Introduction to Computer Algorithmics and Programming

Ceng 113Program Control Statements

Program Control Statements

• Selection

– if and switch

• Iteration

– while, for, do-while

• Jump

– break, continue, return

• Label

• Expression

• Block

– A block begins with a { and ends with a }.

Selection Statements• The general form of the if statement is;if (expression) statement;

else statement;

A statement may consist of a single statement or a block of statements or nothing. The else clause is optional.

/* Magic number program #1 */#include “stdio.h”#include “stdlib.h”void main(void){ int magic; /*magic number */ int guess; /*user’s guess */

magic = rand(); /* generate the magic number */ printf(“guess the magic number: “); scanf(“%d”, &guess); if (guess == magic) printf(“** Right **”);}

Selection Statements

/* Magic number program #2 */

#include “stdio.h”

#include “stdlib.h”

void main(void)

{ int magic; /*magic number */

int guess; /*user’s guess */

magic = rand(); /* generate the magic number */

printf(“guess the magic number: “);

scanf(“%d”, &guess);

if (guess == magic) printf(“** Right **”);

else printf(“Wrong”);

}

Nested ifs

A nested if is an if that is the target of another if or else.

if (i) {

if (j) statement 1;

if (k) statement 2; /* this if */

else statement 3; /* is associated with this else */

}

else statement 4; /* associated with if(i) */

Selection Statements/* Magic number program #3 */#include “stdio.h”#include “stdlib.h”void main(void){ int magic; /*magic number */ int guess; /*user’s guess */

magic = rand(); /* generate the magic number */ printf(“guess the magic number: “); scanf(“%d”, &guess); if (guess == magic)

{ printf(“** Right **”); printf(“%d is the magic number \n”, magic);

} else

{printf(“Wrong, ”); if (guess > magic) printf(“too high \n”);else printf(“too low \n”);}

}

The if-else-if Ladder

General form is;if (expression) statement;else if (expression) statement; else if (expression) statement;

....else statement;

Selection Statements/* Magic number program #4 */#include “stdio.h”#include “stdlib.h”void main(void){ int magic; /*magic number */ int guess; /*user’s guess */

magic = rand(); /* generate the magic number */ printf(“guess the magic number: “); scanf(“%d”, &guess); if (guess == magic)

{ printf(“** Right **”); printf(“%d is the magic number \n”, magic);

} else if (guess > magic) printf(“Wrong, too high \n”); else printf(“Wrong, too low \n”);}

The ? Alternative• We can use the ? Operator to replace if-else statements

of the general form:

if (condition)

expression;

else

expression;

However, the ? is called ternary operator because it requires three operands. Exp1 ? Exp2: Exp3;

x = 10;y = x>9 ? 100: 200;

x = 10;if (x>9) y = 100;else y = 200;

Sample program/* Magic number program #5 */#include “stdio.h”#include “stdlib.h”void main(void){ int magic; /*magic number */ int guess; /*user’s guess */

magic = rand(); /* generate the magic number */ printf(“guess the magic number: “); scanf(“%d”, &guess); if (guess == magic)

{ printf(“** Right **”); printf(“%d is the magic number \n”, magic);

} else guess > magic ? printf(“High”) : printf(“Low”);}

Sample program

/*This program uses the ? operator to square an integer

value entered by the user but preserves the sign (10

squared is 100 and -10 squared is -100).*/

#include “stdio.h”

void main(void)

{ int isqrd, i;

printf(“enter a number :”);

scanf(“%d”, &i);

isqrd = i>0 ? i*i : -(i*i);

printf(“%d squared is %d”, i, isqrd);

}

Sample program#include “stdio.h”int f1(int n);int f2(void);void main(void){ int t; printf(“enter a number: “); scanf(“%d”, &t);/* print proper message */ t ? f1(t) + f2( ) : printf(“zero entered”);}f1(int n){ printf(“%d ”, n); return 0;}

f2(void){ printf(“entered”); return 0; }

if (t!=0) { f1(t); f2; }

else printf(“zero entered”);

switch

C has a built-in multiple branch selection statement, switch, which tests the value of an expression against a list of integer or character constants. When a match is found, the statements associated with that constant are executed.

switch (expression) {case constant1:

statement sequencebreak;

case constant2:statement sequencebreak;

...default:

statement sequence}

•The default statement is executed if no

matches are found. The default is optional.

•A switch statement can be included

maximum 257 case statements.

•When break statement is encountered in a

switch, program execution “jumps” to the line

of code following the switch statement.

switch menu(){ char ch; printf(“1.Check spelling \n”); printf(“2.Correct spelling errors\n”); printf(“3.Display spelling errors\n”); printf(“Strike any other key to skip \n”); printf(“ Enter your choise: ”); ch = getchar(); /* read the selection from the keyboard*/ switch(ch) {

case ‘1’ : check_spelling();break;

case ‘2’:correct_errors(); break;

case ‘3’:display_errors(); break;

default :printf(“No option selected”);

}}

switch/*Process a value */void inp_handler(int i){ int flag;

flag = -1;switch(i) {

case 1:case 2:case 3: flag= 0; break;case 4: flag= 1;case 5: error(flag); break;default : process(i);

}}

Nested switch statement

switch(x) {

case 1:

switch(y) {

case 0: printf(“divide by zero error”);

break;

default: process(x, y);

}

break;

case 2:

...

Example

The programmer has given each medical plan a code:

Plan1 = F, Plan2 = B, Plan3 = K and Plan4=E.

• The company pays for all of Plan1.

• The individual has to pay for part of the others.

Plan2 = amount/4,

Plan3 = amount/7,

Plan4 = amount/5.

• Any other codes are considered in error.

• Write the algorithm and C code to calculate the total amount which the company will be paid to plans.

#include "stdio.h"main(){ int total_investment=0, int amount_of_plan=0;

char plan_code;printf("\n Enter the selected plan code (F, B, K, E) =");scanf("%c", &plan_code);printf("\n Enter the amount of total investment for selected plan =");scanf("%d", &total_investment);switch(plan_code){case 'F':

{amount_of_plan=total_investment;printf("\n The amount of this plan = %d", amount_of_plan);break;}

case 'B': {amount_of_plan=total_investment / 4;printf("\n The amount of this plan = %d", amount_of_plan);break;}

case 'K': {amount_of_plan=total_investment / 7; printf("\n The amount of this plan = %d", amount_of_plan);break;}

case('E'): {amount_of_plan=total_investment / 5;printf("\n The amount of this plan = %d", amount_of_plan);break;}

default: printf("\n You selected invalid plan code.");}

}

#include <stdio.h>main() /* count digits, white space, others */{ char c; int i, nwhite, nother, ndigit[10];nwhite = nother = 0;for (i = 0; i < 10; i++) ndigit[i] = 0;while ((c = getchar()) != ‘.’) { switch (c) { case '0': case '1': case '2': case '3': case '4': case '5': case '6': case '7': case '8': case '9': ndigit[c-'0']++; break; case ' ': case '\n': case '\t': nwhite++; break; default: nother++; break; } }

printf("digits =");for (i = 0; i < 10; i++) printf(" %d", ndigit[i]);printf(", white space = %d, other = %d\n", nwhite, nother);}

Iteration statements

Iteration ( also called loops) statements allow a set of

instructions to be performed until a certain condition is

reached. This condition may be predifened as in the for

loop, or open ended as in the while and do-while loops.

The for loopThe general form of the for statement is;

for (initialization; condition; increment) statement;• The initialization is an assignment statement that is used to set the

loop control variable.• The condition is relational expression that determines when the loop

exist.• The increment defines how the loop control variable changes each

time the loop is repeated.

#include stdio.h

main()

{ int x;

for(x=1; x<=100;x++) printf(“%d “, x);

}

The for loopSample 1;

for(x=100; x!=65; x-=5) {

z = x*x;

printf(“The square of %d, %d”, x, z);

}

Sample 2;

x = 10;

for(y=10; y!=x; y++) printf(“%d”, y);

printf(“%d”, y);

Sample 3;

for(x=0; x!=123; ) scanf(“%d”, &x);

The for loop

The infinity loop;

You can make an endless loop by leaving the conditional expression empty;

for( ; ; ) printf(“ this loop will run forever. \n”);

Sample;

ch = ‘\0’;

for( ; ; ) {

ch=getchar(); /* get a character */

if (ch==‘A’) break; /* exit the loop */

}

printf(“you typed an A”);

ExerciseCreate a function which is convert the 0,5,10,15,20, 25, 30...,90, 95, 100

celsius to fahrenheight values in a table.

F = ((C/5)*9+32)

Exercise/* This program convert to celsius to fahrenheight.*/#include <stdio.h>int main(){// Define variables

int celsius, fahrenheight;

// Print the table headersprintf("\n Program will be list celcius and fahrenheight convertion table.");printf("\n Celcius Fahrenheight");

// Convert the celsius to fahrenheight between 5 and 100.for (celsius = 0; celsius <=100; celsius +=5){

fahrenheight=((celsius/5) * 9 +32);printf("\n %d %d", celsius, fahrenheight);

}

printf("\n");return 0;}

The while loopIts general form is;

while(condition) statement;

The loop iterates while the condition is true. When the condition becomes false, program control passes to the line after the loop code.

wait_for_char(void)

{ char ch;

ch = ‘\0’; /*initialize ch */

while(ch != ‘A’) ch = getchar();

return ch;

}

The while loop/*add spaces to the end of string */#include “stdio.h”#include “string.h”void main(void){ char str[40];

int k;strcpy(str, “this is a test”);k = strlen(str);

printf(“%d”, k);while (k <40) { str[k] = ‘ ‘;

k++;}

str[k] = ‘\0’; /*strings need to be terminated in a null */

printf(“%d”, strlen(str));}

*

The do-while loop

The do-while loop checks its condition at the bottom of the loop. A do-while loop always executes at least once.

do {

statement

} while(condition);

Sample;

do {

scanf(“%d”, &num);

} while(num>100);

menu(){ char ch; printf(“1.Check spelling \n”); printf(“2.Correct spelling errors\n”); printf(“3.Display spelling errors\n”); printf(“Strike any other key to skip \n”); printf(“ Enter your choise: ”); do {

ch = getchar(); /* read the selection from the keyboard*/ switch(ch) {

case ‘1’ : check_spelling();break;

case ‘2’:correct_errors(); break;

case ‘3’:display_errors(); break;

default :printf(“No option selected”);

}} while(ch=‘1’ || ch =‘2’ || ch = ‘3’);

}

Exercise

Mr.Brown has given a test to his class. He would like to have the avarage score for the class, and the higest and lowest scores. Develop a solution to calculate and print out these values. ( Use a significant value to stop the processing of the loop).

# include <stdio.h>

void main ()

{ int counter=0, avarage=0, lowest=100, higest=0, total_grades=0, grade;

printf("\n Please enter -1 to terminate the program.\n");

printf("\n Enter a student grade =");

scanf("%d", &grade);

while (grade != -1)

{

total_grades = total_grades + grade;

counter++;

if (lowest > grade ) lowest = grade;

if (higest < grade ) higest = grade;

printf("\n Enter a student grade =");

scanf("%d", &grade);

}

printf("\n The higest grade is %d", higest);

printf("\n The lowest grade is %d", lowest);

printf("\n The avarege grade is %d", (total_grades/counter));

}

While loop!

# include <stdio.h>void main (){ int counter=0, avarage=0, lowest=100, higest=0, total_grades=0, grade; printf("\n Please enter -1 to terminate the program.\n"); do {

printf("\n Enter a student grade =");scanf("%d", &grade);if (grade != -1) {

total_grades = total_grades + grade;counter++;

if (lowest > grade ) lowest = grade;if (higest < grade ) higest = grade;

} }while (grade != -1); printf("\n The higest grade is %d", higest); printf("\n The lowest grade is %d", lowest); printf("\n The avarege grade is %d", (total_grades/counter));}

Do-While loop!

Unconditional Branch

• The return statement;

The return statement is used to return from a

function. It is a jump statement because it

causes execution to return (jump back) to the

point at which the call to the function was made.

return expression;

The expression is optional.

Unconditional Branch• The break statement;

This statement has two uses;1. to terminate a case in the switch statement,2. to force immediate terminator of a loop, bypassing the normal loop conditional test.

#include “stdio.h”void main(void){

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

printf(“%d”, t);if(t==10) break;

}}

for(t=0; t<100; ++t) { count=1; for( ; ; ) { printf(“%d”, count); count++; if(count==10) break; }}

Unconditional Branch

• The exit() function;

You can break out of a program by using the standart library function

exit() .This function causes immediate termination of the entire

program, forcing a return to the operating system.

void exit(int return_code);

The value of return_code is returned to the calling process, which is

usually the operating system. Zero is generally used as a returned

code to indicate normal program termination.

Unconditional Branch• The continue statement;

Continue forces the next iteration of the loop.

*

/*count spaces */#include “stdio.h”main(){ char s[80], *str; int space; printf(“enter a string: “); gets(s); str =s; for(space=0; *str; str++) {

if(*str!=‘ ‘) continue; space++;

} printf(“%d spaces \n”, space);}

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