Transcript CPS120 Introduction to Computer Science

CPS120 Introduction to
Computer Science
Exam Review
Compilers
• An engine that works on your behalf to
process instructions and allow you to deal
with various basic rules of the language
• The compiler’s job is to make sure you
follow the rules, to require that you provide
enough information that the compiler can
translate you instructions into languages the
components can understand
Fundamental Programming Concepts
• One-after-another (Sequence)
• Decision-making (Selection)
– Making choices between 2 or more alternatives
• Repetition (Iteration)
– Concerned with repetitive tasks (and the termination
conditions of loops)
• Invocation
– Delegation of sub-tasks to functions / procedures
Common Flowchart Symbols
Common Flowchart Symbols
Terminator. Shows the starting and ending points of the program. A terminator has
flow lines in only one direction, either in (a stop node) or out (a start node).
Data Input or Output. Allows the user to input data and results to be displayed.
Processing. Indicates an operation performed by the computer, such as a variable
assignment or mathematical operation. With a heading – an internal subroutine
Decision. The diamond indicates a decision structure. A diamond always has two
flow lines out. One flow lineout is labeled the “yes” branch and the other is labeled the
“no” branch.
Predefined Process. One statement denotes a group of previously defined statements.
Such as a function or a subroutine created externally
Connector. Connectors avoid crossing flow lines, making the flowchart easier to read.
Connectors indicate where flow lines are connected. Connectors come in pairs, one with
a flow line in and the other with a flow line out.
Off-page connector. Even fairly small programs can have flowcharts that extend several
pages. The off-page connector indicates the continuation of the flowchart on another
page. Just like connectors, off-page connectors come in pairs.
Flow line. Flow lines connect the flowchart symbols and show the sequence of operations
during the program execution.
Rules for Pseudocode
1. Make the pseudocode language-independent
2. Indent lines for readability
3. Make key words stick out by showing them
capitalized, in a different color or a different font
4. Punctuation is optional
5. End every IF with ENDIF
6. Begin loop with LOOP and end with ENDLOOP
7. Show MAINLINE first; all others follow
8. TERMINATE all routines with an END
instruction
Syntax & Logic Errors
• A syntax error is simply the violation of the
rules of a language; misuse of structure and
form in programming or a violation of the
compiler’s rules. These errors are detected
by the compiler
• A logic error is a mistake that complies
with the rules of the compiler that causes
the program to generate incorrect output
Compiling and Debugging
• Executable code will not be created until
you correct all of the syntax errors in your
source code
Linker Errors
• Not all syntax errors are detectable by the
compiler
– These errors do not become apparent until files
are put together to create an executable
– These errors are not linked to a specific line of
code
Alphanumeric Codes
• American Standard Code for Information
Interchange (ASCII)
– 7-bit code
– Since the unit of storage is a bit, all ASCII
codes are represented by 8 bits, with a zero in
the most significant digit
– H e l l o W o r l d
– 48 65 6C 6C 6F 20 57 6F 72 6C 64
Decimal Equivalents
•
Assuming the bits are unsigned, the decimal
value represented by the bits of a byte can be
calculated as follows:
1. Number the bits beginning on the right using
superscripts beginning with 0 and increasing as you
move left
•
Note: 20, by definition is 1
2. Use each superscript as an exponent of a power of 2
3. Multiply the value of each bit by its corresponding
power of 2
4. Add the products obtained
Binary to Hex
• Step 1: Form four-bit groups beginning
from the rightmost bit of the binary number
– If the last group (at the leftmost position) has
less than four bits, add extra zeros to the left of
the group to make it a four-bit group
• Step 2: Replace each four-bit group by its
hexadecimal equivalent
– 19EAA7(16
Converting Decimal to Other Bases
• Step 1: Divide the number by the base you are
converting to (r)
• Step 2: Successively divide the quotients by (r)
until a zero quotient is obtained
• Step 3: The decimal equivalent is obtained by
writing the remainders of the successive division
in the opposite order in which they were obtained
– Know as modulus arithmetic
• Step 4: Verify the result by multiplying it out
Representing Negatives
• It is necessary to choose one of the bits of the
“basic unit” as a sign bit
– Usually the leftmost bit
– By convention, 0 is positive and 1 is negative
• Positive values have the same representation in all
conventions
• However, in order to interpret the content of any
memory location correctly, it necessary to know
the convention being used used for negative
numbers
Sign-Magnitude
• For a basic unit of N bits, the leftmost bit is
used exclusively to represent the sign
Sign-magnitude Operations
• Addition of two numbers in sign-magnitude
is carried out using the usual conventions of
binary arithmetic
– If both numbers are the same sign, we add their
magnitude and copy the same sign
– If different signs, determine which number has
the larger magnitude and subtract the other
from it. The sign of the result is the sign of the
operand with the larger magnitude
One’s Complement
• Positive numbers are represented in the usual
way
• For negatives
– STEP 1: Start with the binary representation of the
absolute value
– STEP 2: Complement all of its bits
One's Complement Operations
– Treat the sign bit as any other bit
– For addition, carry out of the leftmost bit is
added to the rightmost bit – end-around carry
Two’s Complement Convention
•
•
A positive number is represented using a
procedure similar to sign-magnitude
To express a negative number
1. Express the absolute value of the number in binary
2. Change all the zeros to ones and all the ones to
zeros (called “complementing the bits”)
3. Add one to the number obtained in Step 2
Two’s Complement Operations
• Addition:
– Treat the numbers as unsigned integers
• The sign bit is treated as any other number
– Ignore any carry on the leftmost position
• Subtraction
– Treat the numbers as unsigned integers
– If a "borrow" is necessary in the leftmost place,
borrow as if there were another “invisible” onebit to the left of the minuend
C++ Usages & Conventions
• C++ is absolutely case sensitive
–For Instance: A is 97 in ASCII and a is 65
–Remember: in ASCII {, [, and ( are not
equivalent
• No keywords in ANSI standard are even
partially uppercase
–‘While’ is not a keyword, ‘while’ is
–Be careful if you define new keywords
Comments
• Document what is happening, why it is happening
and other issues
• Commentary is ignored by the compiler
• C++ has inline, block and documentary comments
–Inline comments are within line of code
• Use the // symbols
–Block comments are long comments delimited with /*
and */
Compiler Directives
• Instructions to the compiler rather than part
of the C++ language
– Most common directive is #include
• #include <iostream.h>
– A .h file is a header file. It serves as a link between
program code and standard C++ code needed to make
programs run
Functions
• A function is a block of code that carries out
a specific task
• Every C++ program has a main function
that executes when a program initiates
– Includes open parenthesis to designate a
function
– Ends with a return 0; statement
Scope Delimiters
• A symbol or pair of symbols used to define
a region or area which is considered a locale
• In programming, many structures need to
have their scope defined because they
should not affect the entire program
– In C++, the symbols ‘{‘ and ‘}’ are used
Semicolons
• There must be a semicolon after every
statement
– To tell the compiler that the statement is
complete
– Function definitions and compiler directives are
exempt
C++ Control Structures
1. "Sequence statements" are imperatives
2. "Selection" is the "if then else" statement
–
AND, OR, NOT and parentheses ( ) can be used for compound
conditions
3. "Iteration" is satisfied by a number of statements
–
–
–
"while"
" do "
"for"
4. The case-type statement is satisfied by the
"switch" statement.
–
CASE statements are used for most non-trivial
selection decisions
Data Types - Characters
• To store a letter or a single character (such
as #, $, *, etc.), we use a variable of the
char data type.
– A char variable only uses 1 byte of memory.
– A char variable can only hold one letter, digit,
or character.
Data Types – Words / Phrases
• To store a word or phrase (string value), we
use a variable that is a string
– Technically string is not a data type but a class
Using Variables in C++
• Variables must be declared before they are
used in C++. Get into the habit of doing this
at the top of your functions
Variable Names
• Choose your own variable names but you
must be careful to use valid ones:
– do not use keywords that are defined in the
programming language (Reserved Words)
– do not include spaces or other disallowed
characters
– do not use more than 255 characters
– do begin the identifier with a letter
Initializing Variables
• C++ does not automatically initialize all
variables
The Assignment Operator
• The assignment operator is the equal
symbol (=)
• The assignment operator changes the value
of the variable to its left after evaluating the
expression on its right
Decision Making In Computers
• When decisions are made in a computer
program, they are simply the result of a
computation in which the final result is
either TRUE or FALSE
• The value zero (0) is considered to be
FALSE by C++. Any positive or negative
value is considered to be TRUE
Using Relational Operators
• Relational operators provide the tools with
which programs make decisions
==
equal to
NOTE: this is two equals symbols next to each
other, not to be confused with the assignment operator, =
>
greater than
<
less than
>=
greater than or equal to
<=
less than or equal to
!=
not equal to
Complete order of operations
• The complete order of operations including all of
the arithmetic, relational, and logical operators
including all of the basic arithmetic, relational, &
logical operators is:
*, /, %
+, <, >, <=, >=, ==, !=
!
&&
||
Character Arrays
• An array is a group of variables of the same
data type that appear together in memory
– In this case each variable holds a character and
the last variable in the string holds the null
terminator (/0)
Input Operations
• The operator >> is known as the input
operator. It is also known as the extraction
operator
• You use the input operator in statements
like,
cin >> numItems;
which would allow the user to input a
value to be stored in the variable
numItems.
Complexities of Word Input
• Some things are done automatically with >>
– get does not skip over line breaks and spaces
• If the user enters a string longer than the length
specified in the call to the get function, the
remaining characters are left in the input stream
• Get always ignores the new line character (‘\n’)
and leaves it in the stream
• Use the ignore function to flush the contents of the
input stream
cin.ignore(80, ‘\n’);
Decision Making in C++
1.
2.
3.
4.
if statement
switch statement
? conditional operator statement
goto statement
General Form
if (test expression)
{
True-block statements;
}
else
{
False-block statements;
}
next statement;
Iterate
• A program loop is a
form of iteration. A
computer can be
instructed to repeat
instructions under
certain conditions.
No
Syntax of a for Loop
for (initializing expression; control expression;
step expression)
{
// one or more statements
}
• The initializing expression sets the counter
variable for the loop to its initial value.
• The control expression ends the loop at the
specified moment.
• The step expression changes the counter variable
• Semi-colons, not commas, divide the expressions
While Loop Syntax
while (control expression)
{
// one or more statements
}
• The control expression must evaluate to
TRUE in order for the while loop to iterate
even once
Do While Syntax
do
{
// body statements would be placed here
}while (control expression);
• Don't forget to include the required semicolon
after the control expression
Break and Continue
• continue causes while, do… while, and for
loops to start over
• break causes while, do … while, for and
switch statements to end
An Example of A Function
#include <iostream.h>
void printMyMessage(int numOfTimes); // PROTOTYPE and NAME
int main( )
{
…
{
printMyMessage (userInput); // CALL STATEMENT WITH ACTUAL PARAMETER
}
…
return 0;
} // end of main
void printMyMessage(int numOfTimes)
// FUNCTION HEADER {
int i=0; // LOCAL VARIABLE WITHIN THE FUNCTION
for (i=0; i<= numOfTimes; i++)
{cout << "Let's Go State!!" << endl;}
} //end of printMyMessage
// BODY
// OF THE
// FUNCTION
Scope of Variables
• The scope of a variable is the area in which it can
be legally referenced
– Variables are either global or local in nature
– Global variables are ones that are declared outside and
above the main function
– They can be used in any function throughout the
program.
• It is not wise to use global variables any more than you have
to.
– Local variables are ones that are declared inside of a
function, including main. They cannot be used or
referred to in other functions
Reading From a File
• Declare a file pointer as an ifstream object
with:
ifstream infile;
– ifstream is a keyword and infile is the name for
the file pointer.
• Open the actual file for reading with:
infile.open("mydata.txt", ios::in);
infile.open("mydata.txt");
Writing Output
• To write data to a sequential-access data file
you would use a statement like:
outfile << "John Doe" << endl;
to print that name to the next line in the data
file pointed to by the file pointer, outfile.
Appending Data
• Adding data to the end of a sequential-access data
file is called appending
• Open the file using the ios::app stream operation
mode as in:
outfile.open("myfile.txt", ios::app);
• where the app is short for append.
• If you accidentally open the file with the ios::out
mode, you will end up overwriting data in the file
because C++ will write the first piece of outputted
data at the beginning of the sequential-access data
file
Declaring an Array
• To declare an array before it is used in the body of
your program, you must use a statement like:
int scores[10];
• This would declare an array of integers, named
"scores".
• In this case, scores can store up to 10 different
integer values.
– The positions of the array are identified by their index
positions which run from 0 to 9 (not 1 to 10.)
• Each one of the 10 variables in scores is called an
element
Initializing an Array
• If you wish to initialize each element of an array
to a specific value, you can use the statement,
int scores[] = {65, 76, 45, 83, 99};
• You don't even have to specify a size of the array
in this case since the initialization statement
would cause the compiler to declare an array of
size 5 since there are five values in the set of curly
braces
Declaring a Multi-dimensional
Array
• To declare an array of integers called
studentGrades to be a 2-dimensional array with 3
rows and 4 columns, you would use the statement:
int studentGrades[3] [4];
where the first integer value is used to specify the
number of rows in the array and the second value
specifies the number of columns
• Think of remote control