C Program to Convert Decimal to Hexadecimal

You can convert a decimal number to hexadecimal in C by repeatedly dividing the number by 16 and storing remainders, then printing them in reverse using code like printf("%X", decimalNumber); or by manual conversion with a loop.

Examples

Input26

Output1A

Input255

OutputFF

Input0

Output0

How to Think About It

To convert decimal to hexadecimal, divide the decimal number by 16 repeatedly. Each remainder corresponds to a hex digit. Collect these remainders and reverse their order to get the hexadecimal number.

Algorithm

1

Get the decimal number input.

2

While the number is greater than 0, divide it by 16 and store the remainder.

3

Map each remainder to its hexadecimal character (0-9, A-F).

4

Reverse the collected characters to form the hexadecimal string.

5

If the number is 0, the hexadecimal is 0.

6

Print the hexadecimal string.

Code

c

#include <stdio.h>
int main() {
    int decimal, temp;
    char hex[20];
    int i = 0;
    printf("Enter a decimal number: ");
    scanf("%d", &decimal);
    if (decimal == 0) {
        printf("Hexadecimal: 0\n");
        return 0;
    }
    temp = decimal;
    while (temp > 0) {
        int rem = temp % 16;
        if (rem < 10)
            hex[i++] = rem + '0';
        else
            hex[i++] = rem - 10 + 'A';
        temp /= 16;
    }
    printf("Hexadecimal: ");
    for (int j = i - 1; j >= 0; j--)
        putchar(hex[j]);
    printf("\n");
    return 0;
}

Output

Enter a decimal number: 26
Hexadecimal: 1A

Dry Run

Let's trace the decimal number 26 through the code.

1

Input

decimal = 26

2

First division

temp = 26; remainder = 26 % 16 = 10; hex[0] = 'A'; temp = 26 / 16 = 1; i = 1

3

Second division

temp = 1; remainder = 1 % 16 = 1; hex[1] = '1'; temp = 1 / 16 = 0; i = 2

4

Stop loop

temp = 0; exit loop

5

Print hex

Print hex in reverse: hex[1] = '1', hex[0] = 'A' => 1A

Iteration	temp	remainder	hex array	i
1	26	10 (A)	['A']	1
2	1	1	['A','1']	2

Why This Works

Step 1: Divide by 16

Dividing the decimal number by 16 gives the remainder which is the hex digit starting from the right.

Step 2: Store remainders

Each remainder is converted to a hex character (0-9 or A-F) and stored in an array.

Step 3: Reverse output

Since the first remainder is the least significant digit, printing the array in reverse order gives the correct hex number.

Alternative Approaches

Using printf with %X format specifier

c

#include <stdio.h>
int main() {
    int decimal;
    printf("Enter a decimal number: ");
    scanf("%d", &decimal);
    printf("Hexadecimal: %X\n", decimal);
    return 0;
}

This is the simplest and fastest way but does not show the manual conversion process.

Using recursion to convert

c

#include <stdio.h>
void decToHex(int n) {
    if (n == 0) return;
    decToHex(n / 16);
    int rem = n % 16;
    if (rem < 10) putchar(rem + '0');
    else putchar(rem - 10 + 'A');
}
int main() {
    int decimal;
    printf("Enter a decimal number: ");
    scanf("%d", &decimal);
    if (decimal == 0) printf("0");
    else decToHex(decimal);
    printf("\n");
    return 0;
}

Uses recursion to print hex digits in correct order without extra storage.

Complexity: O(log n) time, O(log n) space

Time Complexity

The loop runs proportional to the number of hex digits, which is about log base 16 of the decimal number.

Space Complexity

Extra space is used to store hex digits, proportional to the number of digits (log base 16 of the number).

Which Approach is Fastest?

Using printf with %X is fastest and simplest; manual conversion is educational but slower and uses more memory.

Approach	Time	Space	Best For
Manual loop conversion	O(log n)	O(log n)	Learning and understanding conversion
printf with %X	O(1)	O(1)	Quick and practical conversion
Recursive conversion	O(log n)	O(log n)	Elegant code without extra array