Using C Strings in an array

Question

This is a program I wrote (not including "string.h") which is meant to convert a string to upper case. It works with a single string - so far so good.

I run into trouble when I try to create an array of strings so I can test various strings in turn as part of a loop.

So, I don't understand why the program works when my string is declared as char test_string[] = "TEST"; but it does not work when I declare an array of pointers to strings.

This is the working single string version (followed by the non-working array of strings version):

#include <stdio.h>

void toAlpha(char*);
int str_len(char*);

int main()
{
    char test_string[] = "TEST";            /* string to test */
    char *pStr = NULL;                  /* pointer to string */

        pStr = test_string;
        toAlpha(pStr);
        printf("%s\n", pStr);


    return 0;
}


void toAlpha(char *arg)
{
    int i = 0;                  /* counter - original string*/
    int j = 0;                  /* counter - temp string */

    /* check each character in original and save alphabetic characters only */
    for ( i = 0; i < str_len(arg); i++ )
    {
        if( *(arg + i) >= 'a' && *(arg + i) <= 'z' )
            *(arg + j++) = *(arg + i);
        else
            if ( *(arg + i) >= 'A' && *(arg + i) <= 'Z' )
                *(arg + j++) = *(arg + i) - 'A' + 'a';
    }

    /* add a null character terminator */
    *(arg + j) = '\0';

}


int str_len(char *arg)
{
    /*return count of letters in a C string */
    int i = 0;
    if ( arg != NULL )
        while ( arg[i] != '\0' )
            i++;
    return i;
}

This is the non-working version with the failed attempt to use an array (it compiles but crashes at runtime):

#include <stdio.h>

void toAlpha(char*);
int str_len(char*);
void palindrome(char*);

int main()
{
    char *test_strings[1];                      /* strings to test */
    char *pStr = NULL;                          /* pointer to string */
    int i = 0;                                  /* loop counter */

    test_strings[0] = "TEST1";
    test_strings[1] = "TEST2";

    for (i = 0; i < 1; i++){
        pStr = test_strings[i];
        toAlpha(pStr);
        printf("%s\n", pStr);
    }

    return 0;
}


void toAlpha(char *arg)
{
    int i = 0;                  /* counter - original string*/
    int j = 0;                  /* counter - temp string */

    /* check each character in original and save alphabetic characters only */
    for ( i = 0; i < str_len(arg); i++ )
    {
        if( *(arg + i) >= 'a' && *(arg + i) <= 'z' )
            *(arg + j++) = *(arg + i);
        else
            if ( *(arg + i) >= 'A' && *(arg + i) <= 'Z' )
                *(arg + j++) = *(arg + i) - 'A' + 'a';
    }

    /* add a null character terminator */
    *(arg + j) = '\0';

}


int str_len(char *arg)
{
    /*return count of letters in a C string */
    int i = 0;
    if ( arg != NULL )
        while ( arg[i] != '\0' )
            i++;
    return i;
}

Answer 1

I found the error. You can't modify string literal. To fix this you need to substitute the following:

test_strings[0] = "TEST1";
test_strings[1] = "TEST2";

by:

test_strings[0] = (char *) malloc(sizeof(char) * (strlen("TEST1") + 1)); // +1 for the \n
test_strings[1] = (char *) malloc(sizeof(char) * (strlen("TEST2") + 1)); // +1 for the \n
strcpy(test_strings[0], "TEST1");
strcpy(test_strings[1], "TEST2");

Since you don't want to include string.h it seems that you need to implement the function strcpy. Also you need to include stdlib.h (because of malloc).

Answer 2

You have to include "stdio.h" for NULL macro and printf. The runtime problem is because you have an array of 1 element and you are assigning "TEST2" into the non-existing second position. I don't understand what you want to do in toAlpha().

#include <stdio.h>

// ...

int main()
{
    char *test_strings[2];                      /* strings to test */
    char *pStr = NULL;                          /* pointer to string */

    test_strings[0] = "TEST1";
    test_strings[1] = "TEST2";

    for (int i = 0; i < 2; ++i)
    {
        pStr = test_strings[i];
        toAlpha(pStr);
        printf("%s\n", pStr);
    }

    return 0;
}

// ...

Answer 3

Okay. Rewrote both programs. Hopefully better this time. But used string.h library in lieu of writing my own strcpy function. Though I didn't describe toAlpha properly. It's meant to remove any non-alphabetic characters, and to return the result as all lower-case letters. --Topsail

First program (single string):

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

    void toAlpha(char*);
    int str_len(char*);

    int main()
    {
        char test_string[100];              /* string to test */
        char *pStr = NULL;                  /* pointer to string */

            strcpy(test_string, "Test-A");
            pStr = test_string;
            toAlpha(pStr);
            printf("%s\n", pStr);


        return 0;
    }


    void toAlpha(char *arg)
    {
        int i = 0;                  /* counter - original string*/
        int j = 0;                  /* counter - temp string */

        /* check each character in original and save alphabetic characters only */
        for ( i = 0; i < str_len(arg); i++ )
        {
            if( *(arg + i) >= 'a' && *(arg + i) <= 'z' )
                *(arg + j++) = *(arg + i);
            else
                if ( *(arg + i) >= 'A' && *(arg + i) <= 'Z' )
                    *(arg + j++) = *(arg + i) - 'A' + 'a';
        }

        /* add a null character terminator */
        *(arg + j) = '\0';

    }


    int str_len(char *arg)
    {
        /*return count of letters in a C string */
        int i = 0;
        if ( arg != NULL )
            while ( arg[i] != '\0' )
                i++;
        return i;
    }

second program with array of strings:

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

    void toAlpha(char*);
    int str_len(char*);
    void palindrome(char*);

    int main()
    {
        char *test_strings[2];                      /* strings to test */
        char *pStr = NULL;                          /* pointer to string */
        int i = 0;                                  /* loop counter */

        test_strings[0] = (char *) malloc(sizeof(char) * (strlen("TEST-A") + 1));
        strcpy(test_strings[0], "TEST-A");
        test_strings[1] = (char *) malloc(sizeof(char) * (strlen("TEST-1") + 1));
        strcpy(test_strings[1], "TEST-B");


        for (i = 0; i < 2; i++){
            pStr = test_strings[i];
            toAlpha(pStr);
            printf("%s\n", pStr);
            free(pStr);
        }

        return 0;
    }


    void toAlpha(char *arg)
    {
        int i = 0;                  /* counter - original string*/
        int j = 0;                  /* counter - temp string */

        /* check each character in original and save alphabetic characters only */
        for ( i = 0; i < str_len(arg); i++ )
        {
            if( *(arg + i) >= 'a' && *(arg + i) <= 'z' )
                *(arg + j++) = *(arg + i);
            else
                if ( *(arg + i) >= 'A' && *(arg + i) <= 'Z' )
                    *(arg + j++) = *(arg + i) - 'A' + 'a';
        }

        /* add a null character terminator */
        *(arg + j) = '\0';

    }


    int str_len(char *arg)
    {
        /*return count of letters in a C string */
        int i = 0;
        if ( arg != NULL )
            while ( arg[i] != '\0' )
                i++;
        return i;
    }