
processed, these loops use the number of
elements in a user defined dynamic data
structure.
• This topic is a special subject of the lecture
titled Iteration Based on Data Structures.
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Unconditional Branching
• An unconditional branch stmt transfers execution
control to a specific place in the program.
• Heat debate in PL design: should goto be a part
of any HLL, and if so whether its use should be
restricted.
• Goto is the most powerful and flexible stmt.
• Goto is the most dangerous stmt.
• Without restrictions on use, imposed by language
design or by programming standards, goto makes
programs very difficult to read and maintain.
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Unconditional Branching
• A few languages have been designed without
goto – Modula-2, Java
• Kernighan&Ritchie call goto infinitely abusable
but their C PL includes goto.
• C# (a relatively new PL) provides a legitimate
use of goto in the switch statement.
• All of the loop exit statements (break, continue)
are actually camouflaged gotos.
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Exercise 4a
Components of Programming Languages.
Part III.
Practical
• StatementLevel Control Structures
Assignments under discussion are based on lecture 4.
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Task 1
Using counter controlled loop statement, logically pretest
controlled loop statement and logically posttest
controlled loop statements, write in three versions a
program that:
• displays the squares of numbers from 0 to 19 (step size
+1) all at the same line;
• displays squares (x*x) and cubes (x*x*x) of numbers
from 0 to 10 using a three column format:
x
sqr(x)
cube(x)
• displays FahrenheitCelsius table in a range and format
chosen by the student. The relation Celsius/Fahrenheit is
Celsius = 5 / 9 * (Fahrenheit – 32)
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Reminder
Quiz #1
on
Components of PL
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Chapter 8
Statement-Level
Control Structures
ISBN 0-321-49362-1
Chapter 8 Topics
•
•
•
•
•
•
Introduction
Selection Statements
Iterative Statements
Unconditional Branching
Guarded Commands
Conclusions
Copyright © 2009 Addison-Wesley. All rights reserved.
1-67
Levels of Control Flow
–
–
–
Within expressions (Chapter 7)
Among program units (Chapter 9)
Among program statements (this chapter)
Copyright © 2009 Addison-Wesley. All rights reserved.
1-68
Control Statements: Evolution
• FORTRAN I control statements were based
directly on IBM 704 hardware
• Much research and argument in the 1960s
about the issue
– One important result: It was proven that all
algorithms represented by flowcharts can be
coded with only two-way selection and pretest
logical loops
Copyright © 2009 Addison-Wesley. All rights reserved.
1-69
Control Structure
• A control structure is a control statement
and the statements whose execution it
controls
• Design question
– Should a control structure have multiple entries?
Copyright © 2009 Addison-Wesley. All rights reserved.
1-70
Selection Statements
• A selection statement provides the means
of choosing between two or more paths of
execution
• Two general categories:
– Two-way selectors
– Multiple-way selectors
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1-71
Two-Way Selection Statements
• General form:
if control_expression
then clause
else clause
• Design Issues:
– What is the form and type of the control
expression?
– How are the then and else clauses specified?
– How should the meaning of nested selectors be
specified?
Copyright © 2009 Addison-Wesley. All rights reserved.
1-72
The Control Expression
• If the then reserved word or some other
syntactic marker is not used to introduce
the then clause, the control expression is
placed in parentheses
• In C89, C99, Python, and C++, the control
expression can be arithmetic
• In languages such as Ada, Java, Ruby, and
C#, the control expression must be Boolean
Copyright © 2009 Addison-Wesley. All rights reserved.
1-73
Clause Form
• In many contemporary languages, the then and
else clauses can be single statements or compound
statements
• In Perl, all clauses must be delimited by braces
(they must be compound)
• In Fortran 95, Ada, and Ruby, clauses are
statement sequences
• Python uses indentation to define clauses
if x > y :
x = y
print "case 1"
Copyright © 2009 Addison-Wesley. All rights reserved.
1-74
Nesting Selectors
• Java example
if (sum == 0)
if (count == 0)
result = 0;
else result = 1;
• Which if gets the else?
• Java's static semantics rule: else matches
with the nearest if
Copyright © 2009 Addison-Wesley. All rights reserved.
1-75
Nesting Selectors (continued)
• To force an alternative semantics,
compound statements may be used:
if (sum == 0) {
if (count == 0)
result = 0;
}
else result = 1;
• The above solution is used in C, C++, and C#
• Perl requires that all then and else clauses to be
compound
Copyright © 2009 Addison-Wesley. All rights reserved.
1-76
Nesting Selectors (continued)
• Statement sequences as clauses: Ruby
if sum == 0 then
if count == 0 then
result = 0
else
result = 1
end
end
Copyright © 2009 Addison-Wesley. All rights reserved.
1-77
Nesting Selectors (continued)
• Python
if sum == 0 :
if count == 0 :
result = 0
else :
result = 1
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1-78
Multiple-Way Selection Statements
•
•
Allow the selection of one of any number of
statements or statement groups
Design Issues:
1. What is the form and type of the control expression?
2. How are the selectable segments specified?
3. Is execution flow through the structure restricted to
include just a single selectable segment?
4. How are case values specified?
5. What is done about unrepresented expression values?
Copyright © 2009 Addison-Wesley. All rights reserved.
1-79
Multiple-Way Selection: Examples
• C, C++, and Java
switch (expression) {
case const_expr_1: stmt_1;
…
case const_expr_n: stmt_n;
[default: stmt_n+1]
}
Copyright © 2009 Addison-Wesley. All rights reserved.
1-80
Multiple-Way Selection: Examples
•
Design choices for C’s switch statement
1. Control expression can be only an integer type
2. Selectable segments can be statement sequences,
blocks, or compound statements
3. Any number of segments can be executed in one
execution of the construct (there is no implicit
branch at the end of selectable segments)
4. default clause is for unrepresented values (if
there is no default, the whole statement does
nothing)
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Multiple-Way Selection: Examples
• C#
– Differs from C in that it has a static semantics
rule that disallows the implicit execution of
more than one segment
– Each selectable segment must end with an
unconditional branch (goto or break)
– Also, in C# the control expression and the case
constants can be strings
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Multiple-Way Selection: Examples
• Ada
case expression is
when choice list => stmt_sequence;
…
when choice list => stmt_sequence;
when others => stmt_sequence;]
end case;
• More reliable than C’s switch (once a
stmt_sequence execution is completed, control is
passed to the first statement after the case
statement
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Multiple-Way Selection: Examples
• Ada design choices:
1. Expression can be any ordinal type
2. Segments can be single or compound
3. Only one segment can be executed per
execution of the construct
4. Unrepresented values are not allowed
• Constant List Forms:
1. A list of constants
2. Can include:
- Subranges
- Boolean OR operators (|)
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Multiple-Way Selection: Examples
• Ruby has two forms of case statements
1. One form uses when conditions
leap = case
when year % 400 == 0 then true
when year % 100 == 0 then false
else year % 4 == 0
end
2. The other uses a case value and when values
case
when
when
when
else
end
in_val
-1 then neg_count++
0 then zero_count++
1 then pos_count++
puts "Error – in_val is out of range"
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Multiple-Way Selection Using if
• Multiple Selectors can appear as direct
extensions to two-way selectors, using
else-if clauses, for example in Python:
if count < 10 :
bag1 = True
elif count < 100 :
bag2 = True
elif count < 1000 :
bag3 = True
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Multiple-Way Selection Using if
• The Python example can be written as a
Ruby case
case
when count < 10 then bag1 = true
when count < 100 then bag2 = true
when count < 1000 then bag3 = true
end
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Iterative Statements
• The repeated execution of a statement or
compound statement is accomplished
either by iteration or recursion
• General design issues for iteration control
statements:
1. How is iteration controlled?
2. Where is the control mechanism in the loop?
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Counter-Controlled Loops
• A counting iterative statement has a loop
variable, and a means of specifying the
initial and terminal, and stepsize values
• Design Issues:
1. What are the type and scope of the loop
variable?
2. Should it be legal for the loop variable or loop
parameters to be changed in the loop body,
and if so, does the change affect loop control?
3. Should the loop parameters be evaluated only
once, or once for every iteration?
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Iterative Statements: Examples
• FORTRAN 95 syntax
DO label var = start, finish [, stepsize]
• Stepsize can be any value but zero
• Parameters can be expressions
• Design choices:
1. Loop variable must be INTEGER
2. The loop variable cannot be changed in the loop, but the
parameters can; because they are evaluated only once, it
does not affect loop control
3. Loop parameters are evaluated only once
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Iterative Statements: Examples
• FORTRAN 95 : a second form:
[name:] Do variable = initial, terminal [,stepsize]
…
End Do [name]
- Cannot branch into either of Fortran’s Do
statements
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1-91
Iterative Statements: Examples
• Ada
for var in [reverse] discrete_range loop
...
end loop
• Design choices:
- Type of the loop variable is that of the discrete
range (A discrete range is a sub-range of an
integer or enumeration type).
- Loop variable does not exist outside the loop
- The loop variable cannot be changed in the loop,
but the discrete range can; it does not affect loop
control
- The discrete range is evaluated just once
• Cannot branch into the loop body
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1-92
Iterative Statements: Examples
• C-based languages
for ([expr_1] ; [expr_2] ; [expr_3]) statement
- The expressions can be whole statements, or even
statement sequences, with the statements separated by
commas
– The value of a multiple-statement expression is the value of the
last statement in the expression
– If the second expression is absent, it is an infinite loop
• Design choices:
- There is no explicit loop variable
- Everything can be changed in the loop
- The first expression is evaluated once, but the other two
are evaluated with each iteration
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Iterative Statements: Examples
• C++ differs from C in two ways:
1. The control expression can also be Boolean
2. The initial expression can include variable
definitions (scope is from the definition to the
end of the loop body)
• Java and C#
– Differs from C++ in that the control
expression must be Boolean
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Iterative Statements: Examples
• Python
for loop_variable in object:
- loop body
[else:
- else clause]
– The object is often a range, which is either a list of values
in brackets ([2, 4, 6]), or a call to the range function
(range(5), which returns 0, 1, 2, 3, 4
– The loop variable takes on the values specified in the
given range, one for each iteration
– The else clause, which is optional, is executed if the loop
terminates normally
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1-95
Iterative Statements: LogicallyControlled Loops
• Repetition control is based on a Boolean
expression
• Design issues:
– Pretest or posttest?
– Should the logically controlled loop be a
special case of the counting loop statement or
a separate statement?
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Iterative Statements: LogicallyControlled Loops: Examples
• C and C++ have both pretest and posttest
forms, in which the control expression can
be arithmetic:
while (ctrl_expr)
loop body
do
loop body
while (ctrl_expr)
• Java is like C and C++, except the control
expression must be Boolean (and the body
can only be entered at the beginning -- Java
has no goto
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Iterative Statements: LogicallyControlled Loops: Examples
• Ada has a pretest version, but no posttest
• FORTRAN 95 has neither
• Perl and Ruby have two pretest logical
loops, while and until. Perl also has two
posttest loops
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Iterative Statements: User-Located Loop
Control Mechanisms
• Sometimes it is convenient for the
programmers to decide a location for loop
control (other than top or bottom of the
loop)
• Simple design for single loops (e.g., break)
• Design issues for nested loops
1. Should the conditional be part of the exit?
2. Should control be transferable out of more
than one loop?
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Iterative Statements: User-Located Loop
Control Mechanisms break and continue
• C , C++, Python, Ruby, and C# have
unconditional unlabeled exits (break)
• Java and Perl have unconditional labeled
exits (break in Java, last in Perl)
• C, C++, and Python have an unlabeled
control statement, continue, that skips the
remainder of the current iteration, but does
not exit the loop
• Java and Perl have labeled versions of
continue
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1-100
Iterative Statements: Iteration Based on
Data Structures
• Number of elements of in a data structure
control loop iteration
• Control mechanism is a call to an iterator
function that returns the next element in
some chosen order, if there is one; else
loop is terminate
• C's for can be used to build a user-defined
iterator:
for (p=root; p==NULL; traverse(p)){
}
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Iterative Statements: Iteration Based on
Data Structures (continued)
PHP
- current points at one element of the array
- next moves current to the next element
- reset moves current to the first element
•
-
Java
For any collection that implements the Iterator interface
next moves the pointer into the collection
hasNext is a predicate
remove deletes an element
• Perl has a built-in iterator for arrays and hashes,
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foreach
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Iterative Statements: Iteration Based on
Data Structures (continued)
• Java 5.0 (uses for, although it is called foreach)
- For arrays and any other class that implements
Iterable interface, e.g., ArrayList
for (String myElement : myList) { … }
• C#’s foreach statement iterates on the elements of arrays and
other collections:
Strings[] = strList = {"Bob", "Carol", "Ted"};
foreach (Strings name in strList)
Console.WriteLine ("Name: {0}", name);
- The notation {0} indicates the position in the string to be displayed
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Iterative Statements: Iteration Based on
Data Structures (continued)
• Lua
– Lua has two forms of its iterative statement, one
like Fortran’s Do, and a more general form:
for variable_1 [, variable_2] in iterator(table) do
…
end
– The most commonly used iterators are pairs
and ipairs
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Unconditional Branching
• Transfers execution control to a specified place in
the program
• Represented one of the most heated debates in
1960’s and 1970’s
• Major concern: Readability
• Some languages do not support goto statement
(e.g., Java)
• C# offers goto statement (can be used in switch
statements)
• Loop exit statements are restricted and somewhat
camouflaged goto’s
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Guarded Commands
• Designed by Dijkstra
• Purpose: to support a new programming
methodology that supported verification
(correctness) during development
• Basis for two linguistic mechanisms for
concurrent programming (in CSP and Ada)
• Basic Idea: if the order of evaluation is not
important, the program should not specify
one
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Selection Guarded Command
• Form
if <Boolean exp> -> <statement>
[] <Boolean exp> -> <statement>
...
[] <Boolean exp> -> <statement>
fi
• Semantics: when construct is reached,
– Evaluate all Boolean expressions
– If more than one are true, choose one nondeterministically
– If none are true, it is a runtime error
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Loop Guarded Command
• Form
do <Boolean> -> <statement>
[] <Boolean> -> <statement>
...
[] <Boolean> -> <statement>
od
• Semantics: for each iteration
– Evaluate all Boolean expressions
– If more than one are true, choose one nondeterministically; then start loop again
– If none are true, exit loop
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Guarded Commands: Rationale
• Connection between control statements
and program verification is intimate
• Verification is impossible with goto
statements
• Verification is possible with only selection
and logical pretest loops
• Verification is relatively simple with only
guarded commands
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Conclusion
• Variety of statement-level structures
• Choice of control statements beyond
selection and logical pretest loops is a
trade-off between language size and
writability
• Functional and logic programming
languages are quite different control
structures
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Thank You
for
Your attention