Transcript Control Structures - Washtenaw Community College

Control Structures
CPS120: Introduction to
Computer Science
Lecture 5
What Can a Program Do?
• A program can only instruct a computer to:
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Read Input
Sequence
Calculate
Store data
Compare and branch
Iterate or Loop
Write Output
Fundamental Programming Concepts
• Assignment of values to a variable
• Iteration (Looping)
– Over a set of set of statements
– With respect to a logical expressions
(conditions)
• Delegation of sub-tasks to functions /
procedures
The Structure Theorem
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The Structure Theorem states that any algorithm
can be built from three basic control structures.
One-after-another (Sequence)
Decision-making (Selection)
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Making choices between 2 or more alternatives
Repetition (Iteration)
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Concerned with repetitive tasks (and the termination
conditions of loops)
C++ Control Structures
1. "Sequence statements" are imperatives
2. "Selection" is the "if then else" statement
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AND, OR, NOT and parentheses ( ) can be used for compound
conditions
3. "Iteration" is satisfied by a number of statements
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"while"
" do "
"for"
4. The case-type statement is satisfied by the
"switch" statement.
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CASE statements are used for most non-trivial
selection decisions
Sequence Control Structures
• Sequence control structures direct the order
of program instructions.
• The fact that one instruction follows
another—in sequence—establishes the
control and order of operations.
Calculate
• A program can
instruct a computer
to perform
mathematical
operations.
Add 1 to
Counter
Store
• A program will often
instruct a computer to
store intermediate
results.
Place 1
in
Counter
Compare and Branch
• A program can instruct a computer to compare
two items and do something based on a match
or mismatch which, in turn, redirect the
sequence of programming instructions.
– There are two forms:
– IF-THEN
– IF-THEN-ELSE
IF-THEN
Entry
Test
condition p
Exit
false
true
True
statement a
IF-THEN-ELSE
Entry
Test
condition p
false
“false”
statement a
true
Exit
“true”
statement a
Iterate
• A program loop is a
form of iteration. A
computer can be
instructed to repeat
instructions under
certain conditions.
No
Iteration Control Structures
• Iteration control structures are looping
mechanisms.
• Loops repeat an activity until stopped. The
location of the stopping mechanism
determines how the loop will work:
• Leading decisions
• Trailing decisions
Leading Decisions
• If the stop is at the beginning of the
iteration, then the control is called a leading
decision.
• The command DO WHILE performs the
iteration and places the stop at the
beginning.
DO WHILE Loop
Entry
Exit
No
Test
condition p
Yes
Loop
statement a
Trailing Decisions
• If the stop is at the end of the iteration, the
control mechanism is called a trailing
decision.
• The command DO UNTIL performs the
iteration and puts the stop at the end of the
loop.
DO UNTIL Loop
Entry
Loop
statement a
Test
condition p
Exit
No
Yes