Array of pointers with addresses of words

Task: Write a function that takes as parameter a string, an array of pointers and its length and fills the array with addresses to the beginning of each (whitespace-separated) word in the string (similar to the argv[] array of main). Terminate the array with a NULL pointer after the last valid address.
I have some questions. 'An array of pointers' = array of addresses (correct me if I'm wrong I'm learning pointers). From what I know an address is a huge number, so how can I store the addresses of the words in the array, without overflowing it?

Code:

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Code:

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#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void wordAdresses(char *s, int *ptr_arr, int cap)
{
    ptr_arr[0] = '\0';
    cap = 1;
    char* word = strtok(s, " ");
    while(word != NULL)
    {
        int address = *word;
        ptr_arr[cap] = *word;
        cap++;
        if(address+1 > cap)
        {
            printf("String overflow\n");
        }
        cap -= address;
        //printf("Word is %s <--> Value is %X\n",word, word);
        word = strtok(NULL, " ");
    }
    for(int i = 0; i < cap; i++)
    {
        printf("%d\n", ptr_arr[i]);
    }
}
int main()
{
    char s[] = "john goes to school";
    int ptr_arr[100];
    wordAdresses(s, ptr_arr, sizeof ptr_arr);
    return 0;
}

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Output:
String overflow
String overflow
String overflow
String overflow

Now if I put the last for between comments and remove them for the second printf

Code:

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#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void wordAdresses(char *s, int *ptr_arr, int cap)
{
    ptr_arr[0] = '\0';
    cap = 1;
    char* word = strtok(s, " ");
    while(word != NULL)
    {
        int address = *word;
        ptr_arr[cap] = *word;
        cap++;
        if(address+1 > cap)
        {
            printf("String overflow\n");
        }
        cap -= address;
        printf("Word is %s <--> Value is %X\n",word, word);
        word = strtok(NULL, " ");
    }
  /* for(int i = 0; i < cap; i++)
    {
        printf("%d\n", ptr_arr[i]);
    }*/
}
int main()
{
    char s[] = "john goes to school";
    int ptr_arr[100];
    wordAdresses(s, ptr_arr, sizeof ptr_arr);
    return 0;
}

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Output:
String overflow
Word is john <--> Value is 61FE00
String overflow
Word is goes <--> Value is 61FE05
String overflow
Word is to <--> Value is 61FE0A
String overflow
Word is school <--> Value is 61FE0D

So I get the adress of the beginning of each word, but also the message "String overflow" because the numbers are huge. So how can I put the addresses into the array and avoid overflowing?

Quote:

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Originally Posted by rmmstn

fills the array with addresses to the beginning of each (whitespace-separated) word in the string

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That implies that the array of pointers is an array of char pointers (i.e., char*[N]), which means that the parameter should be a pointer to a pointer to char (i.e., char**).

Quote:

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Originally Posted by rmmstn

From what I know an address is a huge number, so how can I store the addresses of the words in the array, without overflowing it?

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By using pointers. The value of a pointer is an address.

Is this correct?

Code:

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#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void wordAdresses(char *s, int *ptr_arr, int cap)
{
    ptr_arr[0] = '\0';
    unsigned i, countWords = 0;
    char* word = strtok(s, " ");
    while(word != NULL)
    {
        cap = 1;
        for(i = 0; i < cap; i++)
        {
            *(ptr_arr+i) = word;
            printf("Word %s has address %X\n",word, *(ptr_arr+i));
        }
      // printf("Word is %s <--> Value is %X\n",word, word);
        word = strtok(NULL, " ");
        cap++;
    }
}
int main()
{
    char s[] = "john goes to school";
    int ptr_arr[100];
    wordAdresses(s, ptr_arr, 100);
    return 0;
}

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Output:
Word john has address 61FE00
Word goes has address 61FE05
Word to has address 61FE0A
Word school has address 61FE0D

Now I would also need to watch for array overflow. What would be the condition for cap(cap < ? ), which if met, would overflow the array?

Sorry, I missed you are passing it in; still think that

Code:

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ptr_arr[0] = '\0';

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defaults to an int array as in the K & R days.

NOTE: It no longer does that.

Tim S.

Damn, yeah I treated it as an array of characters by mistake and I wanted to make sure that it is empty and null-terminated in the beginning. What would be the alternative to make sure my array isn't overflowing? Should I subtract from cap the size of a word in bytes to check if it overflows?

Quote:

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Originally Posted by rmmstn

Damn, yeah I treated it as an array of characters by mistake and I wanted to make sure that it is empty and null-terminated in the beginning. What would be the alternative to make sure my array isn't overflowing? Should I subtract from cap the size of a word in bytes to check if it overflows?

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It should be okay; unless you have a real old compiler!
If you got a warning about shadowing you do have that problem.

Tim S.

Quote:

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Originally Posted by rmmstn

Is this correct?

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No, let's take a look at this line:

Code:

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*(ptr_arr+i) = word;

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it would be slightly easier if we transformed it to use array notation:

Code:

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ptr_arr[i] = word;

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ptr_arr[i] is an int, whereas word is a char*. You have a potential loss of information here because sizeof(int) >= sizeof(char*) is not guaranteed. In fact, it is quite possible that sizeof(int) == 4 whereas sizeof(char*) == 8.

What I had in mind was something like this:

Code:

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

void splitIntoWords(char *text, char **words, size_t cap)
{
    size_t i = 0;
    char *word = strtok(text, " ");
    while (word && i < cap - 1)
    {
        words[i++] = word;
        word = strtok(NULL, " ");
    }
    words[i] = NULL;
}

int main(void)
{
    char text[] = "john goes to school";
    char *words[100];
    splitIntoWords(text, words, sizeof(words) / sizeof(words[0]));
    for (size_t i = 0; i < sizeof(words) / sizeof(words[0]) && words[i]; ++i)
    {
        printf("%s @ %p\n", words[i], (void*)words[i]);
    }
    return 0;
}

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Notice that as I mentioned in my previous post, the words parameter is a char** and the words local variable in main is an array of char pointers. This allows you to store the addresses of the words in the array.

Thanks a lot, it's so much clearer now! Just to make sure I understood fully, you wrote i < cap - 1 because we don't take the terminator '\0' into a account, right?