Control Statements CSCI 125 & 161 / ENGR 144 Statements that - - PDF document

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CSCI 125 & 161 / ENGR 144 Lecture 7

Martin van Bommel

Control Statements

• Statements that affect the sequence of

execution of other statements

• Normal is sequential
• May want to repeat statements - iteration
• May or may not want particular statement

executed at all - conditional

Control Statement Structure

• Control line - first line (e.g. while)
• Body - statements enclosed in { } that are

repeated

• Body typically indented to show range of

control statement

Iteration and Loops

• Iteration - process of repeating an operation
• Essential for large amounts of data -

repeating statements shortens programs

• Loop - any portion of program repeated via

control statement

• e.g. while loop

The while statement

while (condition) { statements } where condition tested before first and every iteration statements executed each time condition true

while operation

• Condition test performed before every cycle,

including first.

• If condition is initially false,

body of loop not executed at all.

• Condition test performed only at beginning of

each loop cycle.

• Even if condition becomes false during cycle,

cycle is completed.

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while example

• Sum up digits of a number

sum = 0; while (n > 0) { sum = sum + n % 10; n = n / 10; }

Infinite loops

• A loop cycle that never stops – infinite loop
• Condition of while statement never false
• e.g.

while (n >= 0) { sum += n % 10; n /= 10; }

Repeat-N-Times Idiom

• for (i = 0; i < N; i++)

{ statements to be repeated }

• May want to add up 10 input numbers

for (i = 0; i < 10; i++) { cin >> value; sum = sum + value; }

Index Variable in Counting Loop

for (i = 0; i < N; i++)

• Variable i is called the index variable
• Any variable may be used - traditionally i
• Must be declared at beginning of function
• Initial value of i in loop is 0 - first cycle
• Next cycles value is 1, 2, etc.
• On last cycle, value is N – 1
• After loop, value of i is N

Non-Zero Counting Loop

• It is possible to modify for loop to begin at

another number for (i = first; i <= last; i++)

• Note the <= operator instead of <
• Value of i ranges from first to last
• Used if value of i required in body of loop

for statement

for (init; test; step) { statements } where init is an expression evaluated at beginning test is a condition for continuing the loop step is an expression to prepare next cycle

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for example

• Countdown from 10 to 0

for (t=10; t >= 0; t--) { cout << t; } cout << ”Liftoff!\n”;

for more

• Note that expressions init, test, and step are
• ptional, but the semicolons must appear

for (;;)

• infinite loop

for (;t<10;) same as while (t<10)

Nested for loops

• Create a 10 x 10 multiplication table

for (row=1; row<=10; row++) { for (col=1; col<=10; col++) { cout << (row * col); } cout << endl; }

Conditional Execution

• May want to execute a statement only if

some condition applies

• May wish to choose between two

alternative courses of actions depending on some test

• Simplest form - if statement

if Statement

if (conditional-test) { …statements executed if test true... } } else { …statements executed if test false... }

Relational & Logical Operators

== Equal != Not equal > Greater < Less >= Greater or equal <= Less or equal

• Logical operate on results of comparisons

! Not (true if operand false – one operand) && And (true if both true) || Or (true if either true)