C program to check whether triangle is valid or not if sides are given

Write a C program to input side of a triangle and check whether triangle is valid or not using if else. How to check whether a triangle can be formed or not if sides of triangle is given using if else in C programming. Logic to check triangle validity if sides are given in C program.

Input first side: 7
Input second side: 10
Input third side: 5

Triangle is valid

Required knowledge

Basic C programming, Relational operators, Nested If else

Property of triangle

A triangle is valid if sum of its two sides is greater than the third side. Means if a, b, c are three sides of a triangle. Then the triangle is valid if all three conditions are satisfied
a + b > c
a + c > b and
b + c > a

Learn more – Program to check triangle validity if angles are given.

Logic to check triangle validity

Step by step descriptive logic to check triangle validity if its sides are given.

1. Input sides of a triangle from user. Store them in some variable say side1, side2 and side1.
2. Given triangle is valid if side1 + side2 > side3 and side1 + side3 > side2 and side2 + side3 > side1.

Program to check triangle validity using nested if...else

/**
 * C program to check whether a triangle is valid or not if its sides are given
 */

#include <stdio.h>

int main()
{
    int side1, side2, side3;

    /* Input three sides of a triangle */
    printf("Enter three sides of triangle: \n");
    scanf("%d%d%d", &side1, &side2, &side3);
    
    if((side1 + side2) > side3)
    {
        if((side2 + side3) > side1)
        {
            if((side1 + side3) > side2) 
            {
                /*
                 * If side1 + side2 > side3 and
                 *    side2 + side3 > side1 and
                 *    side1 + side3 > side2 then
                 * the triangle is valid.
                 */
                printf("Triangle is valid.");
            }
            else
            {
                printf("Triangle is not valid.");
            }
        }
        else
        {
            printf("Triangle is not valid.");
        }
    }
    else
    {
        printf("Triangle is not valid.");
    }

    return 0;
}

Logic to check triangle validity using nested if – best approach

Despite of easiness, the above code is messy and less readable. In above code printf("Triangle is not valid."); statement is unnecessarily repeated for various conditions. You can cut the extra printf("Triangle is not valid."); statement using a flag variable.

Let us suppose a temporary variable valid initialized with 0 indicating triangle is not valid. The main idea is to check triangle validity conditions and if triangle is valid then set valid variable to 1 indicating triangle is valid. Finally, check if(valid == 1) then triangle is valid otherwise not valid.

Program to check valid triangle using nested if – best approach

/** 
 * C program to check whether a triangle is valid using nested if 
 */  
  
#include <stdio.h>  
  
int main()  
{  
    int side1, side2, side3; 

    /* Initially assume that the triangle is not valid */
    int valid = 0;
  
    /* Input all three sides of a triangle */  
    printf("Enter three sides of triangle: \n");  
    scanf("%d%d%d", &side1, &side2, &side3);  
      
    if((side1 + side2) > side3)  
    {  
        if((side2 + side3) > side1)  
        {  
            if((side1 + side3) > side2)
            {  
                /*
                 * If side1 + side2 > side3 and
                 *    side2 + side3 > side1 and
                 *    side1 + side3 > side2 then
                 * the triangle is valid. Hence set
                 * valid variable to 1.
                 */
                valid = 1;
            }  
        }
    }  
    
    /* Check valid flag variable */
    if(valid == 1)
    {
        printf("Triangle is valid.");
    }
    else
    {
        printf("Triangle is not valid.");
    }
  
    return 0;  
}

Another way to cut length of the program is by using logical AND operator &&. Below program illustrates how to use logical AND operator for this program.

Program to check valid triangle using if...else and logical AND operator

/**
 * C program to check whether a triangle is valid or not using logical AND operator
 */

#include <stdio.h>

int main()
{
    int side1, side2, side3;

    /* Input all three sides of a triangle */
    printf("Enter three sides of triangle: \n");
    scanf("%d%d%d", &side1, &side2, &side3);

    if((side1 + side2 > side3) && (side1 + side3 > side2) && (side2 + side3 > side1))
    {
        printf("Triangle is valid.");
    }
    else
    {
        printf("Triangle is not valid.");
    }

    return 0;
}

Enter three sides of triangle: 7
4
10
Triangle is valid.

Happy coding 😉