1

LOOPS

Loops

2

The for Loop• Repetition

– construct to execute a section of code a specified number of times

for (initialization; test-condition; increment){

statement(s)}

• (1) Set initial value of counter. • (2) Perform test to see if loop should continue. • (3) Execute statements. • (4) Update counter. • (5) Repeat from (2).

#include <iostream.h>int main(){

int count; //loop control variablefor (count=1; count<=100; count++)

cout << count << " ";return 0;

}

3

The for Loop• Output?

for (count=10; count < 5; count++) cout << count;

• A square root calculating program #include <iostream.h> #include <cmath> // for older compilers, use <math.h> int main() { int num;

for(num=1; num < 100; num++) {

cout << num << " " << sqrt((double) num) << '\n';}

return 0; }

4

Variations of the for Loop int main() {

for(int i=100; i >= -100; i = i-5) cout << i << ' ';

return 0; } ================================

for(x=0, y=10; x<=10; ++x, --y) cout << x << ' ' << y << '\n'; ================================

#include <iostream.h> #include <conio.h> int main() { int i;

// print numbers until a key is pressed for(i=0; !kbhit(); i++) cout << i << ' ';

return 0; }

5

Variations of the for Loop int x; for(x=0; x != 123; ) { cout << "Enter a number: "; cin >> x; } ================================

int x = 0;for( ; x<10; ){

cout << x << ' '; ++x;

} ================================

for(;;) {

body of loop }

6

The while Loop• Repetition construct

– it’s a for loop stripped of the initialization and update parts

while (test-condition){

statement(s)}

for ( ;test-condition; ) { statement(s) }

initialize counterwhile (test-condition){ statement(s);

update counter;}

7

The while Loop• Might use it if do not know beforehand when to stop

repeating int n = 99; //why initialize? cout << “Guess my fav number”; while(n != 0) cin >> n; cout << “Yes, I was looking for 0 \n”;

================================

unsigned char ch; ch = 1; while(ch) { //when does it terminate? cout << ch; ch++; } //displays what?

8

The do-while Loop

• Similar to while, except that tests condition after the body of the loop i.e. statements executed at least once

do {

statement(s);

} while (test-condition); ================================

int num;

do

{

cout << "Enter a number (100 to stop): ";

cin >> num;

} while (num != 100);

9

Which Loop to Use?

• if (number of repetitions known)

use for;

else if (body to be executed once at least)

use do while;

else {use while;}

• No hard rule; all interchangeable mostly; matter of style

10

The continue Statement• continue, break and goto are jump statements.

• continue used in loops and causes program to skip rest of loop’s body and start the next iteration immediately. Actually takes you to the closing brace of the loop body

long dividend, divisor; char ch;

do

{ cout << "Enter dividend: "; cin >> dividend;

cout << "Enter divisor: "; cin >> divisor;

if (divisor == 0) //divide by zero error

{

cout << "Illegal divisor\n";

continue;

}

cout << "Quotient is " << dividend/divisor;

cout << " , remainder is " << dividend%divisor;

cout << "\nDo another? (y/n): ";

cin >> ch;

} while (ch != 'n');

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The continue Statement• Output of following? #include <iostream.h> int main() { int x;

for(x=0; x<=100; x++) { if(x%2) continue; cout << x << ' '; } return 0; }

12

The break Statement• Causes exit from loop or switch #include <iostream.h> #include <cmath> int main() { double x;

while (1) { cout << "Enter number for square root: "; cin >> x;

if (x <0.0) break; //exit loop if x is -ve cout << sqrt(x) << endl; }// break jumps to here cout << "Sorry, can't do that\n"; return 0; }

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The break Statement int t;

for(t=0; t<100; t++)

{

if(t==10)

break;

cout << t << ' ';

} ================================

for(i=0; i<1000; i++)

{

// do something

if(kbhit())

break;

}// keypress stops execution

• break causes exit from the innermost loop or switch, and not from the enclosing loops or switches

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The goto Statement• Unconditional branch statement that jumps to a label (identifier)• goto label; …

label: statement(s); ================================

int x = 1; loop1: cout << x << endl; x++; if(x < 100) goto loop1; ================================

goto end; int i = 10; //error: goto skips initialization end: ;

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Example

Read and understand the magic number program given at the end of the chapter 4

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while

#include <iostream.h>int main(){ int count_down;

cout << "How many greetings do you want? "; cin >> count_down;

while (count_down > 0) { cout << "Hello "; count_down = count_down - 1; }

cout << endl; cout << "That's all!\n";

return 0;}

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#include <iostream.h>int main(){ char ans;

do { cout << "Hello\n"; cout << "Do you want another greeting?\n" << "Press y for yes, n for no,\n" << "and then press return: "; cin >> ans; } while (ans == 'y' || ans == 'Y');

cout << "Good-Bye\n";

return 0;}

do .. while

18

Counting Space Characters

(while)

void countSpaces1(void){

int ch = 0;int numofspaces = 0;

printf("Enter a sentence: ");ch = getchar();

while ( ch != '\n' ){

if ( ' ' == ch )numofspaces++;

ch = getchar();}

cout << "\nThe number of spaces is: " << numofspaces << endl;}

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Counting Space Characters(do..while)

#include <stdio.h> // getchar()

void countSpaces2(void)

{

int ch = 0;

int numofspaces = 0;

printf("Enter a sentence: ");

do {

ch = getchar();

if ( ' ' == ch )

numofspaces++;

} while ( ch != '\n' );

cout << "\nThe number of spaces is: " << numofspaces << endl;

}

20

Make Number from Digits

#include <stdio.h> // getchar()#include <ctype.h> // isdigit()

void makeInteger(void){

int num=0;int digit=0;

printf("Enter a number: ");

digit = getchar();

for( ; isdigit(digit) ; digit=getchar() ){

num = num * 10;num = num + (digit - '0');

}

cout << "The number is: " << num << endl;}

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Skip Spaces

#include <ctype.h> // header file for isspace()

for (int = getchar(); isspace( c ) ; c = getchar() );

ungetc( c, stdin ); // put the non-space char

// back in the input buffer

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Skip Spaces#include <stdio.h> // getchar()#include <ctype.h> // isdigit(), isspace()

void skipSpaces(void){

int c = 0;int count = 0;

cout << "\nSkipping Spaces\n\n\n";cout.flush();

for ( c=getchar(); isspace(c); c=getchar() ){

count++;}ungetc(c, stdin);

cout << count << " spaces skipped\n";}

•What is the behavior of this program

•What is the function of cout.flush()

•What will happen if it is removed

•What does ungetc() do