using functions in c (return value)

Question

Learning C and having many doubts.

I have a function (lets say function 1) that calls another function (lets say function 2). Function 2 calculates an array of string.

How can I use this array in function 1?

Some code example:

  int find_errors(char* word)
    {
        char error[100];

        /*Given the word, It will find the duplicate chars and store it in the
        error array. */



       return 0;
    }
  int find_word(char* word)
    {

        find_errors (word);

        printf("%s\n", error);

        return 0;
    }

I think you've deleted important parts of the code (like the part that uses error in find_errors). — John
– John, Commented Apr 5, 2011 at 17:52
@John - actually, I don't think he understands what those parts should be. — Brian Roach
– Brian Roach, Commented Apr 5, 2011 at 17:56
@John @Brian. I understand what what the function do. I don't know how to send that error array to the find_word so it can use it. — Favolas
– Favolas, Commented Apr 5, 2011 at 18:10
With the way you have updated your code sample, you cannot get error to find_word(). When the call to find_errors() returns, the memory allocated to char error[] will be cleaned up and no longer usable. You need to use malloc() to dynamically allocate error. — Swiss
– Swiss, Commented Apr 5, 2011 at 18:35

wallyk · Accepted Answer · 2011-04-05 17:52:13Z

5

There are at least three possible approaches:

Use a global variable
pass a parameter between them
return a pointer from the function

answered Apr 5, 2011 at 17:52

wallyk

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3 Comments

ninjalj Over a year ago

In any case, ownership of the array (i.e. who has to allocate the array and who has to free it), and reentrancy issues should be clearly documented. For different ways to do this, see strdup(3), localtime(3), localtime_r(3).

Favolas Over a year ago

Thaks. I don't want to use a global variable. The other two I don't really understand.

Swiss Over a year ago

@Favolas His answers correspond to mine as follows: 1->3 2->2 3->1. Hopefully you will find the elaboration provided helpful.

Swiss · Accepted Answer · 2011-04-05 18:04:30Z

2

There are multiple ways to do this.

1) Create a dynamic array and return a pointer to the array. This will require you to manually free the memory for the array at a later time.

#define NUM_ELEMS 50

// In find_error():
char* error = malloc(NUM_ELEMS * sizeof(char));
return error;

// In find_word():
char *error = find_errors();
// do stuff
free(error);

2) Pass a pointer to find_errors that it can use as the error array. This will not require you to manually free the memory.

// In find_word():
char error[NUM_ELEMS];
find_error(error);

3) Use a global array. May make it more difficult for other people to understand your code. Has other potential problems as well.

// In global scope:
char error[NUM_ELEMS];

edited Apr 5, 2011 at 18:04

answered Apr 5, 2011 at 17:57

Swiss

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2 Comments

Favolas Over a year ago

Thans for the help. First option gave me segmentation fault. Second did not work. Third don't want to use global variables. Will try to do something else. Many thanks.

Swiss Over a year ago

@ Favolas Did you change your function to return a pointer instead of an int for the first option? Please post some code of what you tried to do.

charley · Accepted Answer · 2011-04-05 17:54:39Z

Your question relates to "call-by-reference" and "call-by-value".

char* getNewValsToSet(void)
{
  char* new_vals = (char*) malloc(sizeof(char[5]));
  new_vals[4] = '\0';
  return new_vals;
}

void setValuesEven(char* vals_to_set)
{
  vals_to_set[0] = 'A';
  vals_to_set[2] = 'C';
}

void setValuesOdd(char* vals_to_set)
{
  vals_to_set[1] = 'B';
  vals_to_set[3] = 'D';
}

int main(void)
{
  char* some_vals_to_set = getNewValsToSet();
  setValsEven(some_vals_to_set);
  setValsOdd(some_vals_to_set);

  // ... now has vals "ABCD"

  free(some_vals_to_set);  //cleanup

  return 0;
}

If you have "doubts" about learning C, IMHO it's one of the best things you can do (no matter the language in which you work) because it will explain exactly how things work "under-the-hood" (which all high-level languages try to hide to some degree).

Thiago Cardoso · Accepted Answer · 2011-04-05 18:14:46Z

0

You need to declare the error array globally and use it just like you did.

EDIT: using global variables isn't the best practice in most of the cases, like this one.

edited Apr 5, 2011 at 18:14

answered Apr 5, 2011 at 17:53

Thiago Cardoso

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1 Comment

Brian Roach Over a year ago

There are very few reasons to use global variables. That isn't one of them :) (It will surely work, but it's not a practice you really want people to think is the right way to solve problems)

Joe · Accepted Answer · 2011-04-05 19:18:40Z

Here is an example of what you are looking for with an awesome console output. It dynamically allocates the array to hold any number errors (duplicate characters in your case) that may occur.

//Only free errors if result is > 0
int find_errors(char* word, char** errors)
{
    int num_errors = 0;
    int word_length = strlen(word);
    int ARRAY_SIZE = MIN(8, word_length);
    char existing[word_length];
    int existing_index = 0;

    *errors = NULL;

    for(int i = 0; i < word_length; i++)
    {
        char character = word[i];

        //Search array
        for (int n = 0; n < word_length; ++n ) {
            if(n >= existing_index)
            {
                existing[n] = character;
                existing_index++;
                break;
            }
            if (existing[n] == character) {
                num_errors++;

                if(!*errors)
                    *errors = (char*)malloc(ARRAY_SIZE * sizeof(char));

                //Check if we need to resize array
                if(num_errors >= ARRAY_SIZE)
                {
                    ARRAY_SIZE *= 2;
                    ARRAY_SIZE = MIN(ARRAY_SIZE, word_length);

                    char *tmp = (char*)malloc(ARRAY_SIZE * sizeof(char));
                    memcpy(tmp, *errors, (unsigned long)ARRAY_SIZE);
                    free(*errors);
                    *errors = tmp;
                }
                //Set the error character
                (*errors)[num_errors - 1] = character;
                break;
            } 
        } 
    }

    return num_errors;
}
int find_word(char* word)
{
    char* errors;
    int errCount = find_errors (word, &errors);
    if(errCount > 0)
    {
        printf("Invalid Characters: ");
        for(int i =0; i < errCount; i++)
        {
            printf("%c ", errors[i]);
        }
        printf("\n");

        free(errors);
    }

    return 0;
}

int main(int argc, char *argv[])
{
    find_word("YWPEIT");
    find_word("Hello World");
    find_word("XxxxXXxXXoooooooOOOOOOOOOOOOOOOooooooooOOOOOOOOOOOOooooooOOO");
}

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