C Program to convert a given number to words

In C programming, converting numbers to words is a common problem that involves breaking down a number into its place values and mapping each digit to its corresponding word representation. This program converts numbers from 0 to 9999 into their English word equivalents.

Syntax

void convert(char *num);

Algorithm Overview

The conversion process involves these key steps −

• Parse the input string to determine its length
• Use arrays to store word representations for different place values
• Process digits from left to right based on their position
• Handle special cases for numbers 10-19 and multiples of 10

Example

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

void convert(char *num) {
    int len = strlen(num);
    
    if (len == 0) {
        fprintf(stderr, "empty string
");
        return;
    }
    if (len > 4) {
        fprintf(stderr, "Length more than 4 is not supported
");
        return;
    }
    
    char *single_digit[] = {"zero", "one", "two", "three", "four",
                           "five", "six", "seven", "eight", "nine"};
    
    char *tens_place[] = {"", "ten", "eleven", "twelve", "thirteen", 
                         "fourteen", "fifteen", "sixteen", "seventeen", 
                         "eighteen", "nineteen"};
    
    char *tens_multiple[] = {"", "", "twenty", "thirty", "forty", "fifty",
                            "sixty", "seventy", "eighty", "ninety"};
    
    char *tens_power[] = {"hundred", "thousand"};
    
    printf("%s: ", num);
    
    if (len == 1) {
        printf("%s
", single_digit[*num - '0']);
        return;
    }
    
    while (*num != '\0') {
        if (len >= 3) {
            if (*num - '0' != 0) {
                printf("%s ", single_digit[*num - '0']);
                printf("%s ", tens_power[len-3]);
            }
            --len;
        }
        else {
            if (*num == '1') {
                int sum = *num - '0' + *(num + 1) - '0';
                printf("%s
", tens_place[sum]);
                return;
            }
            else if (*num == '2' && *(num + 1) == '0') {
                printf("twenty
");
                return;
            }
            else {
                int i = *num - '0';
                printf("%s ", i ? tens_multiple[i] : "");
                ++num;
                if (*num != '0')
                    printf("%s", single_digit[*num - '0']);
                printf("
");
                return;
            }
        }
        ++num;
    }
}

int main() {
    convert("1234");
    convert("567");
    convert("89");
    convert("5");
    return 0;
}

1234: one thousand two hundred thirty four
567: five hundred sixty seven
89: eighty nine
5: five

Key Points

• The program handles numbers from 0 to 9999 using string input
• Special handling is required for numbers 10-19 and multiples of 10
• Arrays store word mappings for efficient lookup by index
• The algorithm processes digits based on their positional value

Conclusion

This number-to-words converter demonstrates string manipulation and array usage in C. It efficiently handles place values and special cases to produce accurate word representations of numeric inputs up to four digits.