Convert Decimal & Hex in C: Full Beginner’s Guide

1. Introduction

What Is a Number Base (Radix)

Purpose of This Article

2. How to Convert Decimal to Hexadecimal in C

Using the printf Function

Basic Example

#include <stdio.h>

int main() {
    int number = 255; // decimal
    printf("%xn", number); // lowercase hexadecimal
    printf("%Xn", number); // uppercase hexadecimal
    return 0;
}

Explanation

• %x: lowercase hexadecimal representation
• %X: uppercase hexadecimal representation

• %x: ff
• %X: FF

How to Pad Digits

#include <stdio.h>

int main() {
    int number = 255;
    printf("%04xn", number); // pad to 4 digits
    return 0;
}

Using the sprintf Function

Basic Example

#include <stdio.h>

int main() {
    int number = 255;
    char buffer[10];
    sprintf(buffer, "%x", number);
    printf("Hex: %sn", buffer); // output as a string
    return 0;
}

Explanation

Manual Conversion (Using Bitwise Operations)

Example Code

#include <stdio.h>
#include <string.h>

int main() {
    int number = 255;
    char hex[10];
    int i = 0;

    while (number > 0) {
        int remainder = number % 16;
        hex[i++] = (remainder < 10) ? '0' + remainder : 'A' + (remainder - 10);
        number /= 16;
    }
    hex[i] = '�';

    // reverse the string
    for (int j = 0; j < i / 2; j++) {
        char temp = hex[j];
        hex[j] = hex[i - j - 1];
        hex[i - j - 1] = temp;
    }

    printf("Hex: %sn", hex);
    return 0;
}

Explanation

1. Calculate the remainder with number % 16 and convert it to the corresponding hexadecimal character.
2. Update the quotient with number /= 16 to compute the next digit.
3. Finally, reverse the string to output the digits in the correct order.

3. How to Convert Hexadecimal to Decimal in C

Using the scanf Function

Basic Example

#include <stdio.h>

int main() {
    int number;
    printf("Please enter a hexadecimal number: ");
    scanf("%x", &number); // Receive input as hexadecimal
    printf("Decimal: %dn", number); // Display as decimal
    return 0;
}

Explanation

1. Using the format specifier %x accepts hexadecimal input.
2. The entered value is converted to decimal and stored in the variable number.
3. The printf function uses %d to display the decimal value.

Sample Output

Notes

• The user must input values in hexadecimal format (e.g., FF, 1A3).
• Including a leading “0x” is also handled correctly (e.g., 0xFF).

Using the strtol Function

Basic Example

#include <stdio.h>
#include <stdlib.h>

int main() {
    char hexString[] = "FF"; // Hexadecimal string
    long number = strtol(hexString, NULL, 16); // Convert specifying base 16
    printf("Decimal: %ldn", number);
    return 0;
}

Explanation

1. The first argument of strtol is the string to convert (here hexString).
2. The second argument is a pointer to the end of the parsed string; since we don’t use it, we pass NULL.
3. Specifying the base (16 here) as the third argument converts from hexadecimal to decimal.

Sample Output

Using strtol with Error Handling

Example with Error Handling

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>

int main() {
    char hexString[] = "ZZZ"; // Invalid hexadecimal
    char *endptr;
    errno = 0; // Initialize error

    long number = strtol(hexString, &endptr, 16);

    if (errno != 0 || *endptr != '\0') {
        printf("Invalid input.\n");
    } else {
        printf("Decimal: %ldn", number);
    }

    return 0;
}

Explanation

1. endptr points to the position where conversion stopped. If there are characters that were not converted, *endptr will not be '\0'.
2. Checking errno can detect overflow and other errors.
3. If invalid input is detected, an error message is displayed.

Sample Output

Manual Conversion Method

Example Code

#include <stdio.h>
#include <string.h>
#include <math.h>

int main() {
    char hexString[] = "FF";
    int length = strlen(hexString);
    int decimal = 0;

    for (int i = 0; i < length; i++) {
        char digit = hexString[i];
        int value;

        if (digit >= '0' && digit <= '9') {
            value = digit - '0';
        } else if (digit >= 'A' && digit <= 'F') {
            value = digit - 'A' + 10;
        } else if (digit >= 'a' && digit <= 'f') {
            value = digit - 'a' + 10;
        } else {
            printf("Invalid character.\n");
            return 1;
        }

        decimal += value * pow(16, length - i - 1);
    }

    printf("Decimal: %dn", decimal);
    return 0;
}

Explanation

1. Each character is parsed one digit at a time and converted to its decimal value.
2. Multiply by the positional weight (powers of 16) and sum.

Sample Output

4. Practical Sample Code

Bidirectional Conversion Program

Program Overview

1. Convert decimal to hexadecimal
2. Convert hexadecimal to decimal

Code Example

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

void decimalToHexadecimal() {
    int decimal;
    printf("Please enter a decimal number: ");
    scanf("%d", &decimal);
    printf("Hexadecimal: %Xn", decimal);
}

void hexadecimalToDecimal() {
    char hex[20];
    printf("Please enter a hexadecimal number: ");
    scanf("%s", hex);

    // Validate input string
    for (int i = 0; i < strlen(hex); i++) {
        if (!isxdigit(hex[i])) {
            printf("Invalid hexadecimal number.n");
            return;
        }
    }

    int decimal = (int)strtol(hex, NULL, 16);
    printf("Decimal: %dn", decimal);
}

int main() {
    int choice;

    while (1) {
        printf("nPlease select a conversion option:n");
        printf("1. Convert decimal to hexadecimal n");
        printf("2. Convert hexadecimal to decimal n");
        printf("3. Exit n");
        printf("Selection: ");
        scanf("%d", &choice);

        switch (choice) {
            case 1:
                decimalToHexadecimal();
                break;
            case 2:
                hexadecimalToDecimal();
                break;
            case 3:
                printf("Exiting program.n");
                return 0;
            default:
                printf("Invalid selection. Please try again.n");
        }
    }
    return 0;
}

Execution Example

Please select a conversion option:
1. Convert decimal to hexadecimal
2. Convert hexadecimal to decimal
3. Exit
Selection: 1
Please enter a decimal number: 255
Hexadecimal: FF

Please select a conversion option:
1. Convert decimal to hexadecimal
2. Convert hexadecimal to decimal
3. Exit
Selection: 2
Please enter a hexadecimal number: FF
Decimal: 255

Please select a conversion option:
1. Convert decimal to hexadecimal
2. Convert hexadecimal to decimal
3. Exit
Selection: 2
Please enter a hexadecimal number: ZZZ
Invalid hexadecimal number.

Error Handling Points

Handling Invalid Input

1. Check validity of hexadecimal input:

• Use the isxdigit function to verify each character is a valid hexadecimal digit (0-9, A-F, a-f).

1. Decimal input error handling:

• If the input is not numeric, display an error message and prompt for re‑entry.

Preventing Infinite Loops

Program Extension Ideas

1. Conversion to binary or octal

• Add a menu to select the numeral base.

1. Specify digit width to format conversion results

• Add an option to format the output to a fixed width.

1. Logging to a file

• Save conversion results to a log file.

Conclusion

5. Caveats and Best Practices

Cautions Regarding the Use of Format Specifiers

Aligned Output with Fixed Width

#include <stdio.h>

int main() {
    int number = 255;
    printf("4-digit hexadecimal: %04X\n", number); // Output: 00FF
    return 0;
}

• %04X: Outputs a 4-digit uppercase hexadecimal number, padding missing digits with zeros.
• Using this format ensures aligned and readable output.

Choosing Uppercase or Lowercase

Validating Input Values

Decimal Input

#include <stdio.h>

int main() {
    int number;
    printf("Please enter a decimal number: ");
    if (scanf("%d", &number) != 1) {
        printf("Invalid input.\n");
        return 1;
    }
    printf("Entered value: %d\n", number);
    return 0;
}

Hexadecimal Input

#include <ctype.h>
#include <stdio.h>
#include <string.h>

int isValidHex(const char *hex) {
    for (int i = 0; i < strlen(hex); i++) {
        if (!isxdigit(hex[i])) {
            return 0; // Contains invalid character
        }
    }
    return 1; // Valid hexadecimal
}

int main() {
    char hex[20];
    printf("Please enter a hexadecimal number: ");
    scanf("%s", hex);

    if (!isValidHex(hex)) {
        printf("Invalid hexadecimal number.\n");
        return 1;
    }

    printf("The entered hexadecimal number is valid.\n");
    return 0;
}

Handling Negative Numbers and Overflows

Handling Negative Numbers

#include <stdio.h>

int main() {
    int number = -255;
    printf("Hexadecimal (negative): %X\n", number); // Output is two's complement representation
    return 0;
}

Avoiding Overflows

#include <limits.h>
#include <stdio.h>

int main() {
    printf("Maximum int value: %d\n", INT_MAX);
    printf("Minimum int value: %d\n", INT_MIN);
    return 0;
}

Techniques to Improve Readability and Maintainability

Comments and Naming Conventions

• Give functions and variables meaningful names and add appropriate comments to improve code readability.

Leveraging Standard Functions

• Manual base conversion is useful for learning, but in production, prioritizing standard functions (such as printf and strtol) can improve code maintainability.

Testing the Program

Importance of Test Cases

• Valid input (e.g., decimal 255 → hexadecimal FF)
• Invalid input (e.g., entering string ABC as a decimal)
• Boundary values (e.g., maximum INT_MAX, minimum INT_MIN)
• Negative numbers or zero input

6. FAQ (Frequently Asked Questions)

How to add a leading “0x” to hexadecimal output with printf?

Answer:

Example code:

#include <stdio.h>

int main() {
    int number = 255;
    printf("Hexadecimal (with 0x): 0x%Xn", number); // Output: 0xFF
    return 0;
}

Note:

What happens when you try to convert an invalid string with strtol?

Answer:

Example code:

#include <stdio.h>
#include <stdlib.h>

int main() {
    char hexString[] = "FFG12";
    char *endptr;
    long number = strtol(hexString, &endptr, 16);

    printf("Converted value: %ldn", number);
    printf("Unconverted part: %sn", endptr); // Output: G12
    return 0;
}

Sample output:

Converted value: 255
Unconverted part: G12

How to convert a decimal to hexadecimal without using the printf function?

Answer:

Example code:

#include <stdio.h>

void decimalToHex(int decimal) {
    char hex[20];
    int index = 0;

    while (decimal > 0) {
        int remainder = decimal % 16;
        hex[index++] = (remainder < 10) ? '0' + remainder : 'A' + (remainder - 10);
        decimal /= 16;
    }

    printf("Hexadecimal: ");
    for (int i = index - 1; i >= 0; i--) {
        printf("%c", hex[i]);
    }
    printf("n");
}

int main() {
    int number = 255;
    decimalToHex(number);
    return 0;
}

How to limit the number of digits when reading input with scanf?

Answer:

Example code:

#include <stdio.h>

int main() {
    char hex[5]; // Accept up to 4 digits
    printf("Please enter a hexadecimal number up to 4 digits: ");
    scanf("%4s", hex);
    printf("Entered value: %sn", hex);
    return 0;
}

Note:

How to convert to other bases (e.g., binary or octal)?

Example code: conversion to binary

#include <stdio.h>

void decimalToBinary(int decimal) {
    char binary[32];
    int index = 0;

    while (decimal > 0) {
        binary[index++] = (decimal % 2) + '0';
        decimal /= 2;
    }

    printf("Binary: ");
    for (int i = index - 1; i >= 0; i--) {
        printf("%c", binary[i]);
    }
    printf("n");
}

int main() {
    int number = 255;
    decimalToBinary(number);
    return 0;
}

How to convert uppercase hexadecimal to lowercase?

Answer:

Example code:

#include <stdio.h>
#include <ctype.h>
#include <string.h>

void toLowercase(char *hex) {
    for (int i = 0; i < strlen(hex); i++) {
        hex[i] = tolower(hex[i]);
    }
}

int main() {
    char hex[] = "FF";
    toLowercase(hex);
    printf("Lowercase hexadecimal: %sn", hex);
    return 0;
}

Sample output:

Lowercase hexadecimal: ff

How to output zero-padded hexadecimal format?

Answer:

Example code:

#include <stdio.h>

int main() {
    int number = 15;
    printf("Zero-padded 4-digit hexadecimal: %04Xn", number); // Output: 000F
    return 0;
}

7. Summary

Key Points

Fundamental Concepts of Numeral Systems

• Decimal is the numeric notation we use daily, while hexadecimal is commonly used for specific purposes (e.g., memory addresses or color codes).
• C provides flexible ways to handle numbers in various bases.

Conversion from Decimal to Hexadecimal

• You can convert easily and efficiently using the printf or sprintf functions.
• We also presented a manual conversion method using bitwise operations, which is useful for understanding the conversion logic.

Conversion from Hexadecimal to Decimal

• You can convert a hexadecimal string to decimal using the scanf or strtol functions.
• Including handling of invalid input and errors allows you to create more robust programs.

Practical Program Example

• Through a bidirectional conversion program, we processed user input to achieve decimal‑hexadecimal mutual conversion.
• We also explained how to incorporate error handling to deal with incorrect input.

Cautions and Best Practices

• We described how to use format specifiers and choose uppercase or lowercase representations to format the output nicely.
• We highlighted the importance of handling negative numbers and overflow, and of carefully selecting types and checking ranges.

Frequently Asked Questions (FAQ)

• We addressed practical questions (e.g., how to prepend “0x” with printf, how to handle invalid input) with concrete examples.
• We also touched on how to apply these techniques to other bases (binary or octal).

What You’ll Gain from This Article

1. Fundamental concepts and applications of radix conversion in C
2. Implementation of base conversion using executable code
3. Creating robust programs that include input validation and error handling

For Further Learning

• Methods for converting to other bases (binary and octal)
• Implementing radix conversion in programming languages other than C
• Knowledge related to memory management and binary manipulation

7. Summary

Key Points

Fundamental Concepts of Numeral Systems

• Decimal is the numeric notation we use daily, while hexadecimal is commonly used for specific purposes (e.g., memory addresses or color codes).
• C provides flexible ways to handle numbers in various bases.

Conversion from Decimal to Hexadecimal

• You can convert easily and efficiently using the printf or sprintf functions.
• We also presented a manual conversion method using bitwise operations, which is useful for understanding the conversion logic.

Conversion from Hexadecimal to Decimal

• You can convert a hexadecimal string to decimal using the scanf or strtol functions.
• Including handling of invalid input and errors allows you to create more robust programs.

Practical Program Example

• Through a bidirectional conversion program, we processed user input to achieve decimal‑hexadecimal mutual conversion.
• We also explained how to incorporate error handling to deal with incorrect input.

Cautions and Best Practices

• We described how to use format specifiers and choose uppercase or lowercase representations to format the output nicely.
• We highlighted the importance of handling negative numbers and overflow, and of carefully selecting types and checking ranges.

Frequently Asked Questions (FAQ)

• We addressed practical questions (e.g., how to prepend “0x” with printf, how to handle invalid input) with concrete examples.
• We also touched on how to apply these techniques to other bases (binary or octal).

What You’ll Gain from This Article

1. Fundamental concepts and applications of radix conversion in C
2. Implementation of base conversion using executable code
3. Creating robust programs that include input validation and error handling

For Further Learning

• Methods for converting to other bases (binary and octal)
• Implementing radix conversion in programming languages other than C
• Knowledge related to memory management and binary manipulation