EC-237 Computer System & Programming: Lectures 1-3, Cheat Sheet of Computer Science

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EC-237 Computer System & Programming

Lecture 3

Control Statements

Dr. Arslan Shaukat

Control Structures

▪ Sequential execution

– Statements executed in order

▪ Transfer of control

– Next statement executed not next one in sequence

▪ 3 control structures

– Sequence structure

• Programs executed sequentially by default

– Selection structures

• if, if/else, switch

– Repetition structures

• while, do/while, for

if Selection Structure

▪ Selection structure

– Choose among alternative courses of action

– Pseudocode example:

If student’s grade is greater than or equal to 60

Print “Passed”

– If the condition is true

• Print statement executed, program continues to next statement

– If the condition is false

• Print statement ignored, program continues

– Indenting makes programs easier to read

• C++ ignores whitespace characters (tabs, spaces, etc.)

if Selection Structure

▪ Translation into C++

If student’s grade is greater than or equal to 60

Print “Passed”

if ( grade >= 60 )

cout << "Passed";

▪ Diamond symbol (decision symbol)

– Indicates decision is to be made

– Contains an expression that can be true or false

• Test condition, follow path

if/else Selection Structure

▪ if

– Performs action if condition true

▪ if/else

– Different actions if conditions true or false

▪ Pseudocode

if student’s grade is greater than or equal to 60

print “Passed”

else

print “Failed”

▪ C++ code

if ( grade >= 60 )

cout << "Passed";

else

cout << "Failed";

Selection Structure

▪ Ternary conditional operator ( ?: )

– Three arguments (condition, value if true , value if false )

▪ Code could be written:

cout << ( grade >= 60? “Passed” : “Failed” ); false true print “Failed” print “Passed” grade >= 60

Condition Value if true Value if false

if/else Selection Structure

▪ Example

if ( grade >= 90 ) // 90 and above

cout << "A";

else if ( grade >= 80 ) // 80- 89

cout << "B";

else if ( grade >= 70 ) // 70- 79

cout << "C";

else if ( grade >= 60 ) // 60- 69

cout << "D";

else // less than 60

cout << "F";

if/else Selection Structure

▪ Compound statement

– Set of statements within a pair of braces

if ( grade >= 60 ) cout << "Passed.\n"; else { cout << "Failed.\n"; cout << "You must take this course again.\n";

– Without braces,

cout << "You must take this course again.\n";

always executed

▪ Block

– Set of statements within braces

Dangling Else Problem

if ( x > 5)

if ( y > 5)

cout << “x and y are > 5”;

else

cout << “x is <= 5”;

Dangling Else Problem

if ( x > 5)

if ( y > 5)

cout << “x and y are > 5”;

else

cout << “x is <= 5”;

else associated with the first if

while Repetition Structure

▪ Repetition structure

– Action repeated while some condition remains true

– Psuedocode

while there are more items on my shopping list

Purchase next item and cross it off my list

– while loop repeated until condition becomes false

▪ Example

int product = 2;

while ( product <= 1000 )

product = 2 * product;

Formulating Algorithms (Counter-Controlled

Repetition)

▪ Counter-controlled repetition

– Loop repeated until counter reaches certain value

▪ Definite repetition

– Number of repetitions known

▪ Example

A class of ten students took a quiz. The grades (integers in the

range 0 to 100) for this quiz are available to you. Determine the

class average on the quiz.

Formulating Algorithms (Counter-Controlled

Repetition)

▪ Pseudocode for example:

Set total to zero

Set grade counter to one

While grade counter is less than or equal to ten

Input the next grade

Add the grade into the total

Add one to the grade counter

Set the class average to the total divided by ten

Print the class average

▪ Next: C++ code for this example