Most common character in a file in C

Question

I'm doing my C programming course homework and I need to find a most common character in given file.

My testing with a testfile, emptyfile and other small amount text files works great (or at least I think so), but in the last long testfile something goes wrong and the error message is: "Should have returned 'e' (101) for file rfc791.txt. You returned 'b' (98)" .

So what I'm asking that what might be wrong with my code, when suddenly the most common letter is not what is should be?

int most_common_character(char *filename) {
    FILE *f;
    if ((f = fopen(filename, "r")) == NULL) {
        fprintf(stderr, "Not opened: %s\n", strerror(errno));
        return -1;
    }

    char frequency[26];
    int ch = fgetc(f);
    if (ch == EOF) {
        return 0;
    }

    for (ch = 0; ch < 26; ch++) {
        frequency[ch] = 0;
    }

    while (1) {
        ch = fgetc(f);
        if (ch == EOF) {
            break;
        }
        if ('a' <= ch && ch <= 'z') {
            frequency[ch - 'a']++;
        }
        else if ('A' <= ch && ch <= 'Z') {
            frequency[ch - 'A']++;
        }
    }
    int maxCount = 0;
    int maxChar = 0;
    for (int i = 0; i <= 26; ++i) {
        if (frequency[i] > maxCount) {
            maxCount = frequency[i];
            maxChar = i;
        }
    }
    fclose(f);
    return maxChar + 'a';
}

I would be very grateful if someone has any hints to fix my code :) I've tried to search the solution to this problem from many other related topics but nothing seems to work.

Answer 1

You should use < operator in the second for loop. Because of that when you are checking frequency[i] > maxCount, at frequency[26] it behaves undefined behaviour, meaning the value at that index may be less or higher than the compared value.

Answer 2

Your code do have some problems. However, they are so tiny so the code still works well with small tests.

1. int ch = fgetc(f); drop the first char in the file

2. for (int i = 0; i <= 26; ++i) break out of the array 's range (only from 0-->25)

Beside these small mistakes, your code is awesomely fine. Well done #thumbsup

Answer 3

1. Loop runs out-of-bounds. @Weather Vane

    // for (int i = 0; i <= 26; ++i) { for (int i = 0; i < 26; ++i) { 

2. Code throws away result of the first character. @BLUEPIXY

    int ch = fgetc(f); if (ch == EOF) { return 0; } // This value of ch is not subsequently used. 

Other fixes as below

int most_common_character(char *filename) {
    ...

    // Use a more generous count @Weather Vane
    // char frequency[26];
    // Consider there may be more than 26 different letters
    // fgetc return EOF and value in the unsigned char range
    int frequency[UCHAR_MAX + 1] = { 0 };

    // Not needed as array was initialize above
    // for (ch = 0; ch < 26; ch++) { frequency[ch] = 0; }

    // BTW correct type declaration of int, avoided rookie mistake of using char
    int ch;

    // Codes use tolower(), islower() as that is the portable way to 
    // handle type-of-character detection
    while ((ch = fgetc(f)) != EOF) {
      frequency[tolower(ch)]++;  // could add check to insure frequency[] does not overflow
    } 

    int maxCount = 0;
    int maxChar = -1;
    for (int i = 0; i <= UCHAR_MAX; ++i) {
      if (islower(i) && frequency[i] > maxCount) {
        maxCount = frequency[i];
        maxChar = i;
      }
    }

    fclose(f);
    return maxChar;
}