C Program to Find Largest of Three Numbers

To find the largest of three numbers in C, use if-else statements to compare the numbers like if (a > b && a > c) largest = a; and print the largest value.

Examples

Inputa=5, b=3, c=9

OutputLargest number is 9

Inputa=10, b=10, c=5

OutputLargest number is 10

Inputa=-1, b=-5, c=-3

OutputLargest number is -1

How to Think About It

To find the largest of three numbers, compare the first number with the second and third using && to check if it is greater than both. If true, it is the largest. Otherwise, compare the second number with the third. The one that passes the comparison is the largest.

Algorithm

1

Get input values for three numbers a, b, and c

2

Check if a is greater than both b and c

3

If yes, set largest to a

4

Else check if b is greater than c

5

If yes, set largest to b

6

Otherwise, set largest to c

7

Print the largest number

Code

c

#include <stdio.h>

int main() {
    int a, b, c, largest;
    printf("Enter three numbers: ");
    scanf("%d %d %d", &a, &b, &c);

    if (a >= b && a >= c) {
        largest = a;
    } else if (b >= c) {
        largest = b;
    } else {
        largest = c;
    }

    printf("Largest number is %d\n", largest);
    return 0;
}

Output

Enter three numbers: 5 3 9
Largest number is 9

Dry Run

Let's trace the input a=5, b=3, c=9 through the code

1

Input values

a=5, b=3, c=9

2

Check if a >= b and a >= c

5 >= 3 is true, 5 >= 9 is false, so condition is false

3

Check if b >= c

3 >= 9 is false

4

Set largest to c

largest = 9

5

Print largest

Output: Largest number is 9

Step	Condition	Result	Largest
1	a >= b && a >= c	false
2	b >= c	false
3	Set largest = c		9

Why This Works

Step 1: Compare first number

The code uses if (a >= b && a >= c) to check if the first number is greater than or equal to both others.

Step 2: Compare second number

If the first number is not largest, it checks if the second number is greater than or equal to the third using else if (b >= c).

Step 3: Assign largest

If neither first nor second is largest, the third number is assigned as largest by default.

Alternative Approaches

Using nested if-else

c

#include <stdio.h>

int main() {
    int a, b, c, largest;
    printf("Enter three numbers: ");
    scanf("%d %d %d", &a, &b, &c);

    if (a >= b) {
        if (a >= c) {
            largest = a;
        } else {
            largest = c;
        }
    } else {
        if (b >= c) {
            largest = b;
        } else {
            largest = c;
        }
    }

    printf("Largest number is %d\n", largest);
    return 0;
}

This approach uses nested if-else to reduce the number of comparisons in some cases but is slightly more complex to read.

Using ternary operator

c

#include <stdio.h>

int main() {
    int a, b, c, largest;
    printf("Enter three numbers: ");
    scanf("%d %d %d", &a, &b, &c);

    largest = (a >= b) ? ((a >= c) ? a : c) : ((b >= c) ? b : c);

    printf("Largest number is %d\n", largest);
    return 0;
}

This method uses the ternary operator for a compact one-line assignment but may be harder for beginners to read.

Complexity: O(1) time, O(1) space

Time Complexity

The program uses a fixed number of comparisons (at most two), so it runs in constant time regardless of input.

Space Complexity

Only a few variables are used to store input and the largest number, so space usage is constant.

Which Approach is Fastest?

All approaches run in constant time; ternary operator is concise but nested if-else may be clearer for beginners.

Approach	Time	Space	Best For
Simple if-else	O(1)	O(1)	Beginners, clarity
Nested if-else	O(1)	O(1)	Slightly fewer comparisons in some cases
Ternary operator	O(1)	O(1)	Compact code, experienced programmers