# 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.

Example
Input

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

Output

`Triangle is valid`

## 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`

## 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;
}``````

Output

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

Happy coding 😉

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