Division by zero

Division by zero occurs

Description

This check determines whether the right operand of a division or modulus operation is zero.

Diagnosing This Check

Review and Fix Division by Zero Checks

Examples

Red integer division by zero

#include <stdio.h>

void main() {
    int x=2;
    printf("Quotient=%d",100/(x-2));
}

In this example, the denominator x-2 is zero.

Correction — Check for zero denominator

One possible correction is to check for a zero denominator before division.

In a complex code, it is difficult to keep track of values and avoid zero denominators. Therefore, it is good practice to check for zero denominator before every division.

#include <stdio.h>
int input();
void main() {
    int x=input();
    if(x>0) { //Avoid overflow
        if(x!=2 && x>0)
            printf("Quotient=%d",100/(x-2));
        else
            printf("Zero denominator.");
    }
}

Red integer division by zero after for loop

#include <stdio.h>
void main() {
    int x=-10;
    for (int i=0; i<10; i++)
        x+=3;
    printf("Quotient=%d",100/(x-20));
}

In this example, the denominator x-20 is zero.

Correction — Check for zero denominator

One possible correction is to check for a zero denominator before division.

After several iterations of a for loop, it is difficult to keep track of values and avoid zero denominators. Therefore, it is good practice to check for zero denominator before every division.

#include <stdio.h>
#define MAX 10000
int input();

void main() {
    int x=input();
    for (int i=0; i<10; i++) {
        if(x < MAX) //Avoid overflow
            x+=3;
    }

    if(x>0) { //Avoid overflow
        if(x!=20)
            printf("Quotient=%d",100/(x-20));
        else
            printf("Zero denominator.");
    }
}

Orange integer division by zero inside for loop

#include<stdio.h>

void main() {
    printf("Sequence of ratios: \n");
    for(int count=-100; count<=100; count++)
        printf(" %.2f ", 1/count);
}

In this example, count runs from -100 to 100 through zero. When count is zero, the Division by zero check returns a red error. Because the check returns green in the other for loop runs, the / symbol is orange.

There is also a red Non-terminating loop error on the for loop. This red error indicates a definite error in one of the loop runs.

Correction — Check for zero denominator

One possible correction is to check for a zero denominator before division.

#include<stdio.h>

void main() {
    printf("Sequence of ratios: \n");
    for(int count=-100; count<=100; count++) {
        if(count != 0)
            printf(" %.2f ", 1/count);
        else
            printf(" Infinite ");
    }
}

Orange float division by zero inside for loop

#include <stdio.h>
#include <math.h>

#define stepSize 0.1

void main() {
    float divisor = -1.0;
    int numberOfSteps = (int)((2.0*1.0)/stepSize);

    printf("Divisor running from -1.0 to 1.0\n");
    for(int count = 1; count <= numberOfSteps; count++) {
        divisor+= stepSize;
	divisor = ceil(divisor * 10.) / 10.; // one digit of imprecision
        printf(" .2f ", 1.0/divisor);
    }
}

In this example, divisor runs from –1.0 to 1.0 through 0.0. When divisor is 0.0, the Division by zero check returns a red error. Because the check returns green in the other for loop runs, the / symbol is orange.

There is no red Non-terminating loop error on the for loop. The red error does not appear because Polyspace^® approximates the values of divisor by a broader range. Therefore, Polyspace cannot determine if there is a definite error in one of the loop runs.

Correction — Check for zero denominator

One possible correction is to check for a zero denominator before division. For float variables, do not check if the denominator is exactly zero. Instead, check whether the denominator is in a narrow range around zero.

#include <stdio.h>
#include <math.h>

#define stepSize 0.1

void main() {
    float divisor = -1.0;
    int numberOfSteps = (int)((2*1.0)/stepSize);

    printf("Divisor running from -1.0 to 1.0\n");;
    for(int count = 1; count <= numberOfSteps; count++) {
        divisor += stepSize;
	divisor = ceil(divisor * 10.) / 10.; // one digit of imprecision
        if(divisor < -0.00001 || divisor > 0.00001)
            printf(" .2f ", 1.0/divisor);
        else
            printf(" Infinite ");
    }
}

Check Information

Group: Numerical

Language: C | C++

Acronym: ZDV