C Program to check whether the triangle is equilateral, isosceles or scalene

A triangle consists of three sides and three angles. Based on the relationship between the three sides, triangles can be classified into three types −

• Equilateral triangle: All three sides are equal.
• Isosceles triangle: Any two sides are equal.
• Scalene triangle: No sides are equal.

Syntax

if (side1 == side2 && side2 == side3) {
    // Equilateral triangle
} else if (side1 == side2 || side2 == side3 || side1 == side3) {
    // Isosceles triangle
} else {
    // Scalene triangle
}

Algorithm

Step 1: Declare three sides of triangle.
Step 2: Enter three sides at run time.
Step 3: If side1 == side2 && side2 == side3
        Go to step 6
Step 4: If side1 == side2 || side2 == side3 || side3 == side1
        Go to Step 7
Step 5: Else
        Go to step 8
Step 6: Print the triangle is equilateral.
Step 7: Print the triangle is isosceles.
Step 8: Print the triangle is scalene.

Example

Following is the C program to check whether the triangle is equilateral, isosceles or scalene −

#include <stdio.h>

int main() {
    int side1, side2, side3;
    
    printf("Enter sides of triangle: ");
    scanf("%d %d %d", &side1, &side2, &side3);
    
    if (side1 == side2 && side2 == side3) {
        printf("The given triangle is equilateral<br>");
    } else if (side1 == side2 || side2 == side3 || side3 == side1) {
        printf("The given triangle is isosceles<br>");
    } else {
        printf("The given triangle is scalene<br>");
    }
    
    return 0;
}

Output

When you run the above program with different inputs, it produces the following results −

Run 1:
Enter sides of triangle: 3 4 6
The given triangle is scalene

Run 2:
Enter sides of triangle: 2 2 5
The given triangle is isosceles

Run 3:
Enter sides of triangle: 5 5 5
The given triangle is equilateral

Key Points

• The program uses nested if-else statements to check conditions in order of priority.
• Equilateral check comes first as it's the most restrictive condition.
• The program assumes positive integer inputs representing valid triangle sides.

Conclusion

This program effectively classifies triangles by comparing their side lengths using conditional statements. The logical order of checks ensures accurate identification of triangle types based on geometric properties.