Repetitive Structures BBS514 Structured Programming (Yapısal Programlama)1.

Repetitive Structures

BBS514 Structured Programming (Yapısal Programlama) 1

Repetition Statements

• Repetition statements allow us to execute a statement multiple times

• Often they are referred to as loops

• Like conditional statements, they are controlled by boolean expressions

• C has three kinds of repetition statements:

– while loop

– do loop

– for loop

• The programmer should choose the right kind of loop for the situation


while Statement

• The most general one of these loop statements is while.

while ( boolean-expression )statement

where statement can be any statement including a compound statement, an if-statement, another loop statement, ...

• Statement is repeated as long as the condition (boolean-expression) is true. When the condition gets false, the statement is not executed anymore.

• If the condition never gets false infinite loop• If the condition gets immediately false the loop will be repeated

zero times.


while Statement – Flow Diagram


while ( condition )

statement

condition statementtrue

false

while Statement

Example:

int product = 2;

while ( product <= 100 )product = 2 * product;


product <= 100 product = 2 * producttrue

false

while Statement – Counter Controlled Loop

• Print 5 asterisk characters

• This loop will be repeated for 5 times (count: 0,1,...,4)

count = 0;while ( count < 5 ) {printf(“*\n”);count = count + 1;

}printf(“done\n”);


*****done

while Statement – Counter Controlled Loop

• Read & sum 5 values

sum = 0;count = 0;while ( count < 5 ) {scanf(“%d”, &value);sum = sum + value;count = count + 1;

}printf(“sum is %d \n”, sum);


53741sum is 20

Counter-Controlled Repetition – Example Program

• A class of 10 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

• The algorithm

Set total to zero

Set grade counter to one

While grade counter is less than or equal to 10 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


Counter-Controlled Repetition – Example Program/* Class average program with counter-controlled repetition */

#include <stdio.h>

int main() {int counter, grade, total, average;

/* initialization phase */total = 0;counter = 1;

/* processing phase */while ( counter <= 10 ) {

printf( "Enter grade: " );scanf( "%d", &grade );total = total + grade;counter = counter + 1;

}

/* termination phase */average = total / 10.0;printf( "Class average is %d\n", average );

return 0; /* indicate program ended successfully */}


Enter grade: 98Enter grade: 76Enter grade: 71Enter grade: 87Enter grade: 83Enter grade: 90Enter grade: 57Enter grade: 79Enter grade: 82Enter grade: 94Class average is 81

Infinite Loops

• The body of a while loop eventually must make the condition false

• If not, it is called an infinite loop, which will execute until the user interrupts the program

• This is a common logical error

• You should always double check the logic of a program to ensure that your loops will terminate normally

int count = 1;while (count <= 25){

printf(“%d \n”, count); count = count - 1;

}


Infinite Loops

i = 1;while ( i != 50 ) {

printf(“%d\n”,i);i = i + 2;

}printf(“done”);

scanf(“%d”, &i);while ( i != 1 ) {

if ( i % 2 == 0) i = i / 2;

elsei = 3 * i + 1;

}printf(“done”);


Sometimes, it is difficult to check loop terminating condition.

Sentinel-Controlled Loops

• What happens if we don’t know how many times a loop will run.

• Ex: a loop which reads the scores of the students in an exam and find the average of the scores;

– we don’t know the number of students.

– How are we going to stop the loops?

use a sentinel-value

• We choose a sentinel-value which can not be a score (e.g. –1)

• We read the scores until this sentinel-value has been entered. When this sentinel-value has been read, we stop the loop.


Sentinel-Controlled Loops

• Problem becomes:

Develop a class-averaging program that will process an arbitrary number of grades each time the program is run.

– Unknown number of students

– How will the program know to end?

• Use sentinel value

– Also called signal value, dummy value, or flag value

– Indicates “end of data entry.”

– Loop ends when user inputs the sentinel value

– Sentinel value chosen so it cannot be confused with a regular input (such as -1 in this case)


/* Class average program with sentinel-controlled repetition */#include <stdio.h>int main() {

float average; int counter, grade, total;

/* initialization phase */ total = 0; counter = 0;

/* processing phase */printf( "Enter grade, -1 to end: " );

scanf( "%d", &grade ); while ( grade != -1 ) {

total = total + grade;counter = counter + 1; printf( "Enter grade, -1 to end: " );scanf( "%d", &grade );

} /* termination phase */ if ( counter != 0 ) {

average = ( float ) total / counter;printf( "Class average is %.2f", average ); }

elseprintf( "No grades were entered\n" );

return 0; /* indicate program ended successfully */}


Enter grade, -1 to end: 75Enter grade, -1 to end: 94Enter grade, -1 to end: 97Enter grade, -1 to end: 88Enter grade, -1 to end: 70Enter grade, -1 to end: 64Enter grade, -1 to end: 83Enter grade, -1 to end: 89Enter grade, -1 to end: -1Class average is 82.50

for Statement

• Another loop statement in C is for-statement.

• for-statement is more suitable for counter-controlled loops.

for ( initialization ; condition ; increment )

statement

which is equivalent to

initialization ;

while (condition ){

statement ;

increment ;

}


for Statement – Flow Diagram


initialization

statement

increment

condition false

true

for ( initialization ; condition ; increment ) statement

for Statement

• Example:

for(counter = 1; counter <= 10; counter++ )

printf( "%d\n", counter );– Prints the integers from one to ten

• Initialization and increment

– Can be comma-separated lists

– Example:

for (i = 0, j = 0; j + i <= 10; j++, i++)

printf( "%d\n", j + i );


No semicolon (;) after last expression

for statement -- Example

int i; int i;int sum = 0; int sum = 0;for (i=1; i<=20; i++) i = 1;

sum = sum + i; while (i<=20) { sum = sum + i; i++;}

---

int i; int i; int sum = 0; int sum = 0; for (i=100; i>=1; i--) i = 100;

sum = sum + i; while (i>=1) { sum = sum + i; i++;}


for statement -- Example

int i, j;int count=0;for (i=1, j=10; i<j; i++, j--) count is 5

count++;

int i, j;int count=0;i=1;j=10;for (; i<j; ) {

count++; i++; j--;}


The do/while Repetition Structure

• The do/while repetition structure

– Similar to the while structure

– Condition for repetition tested after the body of the loop is performed

• All actions are performed at least once

– Format:

do {

statement;

} while ( condition );


do-while statement – Flow Diagram


statement

condition

false

true

do-while statement – Example

/*Using the do/while repetition structure */

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

int counter = 1;

do {printf( "%d ", counter );counter = counter + 1;

} while ( counter <= 10 );

return 0;}

Program Output:


1 2 3 4 5 6 7 8 9 10

Nested Loops

• When a loop body includes another loop construct this is called a nested loop.

• In a nested loop structure the inner loop is executed from the beginning every time the body of the outer loop is executed.

• Example 1:

value = 0;

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

for (j=1; j<=5; j=j+1)

value = value + 1;• How many times the inner loop is executed?

value is 10*5=50


Nested Loops

• Example 2:

value = 0;

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

for (j=1; j<=i; j=j+1)

value = value + 1;

How many times the inner loop is executed?

value is 1+2+3+4+5+6+7+8+9+10=55


Nested Loops - Printing a triangle

• Write a program to draw a triangle like the following: (The number of lines is an input)

* ** *** **** *****• We can use a nested for-loop: for (i=1; i<=num_lines; ++i){ for (j=1; j<=i; ++j) printf("*"); printf("\n");

}BBS514 Structured Programming (Yapısal Programlama) 25

Nested Loops – Example2

• a triangle shape (1st line contains 1 star, 2nd line contains 3 star,..., nth line contains 2n-1 stars)

* for (i=1; i<=n; i++) { // for each line

*** for (j=1; j<=(n-i); j++)

***** printf(“ ”);

. for (j=1; j<=(2*i-1); j++)

. printf(“*”);

***.....*** printf(“\n”);

}



/* This program reads numbers until the user enters a negative number. For each number read, it prints the number and the summation of all values between 1 and the given number. */

int main() { int num, count, total = 0; printf( "Enter a value or a negative number to end: " ); scanf( "%d", &num ); while ( num >= 0 ) {

for (count = 1; count <= num; count++) total = total + count;printf(“%5d%5d”,num, total);printf( "Enter a value or a negative number to end:");scanf( "%d", &num );total = 0;

} return 0;}



• Nesting can be more than one level

int sum=0;

for (i=1; i<=5; i++)

for (j=1; j<=5; j++)

for (k=1; k<=5; k++)

sum=sum+1; sum is 125


Loop - Exercises

• Write a program that reads an integer N and computes N!

fact = 1;

for (j = 1; j <= n; j++)

fact = fact * j;

• Write a program for finding the sum of the first n terms of the series1 + 1/2+ 1/3 + ….

float sum = 0;

for (j = 1; j <= n; j++)

sum = sum + 1.0 / j;


Loop - Exercises

• Write a program that reads numbers until a negative number is read and prints the number of values read, the largest value, the smallest value and the range.

count =0;

scanf (“%d”,&num);

min = max = num;

while (num >= 0){

count ++;

if (num < min)

min = num;

else if (num > max)

max = num;

scanf (“%d”,&num);

}

printf(“count:%d max:%d min:%d range:[%d .. %d]”,

count,max,min,min,max);


Loop - ExercisesConsider the Fibonacci sequence: 0 1 1 2 3 5 8 13 …

Write a program to find the nth Fibonnacci number.

int main () {

int n, prev1=1, prev2=0, temp, j;

printf(“Enter n > “);

scanf(“%d”, &n);

if (n==0 || n== 1)

printf(“%d th Fib number is %d”, n, n);

else {

prev1 = 0;

prev2 = 1;

for (j=1; j <= n; j++) {

temp = prev1 + prev2;

prev2 = prev1;

prev1 = temp;

}

printf(“%d th Fib number is %d”, n, prev1);

}

}


Loop - Exercises

Write a program to test whether a given positive integer (> 1) is a prime number of not.

if (n == 2)

printf(“%d is prime”, n);

else if (n%2 == 0)

printf(“%d is NOT prime”, n);

else {

flag = 1;

for (i = 3; i < (n/2) && flag ); i += 2) {

if (n%i == 0)

flag = 0;

}

if (flag == 1)

printf(“%d is prime”, n);

else

printf(“%d is NOT prime”, n);

}


Repetitive Structures BBS514 Structured Programming (Yapısal Programlama)1.

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