Question

How to extract and sort numbers from a string containing numbers and text in C

Answer 1

#include <stdio.h>      // for printf
#include <stdlib.h>     // for malloc, realloc, qsort, strtol
#include <ctype.h>      // for isdigit
#include <string.h>     // for strlen

/* 
    Extract all integer values from a mixed string.
    This function walks through the characters, collects digits into a temporary buffer,
    and converts each completed number into an integer.
*/
int* extract_numbers(const char* input, size_t* out_count) {
    size_t capacity = 8;          // initial capacity for dynamic array
    size_t count = 0;             // how many numbers we have collected
    int* numbers = malloc(capacity * sizeof(int));

    char buffer[64] = "";         // temporary buffer for digits
    size_t buf_len = 0;

    for (size_t i = 0; input[i] != '\0'; ++i) {
        if (isdigit((unsigned char)input[i])) {
            /* accumulate digits */
            if (buf_len < sizeof(buffer) - 1) {
                buffer[buf_len++] = input[i];
            }
        } else {
            /* flush buffer if it contains a number */
            if (buf_len > 0) {
                buffer[buf_len] = '\0';
                int value = (int)strtol(buffer, NULL, 10);

                if (count == capacity) {
                    /* grow the dynamic array */
                    capacity *= 2;
                    numbers = realloc(numbers, capacity * sizeof(int));
                }

                numbers[count++] = value;
                buf_len = 0;
            }
        }
    }

    /* flush any remaining number at the end */
    if (buf_len > 0) {
        buffer[buf_len] = '\0';
        int value = (int)strtol(buffer, NULL, 10);

        if (count == capacity) {
            capacity *= 2;
            numbers = realloc(numbers, capacity * sizeof(int));
        }

        numbers[count++] = value;
    }

    *out_count = count;

    return numbers;
}

/* 
    Comparison function for qsort.
    qsort is efficient and widely used for sorting in C.
*/
int compare_ints(const void* a, const void* b) {
    int ia = *(const int*)a;
    int ib = *(const int*)b;

    return (ia > ib) - (ia < ib);
}

/* 
    Print all numbers in a simple space-separated format.
*/
void print_numbers(const int* numbers, size_t count) {
    for (size_t i = 0; i < count; ++i) {
        printf("%d", numbers[i]);
        if (i + 1 < count) {
            printf(" ");
        }
    }
    printf("\n");
}

int main(void) {
    const char* input = "1000withz7 and3 or 99 give42";

    /* extract numbers */
    size_t count = 0;
    int* numbers = extract_numbers(input, &count);

    /* sort numbers */
    qsort(numbers, count, sizeof(int), compare_ints);

    /* display result */
    printf("Sorted numbers: ");
    print_numbers(numbers, count);

    free(numbers);
    
    return 0;
}


/*
run:

Sorted numbers: 3 7 42 99 1000

*/

 

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