Thread: Using Dynamic Memory allocation for arrays

How am I supposed to use dynamic memory allocations for arrays?

For example here is the following array in which i read individual words from a .txt file and save them word by word in the array:

Code:

char words[1000][15];

Here 1000 defines the number of words the array can save and each word may comprise of not more than 15 characters.

Now I want that that program should dynamically allocate the memory for the number of words it counts. For example, a .txt file may contain words greater that 1000. Now I want that the program should count the number of words and allocate the memory accordingly.

Since we cannot use a variable in place of [1000], I am completely blank at how to implement my logic. Please help me in this regard.

How can I apply malloc to multidimensional arrays ??

Code:

words[1000][15];

I want to dynamically allocate this [1000] part and the [15] part may remain constant.
So that the program is not limited to 1000 words only. I hope you got wat I am trying to ask.

I don't know what typedef is and I have studied malloc a bit.....So it wud be very kind of u if u wud elaborate....Thanks in advance..

Originally Posted by mrafay

How can I apply malloc to multidimensional arrays ??

Code:

words[1000][15];

I want to dynamically allocate this [1000] part and the [15] part may remain constant.
So that the program is not limited to 1000 words only. I hope you got wat I am trying to ask.

Hmmm. Are you sure you want to make the first dimension dynamic? You will need
some place to store the pointers for the dynamically allocated memory. If you need
this to be all dynamic, maybe a linked list would be a better solution.

Originally Posted by laserlight

Considering that you only need one pointer given the requirements, that's rather easy

With a single pointer, how would he know how much memory to malloc? I seems like
he wants this array to grow dynamically (no bounds on the first dimension).

Originally Posted by kona49er

With a single pointer, how would he know how much memory to malloc? I seems like
he wants this array to grow dynamically (no bounds on the first dimension).

Since the second dimension is fixed at 15 (and more importantly, fixed at compile-time) no further pointers are needed. ptr[0] is at the returned value from malloc. ptr[1] is fifteen bytes further along. ptr[2] is fifteen bytes further along from that, etc. Also: since ptr is of type pointer-to-array, ptr[0] is of type array, meaning further subscripts can be used.

Originally Posted by tabstop

Since the second dimension is fixed at 15 (and more importantly, fixed at compile-time) no further pointers are needed. ptr[0] is at the returned value from malloc. ptr[1] is fifteen bytes further along. ptr[2] is fifteen bytes further along from that, etc. Also: since ptr is of type pointer-to-array, ptr[0] is of type array, meaning further subscripts can be used.

Sure but ... I'm still wondering how the guy would know how much memory to malloc.
He does not know how many words he will be reading.

Originally Posted by kona49er

Sure but ... I'm still wondering how the guy would know how much memory to malloc.
He does not know how many words he will be reading.

Code:

number_of_words = 0;
while (1==fscanf(infile, "%s", &scratch)) number_of_words++;

You won't know until you read them in. (Alternatively, you could start with 10, then if you need more get 20, then get 40, then get 80, etc. using realloc.)

Originally Posted by tabstop

Code:

number_of_words = 0;
while (1==fscanf(infile, "%s", &scratch)) number_of_words++;

You won't know until you read them in. (Alternatively, you could start with 10, then if you need more get 20, then get 40, then get 80, etc. using realloc.)

Yes, very good suggestions. Thanks.

Isn't that a bit of a slow way? You read the entire file, then allocate an array big enough for the file, and THEN read the file into the array?

That basically means reading over this thing 3 times. That is some pretty bad time complexity.

Originally Posted by \007

Isn't that a bit of a slow way? You read the entire file, then allocate an array big enough for the file, and THEN read the file into the array?

That basically means reading over this thing 3 times. That is some pretty bad time complexity.

I would think twice. (And while it is inefficient, the time complexity would naturally be the same O(n) as nearly anything else, assuming n is the number of lines. The realloc method I gave is also O(n).)

It would be nice if the first line of the file said how many other lines there were, but that doesn't appear to be the case.