Memory Allocation with 2d Character Array

Question

So I need to scan in a dictionary of words, max length 19 and dynamically allocate memory to set the size of the dictionary array. I am stuck on how to do this.

fscanf(ifp, "%d", &numwords); //Number of words in dictionary

char ** dictionary;


for(i = 0; i < numwords; i++){
    for(j = 0; j < 20; j++){
        dictionary[i][j] = (char *) malloc(20 * sizeof(char));
        fscanf(ifp, "%s", &dictionary[i][j]);
        //printf("%s\n", dictionary[i]); //tests that the letter is read in correctly
    }
}

I am lost on what is wrong. Any help would be greatly appreciated.

This is not a 2D array, but a pointer of pointers. Please read the C FAQ on these things. — Jens Gustedt
– Jens Gustedt, Commented Oct 12, 2012 at 21:44

hmjd · Accepted Answer · 2012-10-12 21:45:54Z

4

You need to allocate memory to hold the list of char*:

dictionary = malloc(sizeof(char*) * numwords);

and when you are allocating the char array:

dictionary[i] = malloc(20); /* No [j] */

Note that sizeof(char) is guaranteed to be 1 so it can be omitted from the malloc() argument. When reading the strings, prevent buffer overrun by specifying the maximum width allowed:

fscanf(ifp, "%19s", dictionary[i]);

There is no requirement for the inner loop. The program needs to read numwords from the file, only the outer for is required.

Check return values from functions (malloc() does not return NULL for example and fscanf() returns the number of expected assignments).

edited Oct 12, 2012 at 21:45

answered Oct 12, 2012 at 21:38

hmjd

122k21 gold badges216 silver badges259 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

user529758 Over a year ago

I'd rather use sizeof(*dictionary). Also, you forgot to point out that it's not dictionary[i][j] that needs to be assigned the result of malloc() but only dictionary[i].

Ryan Over a year ago

So what you are saying is that before the for loops to read in the dictionary I need to put dictionary = malloc(sizeof(char*) * numwords); and that dictionary[i] = malloc(20); goes in the for loops?

Ryan Over a year ago

Ok so even though it has to be a 2D array it will only need that one loop?

hmjd Over a year ago

@Ryan, it isn't really 2d array but a list of pointers, but yes just one loop.

Ryan Over a year ago

The reason I ask is because I need to make a binary search now, requiring a 2d array to access specific letters of the words in the dictionary. Sorry to be such a pain on this, just new with memory allocation

Collectives™ on Stack Overflow

Memory Allocation with 2d Character Array

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related