Convert every letter from a paragraph (string) to lowercase before storing it back to the string (C Programming)

Question

I'm trying to use the following code to convert every letters from a paragraph (strings) into lowercase letters before storing it back to the string. But the compiler wouldn't compile =(

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

void spellCheck(char article[], char dictionary[]) {
    int i = 0;
    char* tempArticle;

    tempArticle = malloc((strlen(article)+1) * sizeof(char));

     strcpy(tempArticle, article);
     *tempArticle = toLower(*tempArticle);
     printf("%s", tempArticle);
}


char toLower(char letter){
    if (letter >= 'A' && letter <='Z'){
     letter = letter + 32;
    }
    return letter;
}

Answer 1

So, there is a standard function called tolower to do this. What this new code does is declare a char pointer, sets it to point at the first element in the string, and goes character by character until the null terminator converting it to lower case.

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


void spellCheck(char article[], char dictionary[]) {
    int i = 0;
    char* tempArticle;
    char *c;

    tempArticle = malloc((strlen(article)+1) * sizeof(char));

     strcpy(tempArticle, article);
    /* *tempArticle = toLower(*tempArticle);*/

     for(c=tempArticle; *c != '\0'; c++)
          *c = (char)tolower(*c);

     printf("%s", tempArticle);
}

/* there is a standard function for this */
/*char toLower(char letter){
    if (letter >= 'A' && letter <='Z'){
     letter = letter + 32;
    }
    return letter;
}*/

Answer 2

The only compile error I get from that is from a failure to declare toLower.

See http://codepad.org/3R4ChOMe

Answer 3

char toLower(char) takes a character and returns a character which is fine, but you need to iterate over all of the characters in your string using a for loop.

The compile problem is probably from a missing function prototype for toLower(). Try something like:

char toLower(char letter);

void spellCheck(char article[], char dictionary[]) {
    size_t len = strlen(article) + 1;
    char* tempArticle = malloc(len * sizeof(char));
    if (tempArticle != NULL) {
        size_t i = 0;
        for (i=0; i<len; ++len)
            tempArticle[i] = toLower(article[i]);
        printf("%s", tempArticle);
        free(tempArticle);
    } else {
        perror("malloc");
    }
}

If you have to iterate over the string, then there is no need to call strcpy(). You were also missing a call to free() - you need to make sure that you deallocate any memory that you have allocated.

As others have noted, you should probably replace your toLower() function with the Standard supplied tolower(). It is declared in <ctype.h> and will do the case-folding task in a much safer way.

Answer 4

BobbyShaftoe has the correct idea, but there is one minor change I would make:

void spellCheck(char article[], char dictionary[]) {
    int len = 0;
    int i = 0;
    char* tempArticle;
    char *c;

    len = strlen(article);
    tempArticle = malloc((len+1) * sizeof(char));

    for(i = 0; i < len; i++) 
      tempArticle[i] = (char)tolower(article[i]);
    tempArticle[i] = '\0';

     printf("%s", tempArticle);
}

The difference is why do a string copy when you can just convert as you move them over.

Answer 5

The Executive Summary (and Notes)

(I'm marking this as Community Wiki because, with a few exceptions, I'm just pulling all the answers and comments into one answer.)

The code:

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

void spellCheck(char article[], char dictionary[]) {
    size_t len = strlen(article) + 1;
    size_t i;
    char* tempArticle;

    tempArticle = malloc(len * sizeof *tempArticle);

    if (tempArticle == NULL) {
        fprintf(stderr, "spellcheck: Memory allocation failed.\n");
        return;
    }

    for(i = 0; i < len; i++)
        tempArticle[i] = tolower((unsigned char)article[i]);

    printf("%s", tempArticle);

    free(tempArticle);
}

The changes:

1. Your toLower() function is identical to the C standard library tolower() function (except that it's not as portable and probably less efficient), so let's use that instead.
2. tolower() works on single characters, not full strings. To convert an entire string to lowercase, you have to use it in a loop.
3. strlen(), strcpy(), and the tolower() loop all have to loop through the string. We can eliminate the strcpy() loop by copying and tolower()ing at the same time. This will result in a slight increase in efficiency.
4. Changed sizeof(char) to sizeof(*tempArticle). There is no difference - sizeof(char) is guaranteed by the standard to be 1, and *tempArticle is a char, so it's size will always be 1 - but if in the future you change tempArticle to be a wchar_t or some other type, the malloc() line will still work correctly. Some would argue to leave off the sizeof() altogether, since sizeof(char) is guaranteed to be 1. I would leave it.
5. Add a call to free() at the end of the function - otherwise, the function will leak memory.
6. Check for malloc() failure.

Improvements over other posts in this thread:

1. strlen() returns a size_t, not an int. The difference is more than semantic - int is often signed, while size_t is unsigned, and is guaranteed by the standard to be what you want. size_t should be used as a variable to hold array/pointer indexes, not int.
2. Calling strlen() is an O(n) operation, so we should save it's result, particularly if we're going to need it later.
3. The argument to tolower() is cast to unsigned char, as required by the rather odd calling convention of this function.

Answer 6

Forgive me if I misunderstood the problem, but wouldn't it be better just to do the conversion in place rather than copying the string into a temporary buffer? After all, the questioner claims to want to store the result back in the original string.

I have in mind something like this:

#include <stdio.h>
#include <ctype.h>

void spellCheck(char article[], char dictionary[]) {
    for (char *pos = article; *pos != '\0'; pos++) {
        *pos = tolower(*pos);
    }

    // Remove the following line if you don't want to print the result.
    printf("%s", article);
}

Note that depending on the C standard you're using, you may need to move the declaration of pos out of the for loop header.

Answer 7

I would do it this way:

void spellCheck(char *article, char *dictionary)
{
  while(*article)
  {
    *article = tolower(*article);
    article++;
  }
}

As this is the "standard" "iterate over a string" thing.

But I don't really think the OP wants that, spellCheck seems quite a name for something more evolved.

Also, beware that tolower uses the current locale and is restricted to unsigned char range (even if it takes an int).

Answer 8

Did you ever heard of <ctype.h>