Case insensitive sorting in C

Question

I have a .txt file and I want to sort it using C programming. I have this code for sorting a .txt file:

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

#define MAX_LEN 100 // Length of each line in input file.

int main(void)
{
    char *strFileName = "/home/milad/Desktop/ddd.txt";
    char *strFileSummary = "/home/milad/Desktop/ddd2.txt";
    char strTempData[MAX_LEN];
    char **strData = NULL; // String List
    int i, j;
    int noOfLines = 0;

    FILE * ptrFileLog = NULL;
    FILE * ptrSummary = NULL;

    if ( (ptrFileLog = fopen(strFileName, "r")) == NULL ) {
        fprintf(stderr,"Error: Could not open %s\n",strFileName);
        return 1;
    }
    if ( (ptrSummary = fopen(strFileSummary, "a")) == NULL ) {
        fprintf(stderr,"Error: Could not open %s\n",strFileSummary);
        return 1;
    }

    // Read and store in a string list.
    while(fgets(strTempData, MAX_LEN, ptrFileLog) != NULL) {
        // Remove the trailing newline character
        if(strchr(strTempData,'\n'))
            strTempData[strlen(strTempData)-1] = '\0';
        strData = (char**)realloc(strData, sizeof(char**)*(noOfLines+1));
        strData[noOfLines] = (char*)calloc(MAX_LEN,sizeof(char));
        strcpy(strData[noOfLines], strTempData);
        noOfLines++;
    }
    // Sort the array.
    for(i= 0; i < (noOfLines - 1); ++i) {
        for(j = 0; j < ( noOfLines - i - 1); ++j) {
            if(strcmp(strData[j], strData[j+1]) > 0) {
                strcpy(strTempData, strData[j]);
                strcpy(strData[j], strData[j+1]);
                strcpy(strData[j+1], strTempData);
            }
        }
    }
    // Write it to outfile. file.
    for(i = 0; i < noOfLines; i++)
        fprintf(ptrSummary,"%s\n",strData[i]);
    // free each string
    for(i = 0; i < noOfLines; i++)
        free(strData[i]);
    // free string list.
    free(strData);
    fclose(ptrFileLog);
    fclose(ptrSummary);
    return 0;
} 

This code is case sensitive and it sorts the upper-case letters first, then the lower case letters and That is not what I want. I want it to sort letters alphabetically and without any sensitivity to the case of the letter. I know about ASCII codes and why I have this problem, but I can't find a way to fix it.

How can I change the code to make it case insensitive?

Answer 1

If your system has POSIX strcasecmp function available, replace strcmp with strcasecmp for a drop-in replacement.

Otherwise, implement case-insensitive string comparison in your own code, and replace the call of strcmp with it. You can implement the required functionality by comparing strings character-by-character after converting characters on both sides to the same case (upper or lower).

Note 1: Your algorithm has an inefficiency: you allocate all your strings at max length to avoid undefined behavior when the strings are of unequal length. You can allocate strings at exact length and avoid undefined behavior, because you do not need copying the content at all: swapping string pointers will do the job, and it would be a lot faster.

if(strcasecmp(strData[j], strData[j+1]) > 0) {
    char *tmp = strData[j];
    strData[j] = strData[j+1];
    strData[j+1] = tmp;
}

Note 2: Bubble sort algorithm is pretty slow, unless the array is nearly sorted to begin with. For larger files, you can improve the speed considerably by using qsort.