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C/C++ Programming for Engineers:Loops

John T. Bell

Department of Computer ScienceUniversity of Illinois, Chicago

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Review

What would be the best data type to use to record the concentration of a solution in a chemistry laboratory experiment?

A. boolB. charC. doubleD. intE. long int

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The Power of Loops

• Computers are dumb machines, that only do what they are told.

• Their power lies in that they follow instructions very quickly, and don’t mind repeating the same instruction millions of times per second. Hence the power of the loop.

• The programmers job is then to tell the computer how to loop, when to stop looping, and what to do each pass through the loop.

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While Loops

• The simplest loops to understand are whileloops, which continue looping as long as some condition remains true.

• The condition is evaluated BEFORE each iteration of the loop, and the loop is executed only if the condition is true.

• ( If the condition is initially false, then the loop will execute zero times, i.e. not at all. )

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While Loop Syntax

while( condition ) {

// code in body of loop

}

• If braces are omitted, then a single statement comprises the body of the loop.

condition?

body

Followingcode

False

True

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While Example

int guess = -1, answer = rand( ) % 10 + 1;

while( guess != answer ) {

cout << “Enter a guess from 1 to 10: “;

cin >> guess;

}

cout << “That’s right! Congratulations!\n”;

Must be initialized to ensure loop entry

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Looping With a Counter

• Although a while loop could be used with a counter to loop a specified number of times, that is not good practice.

• The best approach when a loop must execute a specified number of times is with a for loop, which initializes the counter, tests it on each iteration, and ( typically ) increments it on each iteration.

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For Loop Syntax

for( init; condition; incr ) {

// code in body of loop

}

• If braces are omitted, then a single statement comprises the body of the loop.

• Note “incr” always happens after executing the body.

condition?

body

Followingcode

False

True

incr

init

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The for incrementation component most commonly uses auto-increment:

• “i++;”, as a stand-alone statement, is equivalent to “i += 1;” or “i = i + 1;”

• “i--;”, as a stand-alone statement, is equivalent to “i -= 1;” or “i = i - 1;”

• There is more to auto-increment ( and auto-decrement ), which we cover elsewhere.

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For Loop Example

cout << “ i i^2 i^3” << endl;

int i;

for( i = 1; i <= 10; i++ ) {

cout << i << “ “ << i * i << “ “ << i * i * i;

cout << endl;

}

cout << “After the loop, i = “ << i << endl;

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What will be printed after the loop ends?

for( i = 1; i <= 10; i++ ) {// Assume loop body does not change i

}cout << “After the loop, i = “ << i << endl;A. 0B. 1C. 10D. 11E. Undefined. It may depend on the compiler.

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Do-While Loops

• Do-while loops are nearly identical to whiles.

• The condition is evaluated AFTER each iteration of the loop, and the loop is repeated only if the condition is true.

• A do-while loop will always execute at least once. After that it functions identically to a while loop.

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Do-While Loop Syntax

do {

// code in body of loop

} while( condition );

• Note required semi-colon

• If braces are omitted, then a single statement comprises the body of the loop. ( Very rarely omitted. )

condition?

body

Followingcode

False

True

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Do-While Example

int guess, answer = rand( ) % 10 + 1;

do {

cout << “Enter a guess from 1 to 10: “;

cin >> guess;

} while( guess != answer ) ;

cout << “That’s right! Congratulations!\n”;

No initialization needed

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Break and Continue

• “break” causes a loop to finish immediately, continuing execution with the code following the loop body. ( Also used in switches. )

• “continue” causes the current iteration of a loop to finish, starting the next iteration.– While or do-while loops will jump to the

evaluation of the loop condition.

– For loops will execute the “incrementation”, and then jump to the evaluation of the loop condition.

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For Loop Flowchart Illustrating break and continue

for( init; condition; incr ) {

// code 1

if( test_C )

continue;

// code 2

if( test_B )

break;

// code 3

}

condition?

code 1

Following code

False

True

incr

init

code 2

code 3

test_C

test_B

True

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Input Checking With a While Loop

int age = -1; // Initialized to guarantee loop entry

while( age < 0 || age > 120 ) {

cout << “Please enter your age: “;

cin >> age;

if( age < 0 || age > 120 )

cout << age << “ is invalid. Try again.\n”;

} // Continue while input is bad

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Input Checking With a Do-While Loop

int age; // No initialization needed

do { cout << “Please enter your age: “;

cin >> age;

if( age < 0 || age > 120 )

cout << age << “ is invalid. Try again.\n”;

} while( age < 0 || age > 120 ); //Continue while bad

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Input Checking With an Infinite Loop

int age; // No initialization needed

while( true ) {

cout << “Please enter your age: “;

cin >> age;

if( age > 0 && age <= 120 ) // Exit loop if good

break;

cout << age << “ is invalid. Try again.\n”;

} // Loops infinitely while input is bad

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Review

In the following statement, which of the following is the correct order in which the operators will be evaluated?

A *= B + C / D – E++;

A. *=, +, /, -, ++

B. /, +, -, *=, ++

C. ++, /, +, -, *=

D. ++, +, /, -, *=

E. None of the above. The correct order of operations is not listed here.

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When to Use What Kind of Loop

• If you can count ( or calculate ) how many times the loop must execute, use a for loop, always with an INTEGER counter.

• Else if you need to ensure the loop executes at least once, ( and can’t rig a while to do so ), use a do-while loop. ( A comment at the top of the loop improves readability. )

• Else use a while loop.

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Review

What type of loop is guaranteed to execute at least once, and then continue to repeat until a condition becomes false?

A. forB. untilC. do untilD. whileE. do while

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Nested Loops

• The body of a loop can contain any valid code, including other loops, termed nested loops.– The internal loop does not have to be the same type

as the enclosing outer loop.

– A nested “for” loop is a common way to iterate over two variables and/or produce a table:for( i = 0; i < iMax; i++ ) {

for( j = 0; j < jMax; j++ )

cout << i * j << “ “; // Prints numbers on a line

cout << endl; // New line when j loop exits.

}

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Empty Loops

• Loops can also be empty.

– This is very rarely done intentionally.

– Comments and braces are needed if this is done on purpose.

– Error example: An infinite empty loop:int i = 0;

while( i < 5 )

i = i + 1;

Error here. The body of the loop is between the closing ) and the ;

“i = i + 1;” is after the loop, not inside it, and will never be executed.

;

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Always Use an INTEGER Loop Counter

int i, nLoops; // ( Assume all vars are given values. )

double x, xMin, xMax, delataX;

// WRONG:

for( x = xMin; x <= xMax; x += deltaX ) { . . .

// RIGHT:

for( i = 0; i < nLoops; i++ ) {

x = xMin + i * deltaX;

. . .

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Review

What value will be stored by the following C++ code:

double answer = 3 + 5 / 2 * 4 / 5.0;

A. 0.2

B. 3.2

C. 4.0

D. 4.6

E. 5.0

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Review

Given the following code, what will be stored in k?int i = 5, j = 4, k = 3;k /= i++ % --j;

A. 0B. 1C. 1.5D. 3E. infinity ( divide by zero error. )

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Infinite Loop Knock Knock Joke

Knock knock

Knock who?

Who’s there?

Knock