C Program To Convert Decimal To Hexadecimal Using Functions

In this article, you will learn how to convert decimal numbers to their hexadecimal equivalents in C, specifically focusing on using custom functions for clear, modular code. We will explore the underlying logic and also touch upon a simpler library-based approach.

Problem Statement

Converting numbers between different bases is a fundamental concept in computing. Decimal (base-10) is our everyday number system, while hexadecimal (base-16) is widely used in programming for representing data like memory addresses, color codes, and byte values due to its compact nature. A single hexadecimal digit can represent four binary bits, making it much more concise than binary and easier to read than long decimal numbers. The challenge lies in converting a decimal integer into its string representation using hexadecimal digits (0-9 and A-F).

Example

Consider the decimal number 255. When converted to hexadecimal, it becomes FF.

Background & Knowledge Prerequisites

To fully grasp this article, a basic understanding of the following C concepts is recommended:

• C Syntax: Variables, data types (int, char), loops (while), and conditional statements (if-else).
• Functions: How to declare, define, and call functions, including passing parameters.
• Arrays and Strings: Basic manipulation of character arrays to form strings, including null-termination (\0).
• Number Systems: A general idea of decimal and hexadecimal systems, particularly how hexadecimal uses digits 0-9 and letters A-F to represent values 10-15.

Use Cases or Case Studies

Decimal to hexadecimal conversion is a common operation with several practical applications:

• Memory Addressing: Hexadecimal is standard for representing memory locations in debuggers and system programming.
• Color Codes: In web development and graphic design, RGB (Red, Green, Blue) color values are frequently expressed in hexadecimal (e.g., #FF0000 for red).
• Data Representation: Displaying raw binary data, such as network packet contents or file headers, in a human-readable format.
• Debugging and Diagnostics: Analyzing memory dumps, CPU registers, or error codes often involves hexadecimal values.
• Cryptography: Many cryptographic hashes and keys are represented as hexadecimal strings.

Solution Approaches

We will explore two primary approaches to convert decimal to hexadecimal in C: implementing a custom function and utilizing the standard library's sprintf function.

Approach 1: Implementing a Custom Conversion Function

Developing a custom function provides a deep understanding of the conversion logic and offers greater control.

• One-line summary: Create a user-defined function that converts a decimal integer to its hexadecimal string representation by repeatedly dividing by 16 and mapping remainders to appropriate hex characters.

• Code example:

// Decimal to Hexadecimal Converter using Custom Function
    #include <stdio.h> // For printf
    #include <string.h> // For string manipulation (optional for manual reverse)

    // Function to convert a decimal number to hexadecimal string
    // Stores the result in the provided char array 'hexResult'
    void decimalToHex(int decimalNum, char hexResult[]) {
        int i = 0;
        // Handle the case where the decimal number is 0 separately
        if (decimalNum == 0) {
            hexResult[i++] = '0';
            hexResult[i] = '\\0';
            return;
        }

        while (decimalNum > 0) {
            int remainder = decimalNum % 16;
            if (remainder < 10) {
                // Convert 0-9 to ASCII '0'-'9'
                hexResult[i++] = remainder + '0';
            } else {
                // Convert 10-15 to ASCII 'A'-'F'
                hexResult[i++] = remainder + 'A' - 10;
            }
            decimalNum /= 16; // Divide by 16 for the next digit
        }
        hexResult[i] = '\\0'; // Null-terminate the string

        // The hexadecimal digits are accumulated in reverse order
        // (least significant digit first), so we need to reverse the string.
        int start = 0;
        int end = i - 1;
        while (start < end) {
            char temp = hexResult[start];
            hexResult[start] = hexResult[end];
            hexResult[end] = temp;
            start++;
            end--;
        }
    }

    int main() {
        int decimal1 = 255;
        char hexResult1[100]; // Buffer to store the hexadecimal string
        decimalToHex(decimal1, hexResult1);
        printf("Decimal %d -> Hexadecimal %s\\n", decimal1, hexResult1);

        int decimal2 = 10;
        char hexResult2[100];
        decimalToHex(decimal2, hexResult2);
        printf("Decimal %d -> Hexadecimal %s\\n", decimal2, hexResult2);

        int decimal3 = 256;
        char hexResult3[100];
        decimalToHex(decimal3, hexResult3);
        printf("Decimal %d -> Hexadecimal %s\\n", decimal3, hexResult3);

        int decimal4 = 0;
        char hexResult4[100];
        decimalToHex(decimal4, hexResult4);
        printf("Decimal %d -> Hexadecimal %s\\n", decimal4, hexResult4);

        int decimal5 = 4096;
        char hexResult5[100];
        decimalToHex(decimal5, hexResult5);
        printf("Decimal %d -> Hexadecimal %s\\n", decimal5, hexResult5);

        return 0;
    }

Run

• Sample output:

Decimal 255 -> Hexadecimal FF
    Decimal 10 -> Hexadecimal A
    Decimal 256 -> Hexadecimal 100
    Decimal 0 -> Hexadecimal 0
    Decimal 4096 -> Hexadecimal 1000

• Stepwise explanation:

1. decimalToHex Function Definition: This function takes an int decimalNum (the number to convert) and a char array hexResult[] (where the hexadecimal string will be stored).
2. Handle Zero: A special case is included for decimalNum == 0, immediately storing '0' in hexResult and returning.
3. Conversion Loop: A while loop runs as long as decimalNum is greater than 0.

• Calculate Remainder: remainder = decimalNum % 16 calculates the remainder, which corresponds to the current hexadecimal digit.
• Map to Character:
• If remainder is less than 10, it's a digit 0-9. Adding '0' converts it to its ASCII character equivalent (e.g., 5 becomes '5').
• If remainder is 10 or greater (10-15), it corresponds to 'A'-'F'. Adding 'A' - 10 maps these remainders correctly (e.g., 10 becomes 'A', 11 becomes 'B').
• Store Digit: The converted character is stored in the hexResult array at index i, and i is incremented.
• Update Decimal: decimalNum /= 16 performs integer division, effectively moving to the next hexadecimal digit position.

1. Null Termination: After the loop, hexResult[i] = '\0' adds the null-terminator character, marking the end of the C string. This is crucial for functions like printf.
2. Reverse String: The hexadecimal digits are generated from right to left (least significant to most significant). Therefore, the hexResult array needs to be reversed to display the correct hexadecimal number. A while loop with two pointers (start and end) efficiently swaps characters from both ends until they meet in the middle.
3. main Function: This function demonstrates how to call decimalToHex with various decimal inputs and prints the original decimal number along with its converted hexadecimal representation.

Approach 2: Using the sprintf Function

For a more concise solution that relies on standard library functions, sprintf is an excellent choice.

• One-line summary: Utilize the standard library function sprintf with the %X or %x format specifier to directly format an integer as a hexadecimal string.

• Code example:

// Decimal to Hexadecimal Converter using sprintf
    #include <stdio.h> // For printf and sprintf

    int main() {
        int decimal1 = 255;
        char hexResult1[100]; // Buffer to store the hexadecimal string
        sprintf(hexResult1, "%X", decimal1); // %X for uppercase hex
        printf("Decimal %d -> Hexadecimal %s\\n", decimal1, hexResult1);

        int decimal2 = 10;
        char hexResult2[100];
        sprintf(hexResult2, "%x", decimal2); // %x for lowercase hex
        printf("Decimal %d -> Hexadecimal %s\\n", decimal2, hexResult2);

        int decimal3 = 256;
        char hexResult3[100];
        sprintf(hexResult3, "%X", decimal3);
        printf("Decimal %d -> Hexadecimal %s\\n", decimal3, hexResult3);

        int decimal4 = 0;
        char hexResult4[100];
        sprintf(hexResult4, "%X", decimal4);
        printf("Decimal %d -> Hexadecimal %s\\n", decimal4, hexResult4);

        return 0;
    }

Run

• Sample output:

Decimal 255 -> Hexadecimal FF
    Decimal 10 -> Hexadecimal a
    Decimal 256 -> Hexadecimal 100
    Decimal 0 -> Hexadecimal 0

• Stepwise explanation:

1. Include stdio.h: This header file provides the sprintf and printf functions.
2. Declare Buffer: A char array (e.g., hexResult1) is declared to serve as a buffer where sprintf will write the resulting hexadecimal string.
3. sprintf Function:

• sprintf(hexResult1, "%X", decimal1); performs the conversion. sprintf works similarly to printf but writes to a string buffer instead of the console.
• The %X format specifier instructs sprintf to interpret decimal1 as an integer and format it as an uppercase hexadecimal string.
• Using %x would produce lowercase hexadecimal characters (e.g., a instead of A).

1. Print Result: The printf function is then used to display the original decimal number and the hexadecimal string stored in hexResult.

Conclusion

Converting decimal numbers to hexadecimal is a common task in C programming, essential for various computing applications. By implementing a custom function, you gain a deeper understanding of number base conversion algorithms, involving repeated division by 16 and mapping remainders to appropriate hexadecimal characters, followed by string reversal. For simpler, more straightforward conversions, the standard library's sprintf function offers a convenient and powerful alternative using the %X or %x format specifiers. Understanding both approaches equips you with flexible tools for handling number system conversions in your C projects.

Summary

• Decimal to hexadecimal conversion is crucial for memory representation, color codes, and debugging.
• A custom C function can perform this conversion by repeatedly dividing the decimal number by 16.
• The remainders (0-15) are mapped to hexadecimal digits (0-9, A-F) and collected in a character array.
• The collected digits must be reversed to form the correct hexadecimal string.
• The C standard library's sprintf function provides a simpler, built-in way to convert integers to hexadecimal strings using %X (uppercase) or %x (lowercase) format specifiers.
• Both custom implementations and library functions offer valid solutions, with the choice depending on the need for control versus convenience.