Converting Large Numbers to Words in C

Handling and converting large numbers to words in programming is a challenging yet exciting task, especially in C where the typical data types can only store numbers up to a certain limit. In this article, we’ll walk through creating a C program that converts numbers of arbitrary lengths—up to 100 digits—into English words. We will break the process down into sections, covering everything from breaking the number into manageable parts to handling edge cases and common hacks.

Table of Contents

1. Introduction

2. Understanding the Problem

3. Key Challenges

4. Algorithm Process

5. C Code Implementation

6. Equations and Calculations

7. Hacks and Optimizations

8. Conclusion

1. Introduction

Numbers are fundamental to computing, and converting them into human-readable formats is a common task in many applications, such as financial reporting and voice systems. Typically, most languages provide standard functions for handling numbers within a specific range. However, if we need to convert very large numbers—like those exceeding 64-bit limits—into words, things get trickier. This article explores a solution in C to convert numbers up to 100 digits into English words.

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2. Understanding the Problem

C does not have a built-in data type that can handle very large numbers directly. For instance:

• The largest built-in type, long long int, can only handle numbers up to 19 digits.

• Thus, converting a number up to 100 digits into words involves working with string processing instead of numerical operations.

Problem Statement

Given a number as a string (of up to 100 digits), convert it into its equivalent English word representation. For example:

• Input: 12345678901234567890

• Output: "twelve quintillion three hundred forty-five quadrillion six hundred seventy-eight trillion nine hundred one billion two hundred thirty-four million five hundred sixty-seven thousand eight hundred ninety"

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3. Key Challenges

1. Handling String Input: We must read the number as a string to handle more than 19 digits.

2. Chunking Numbers: The number must be broken into groups of three digits, since English words are grouped this way (thousands, millions, billions, etc.).

3. Efficiency: Converting such large numbers can be computationally expensive if not handled efficiently.

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4. Algorithm Process

Step-by-Step Breakdown

To solve this problem, we take the following approach:

1. Read Input as a String: Since standard data types in C cannot hold numbers larger than 19 digits, we read the number as a string of characters.

2. Chunk the Number: Split the number into groups of three digits, starting from the right-hand side. This is because English words use groupings of thousands, millions, etc.

3. Convert Each Chunk: Convert each 3-digit chunk into words using pre-defined mappings (like "one," "hundred," "thousand," etc.).

4. Add Suffix: For each chunk, add a suffix like "thousand," "million," or "billion" based on its position in the number.

5. Combine the Words: Merge the words for each chunk to form the final string representing the number in words.

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5. C Code Implementation

Here is a sample implementation in C that converts numbers up to 100 digits into words.

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

char *one_to_nineteen[] = {"", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten",
                           "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen",
                           "eighteen", "nineteen"};
char *tens[] = {"", "", "twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety"};
char *thousands[] = {"", "thousand", "million", "billion", "trillion", "quadrillion", "quintillion", "sextillion", "septillion"};

void convert_to_words(char *num);
void convert_chunk(int num, char *result);

int main() {
    char number[101]; // for numbers up to 100 digits

    printf("Enter a number (up to 100 digits): ");
    scanf("%100s", number); // read the number as a string

    if (strlen(number) == 0 || strcmp(number, "0") == 0) {
        printf("zero\n");
    } else {
        convert_to_words(number);
    }

    return 0;
}

void convert_to_words(char *num) {
    int len = strlen(num);
    int chunk_count = (len + 2) / 3; // Number of 3-digit chunks
    int start_index = len % 3 == 0 ? 0 : len % 3; // Calculate how many digits before first chunk

    // We go through each chunk
    for (int i = 0; i < chunk_count; i++) {
        int chunk_value = 0;

        // Process the first (potentially smaller) chunk
        if (i == 0 && start_index != 0) {
            for (int j = 0; j < start_index; j++) {
                chunk_value = chunk_value * 10 + (num[j] - '0');
            }
            // Move the starting index forward after processing
            num += start_index;
        } else { // Process normal 3-digit chunks
            for (int j = 0; j < 3; j++) {
                chunk_value = chunk_value * 10 + (num[j] - '0');
            }
            num += 3; // Move the string pointer forward
        }

        // Convert the chunk into words
        char chunk_words[50] = "";
        convert_chunk(chunk_value, chunk_words);

        // Print the words with the appropriate thousands suffix
        if (chunk_value > 0) {
            printf("%s %s ", chunk_words, thousands[chunk_count - i - 1]);
        }
    }

    printf("\n");
}

void convert_chunk(int num, char *result) {
    if (num >= 100) {
        sprintf(result + strlen(result), "%s hundred ", one_to_nineteen[num / 100]);
        num %= 100;
    }

    if (num >= 20) {
        sprintf(result + strlen(result), "%s ", tens[num / 10]);
        num %= 10;
    }

    if (num > 0) {
        sprintf(result + strlen(result), "%s ", one_to_nineteen[num]);
    }
}

6. Equations and Calculations

To better understand how to break the numbers into chunks, let's define the process mathematically:

Let N represent the total number of digits in the input string. The number of chunks can be calculated by:

$$\text{{chunks}} = \left\lceil \frac{N}{3} \right\rceil$$

For each chunk, we extract the 3-digit number:

$$\text{{chunk_value}} = \text{{digits}}[3(i - 1):3i]$$

Each chunk is then mapped to its corresponding English words based on predefined arrays for numbers 1 to 19, tens, and hundreds.

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7. Hacks and Optimizations

Memory Usage

Instead of using recursion or multiple arrays for each part of the number, the program can reuse a buffer to store the words temporarily.

Edge Case Handling

Ensure that inputs like "0000" or "000000" don't accidentally result in incorrect word outputs. Handle cases where the number has fewer than 3 digits in any chunk correctly.

Optimization Hack

If the number is known to be large and complex, pre-parse the string to detect any leading zeroes and avoid unnecessary computations for all-zero groups.

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8. Conclusion

Converting a large number to words in C is an interesting problem that touches on multiple aspects of programming: string manipulation, logic design, and optimization. By working through this problem step-by-step and carefully breaking it into smaller chunks, it is possible to write an efficient and clear program capable of handling large numbers.