A function that changes all lowercase letters of a string to uppercase-CodePudding

I am trying to write a function that changes all lowercase letters of a string to uppercase. Here is my code:

/**
 * string_toupper - This function will replace all lowercase letters in
 * the string pointed by str to uppercase
 * @str: The string that will be checked for lowercase letters
 *
 * Return: The resulting string str, where all the letters are uppercase
 */

char *string_toupper(char *str)
{
        int i;

        for (i = 0; *str != '\0'; i  )
        {
                if (str[i] >= 'a' && str[i] <= 'z')
                        str[i] -= 32;
        }
        return (str);
}

And I tried it using:

#include <stdio.h>

int main(void)
{
    char str[] = "Hello World!\n";
    char *ptr;

    ptr = string_toupper(str);
    printf("%s\n", ptr);
    printf("%s\n", str);
    return (0);
}

But I get the following output:

Segmentation fault(core dumped)

My approach --> I will check the string if it has a lowercase letter. Then I will subtract 32 from the character if it matches to a lowercase character. I did this to make the character to uppercase, by subtracting 32 I am able to get the uppercase letter of the corresponding lowercase character I have found in the string.

But I am getting a Segmentation fault error, why is it happening?

CodePudding user response：

change the for loop condition to for (i = 0; str[i] != '\0'; i ) since it should check every index.

char *string_toupper(char *str)
{
    int i;
    for (i = 0; str[i] != '\0'; i  )
    {
            if (str[i] >= 'a' && str[i] <= 'z')
                    str[i] =(int)str[i] - 32;
    }
    return (str);
 }

CodePudding user response：

By request, this is offered for education and debate.

Some workplaces or institutes insist on a particular style wrt curly braces, etc. I freelance...

Notice that the function name is not reproduced in a comment block. Bad habit that leads to satisfying supervisors with copy/paste of comment blocks that are WRONG and certainly misleading. Better to let the code explain itself by using conventional idioms and standard libraries.

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

char *string_toupper( char *str ) {
    // Uppercase all lowercase letters found in 'str'.
    // Return str after processing.

    assert( str != NULL ); // Trust no-one, especially yourself

    // Alternative for()::  for( int i = 0; str[ i ]; i   )
    for( int i = 0; str[ i ] != '\0'; i   )
        str[ i ] = (char)toupper( str[ i ] ); // note casting.

    return str;
}

int main( void ) {
    char str[] = "Hello World!";

    // No further use? Don't store return value; just use it.
    printf( "%s\n", string_toupper( str ) );
    printf( "%s\n", str );

    return 0;
}

CodePudding user response：

OP's key problem is well explained by Prithvish: wrong loop test.

// for (i = 0; *str != '\0'; i  )
for (i = 0; str[i] != '\0'; i  )

To help OP with How can I make my code work on every environment?, some thoughts for later consideration.

Future names

"Function names that begin with str, mem, or wcs and a lowercase letter may be added to the declarations in the <string.h> header." C17dr § 7.31.13

So do not code function names that begin str<lowercase> to avoid future collisions.

Indexing type

int i; is too narrow a type for long lines. Use size_t for array indexing.

Alternatively simply increment the pointer.

Test case with classification is...() functions

str[i] >= 'a' && str[i] <= 'z' is incorrect on systems where [a...z] are not continuous. (Uncommon these days - example EBCDIC).

Simplify with topper()

To convert any character to its uppercase equivalent:

str[i] = toupper(str[i]);

Use unsigned access

is..(x) and toupper(x) functions need unsigned char character values (or EOF) for x.

On soon to be obsolete rare non-2's complement systems, character string should be accessed as unsigned char to avoid stopping on -0.

Putting this together:

#include <ctype.h>

char *str_toupper(char *str) {
  unsigned char *ustr = (unsigned char *) str;
  while (*ustr) {
    *ustr = toupper(*ustr);
    ustr  ;
  }
  return str;
}