Converting a C-String to all lower

Question

I'm trying to convert a C-String to all lower case wihout the use of tolower from ctype.h . Hower my code does not seem to work: I'm receiving a runtime error. What I'm trying to do is changing the ASCII value of the capitalized letters bij 'a' - 'A' which should convert those values to the ones for lower case as far as I'm aware.

#include <stdio.h>
void to_lower(char* k) {
    char * temp = k;
    while(*temp != 0) {
        if(*temp > 'A' && *temp < 'Z') {
            *temp += ('a' - 'A');
        }
        temp++;
    }
}

int main() {
    char * s = "ThiS Is AN eXaMpLe";
    to_lower(s);
    printf("%s",s);
}

What are the errors you're receiving? It looks like you have an extra `}` after `to_lower`. — SimpleJ, Jan 06 '15 at 23:28
It also looks like you're modifying a string literal which is undefined behavior. — Mooing Duck, Jan 06 '15 at 23:29
@SimpleJ: He doesn't have an extra `}`, it's just unformatted code. — Mooing Duck, Jan 06 '15 at 23:30

score 9 · Accepted Answer · answered Jan 06 '15 at 23:30

9

Two errors.

This code won't convert A and Z to lowercase :

if(*temp > 'A' && *temp < 'Z') {

Use >= and <= instead.

And it's not legal to try to modify a string literal ! Arrays can be modified, string literals can't.

Change that char * s = "ThiS Is AN eXaMpLe"; to char s[] = "ThiS Is AN eXaMpLe";

answered Jan 06 '15 at 23:30

tux3

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The contents of an array can be modified. – Mooing Duck Jan 06 '15 at 23:31
1

I think of an array as its content, and nothing more. It's implicitly convertible to a pointer, but it really isn't the same thing as a const pointer. But I suppose that's a valid way to see it too. – tux3 Jan 06 '15 at 23:32
@MooingDuck In some cases, the string literal will be located in read only program memory. Trying to modify it in that case would cause the program to crash. I've run into this issue while programming AVR microcontrollers – SimpleJ Jan 06 '15 at 23:33

luser droog · Answer 2 · 2015-01-07T08:11:41.263

1

Even if you don't use the existing standard library function, it may still be useful to follow its interface. tolower converts an individual character. Applying this function to a string can be written as a de-coupled function.

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

int to_lower (int c) {
    if (strchr("ABCDEFGHIJKLMNOPQRSTUVWXYZ", c))
        c = c - 'A' + 'a';
    return c;         
}

void mapstring (char *str, int (*f)(int)) {
    for (; *str; str++)
        *str = f(*str);
}

int main() {
    char s[] = "THIS IS MY STRING";

    mapstring(s, to_lower);
    printf("%s\n", s); 
    return 0;
}

edited Jan 07 '15 at 08:11

answered Jan 07 '15 at 00:04

luser droog

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The first line of the question explicitely says that it's trying to do it "without the use of tolower from ctype.h", though. – tux3 Jan 07 '15 at 00:56
@tux3 Thanks. I've re-written my suggestion. The point was separating the functions, not just re-using the library but imitating it. – luser droog Jan 07 '15 at 07:47

score 0 · Answer 3 · answered Jan 06 '15 at 23:32

0

Two problems that I can see immediately: (1) char *s = "This..." creates a non-writable string. You need to use a character array and copy a string into it. (2) if (*temp > 'A' && *temp < 'Z') skips A and Z. You need >= and <=.

answered Jan 06 '15 at 23:32

Lee Daniel Crocker

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Converting a C-String to all lower

3 Answers3