Check Triangle Type: Equilateral, Isosceles, Scalene, or Invalid in C

This program reads the three sides of a triangle and determines whether it is equilateral, isosceles, scalene, or not a valid triangle at all — using nested if-else statements.

C Program

#include <stdio.h>

int main() {
    float a, b, c;

    printf("Enter the three sides of the triangle: ");
    scanf("%f %f %f", &a, &b, &c);

    if (a <= 0 || b <= 0 || c <= 0) {
        printf("Not a valid triangle.\n");
    }
    else if ((a + b <= c) || (b + c <= a) || (a + c <= b)) {
        printf("Not a valid triangle.\n");
    }
    else {
        if (a == b && b == c) {
            printf("Equilateral triangle.\n");
        }
        else if (a == b || b == c || a == c) {
            printf("Isosceles triangle.\n");
        }
        else {
            printf("Scalene triangle.\n");
        }
    }

    return 0;
}

Explanation

• First, the program rejects non-positive side lengths.
• The triangle inequality theorem is checked: the sum of any two sides must be strictly greater than the third side. If this fails, it isn't a valid triangle.
• If valid, nested if-else checks:
  • All three sides equal → Equilateral
  • Exactly two sides equal → Isosceles
  • All sides different → Scalene

Sample Output

Enter the three sides of the triangle: 5 5 5
Equilateral triangle.

Enter the three sides of the triangle: 1 2 10
Not a valid triangle.