dynamic array of static sized strings

Posted on 2002-05-31
Last Modified: 2010-04-15
I want to create a dynamic array of strings, each string of size 100.  I know I can use char **ppMyArray, allocate the number I want, and then allocate each pointer to size 100.  My question: Is there a way to create the array without the need to loop through and allocate each pointer to size 100?
Question by:marvinm
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LVL 16

Accepted Solution

imladris earned 200 total points
ID: 7047118
Sure. If you declare the array:

char (* dynarray)[100];

Then you can allocate as many elements as you want with:


Author Comment

ID: 7047144
how can I do the allocation to avoid
warning: assignment from incompatible pointer type?

Expert Comment

ID: 7047153
Can you put the string in a structure and dynamically create them?

typedef struct tagSTRING_THING
     char     strData[100];
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Author Comment

ID: 7047154
a = (char (*)[100])malloc() seems to work
If I declare a char **, then I need to malloc num_elements * sizeof (char *), and then for each one num_elements*100
do I need to allocate num_elements * ((sizeof(char *)+100)?
Thank You
(this is the last part of my question)
LVL 16

Expert Comment

ID: 7047487
Almost. The declaration:

char (*a)[100];

is a pointer to an array of 100 characters. So what you are allocating is multiples of arrays of 100 characters. So the malloc would be:

a=(char (*)[100])malloc(numelements*sizeof(char)*100);

For calloc it would be:

a=(char (*)[100])calloc(numelements,sizeof(char)*100);

This is, for completeness, in contrast to:

char **a;

With this declaration a is a pointer to a pointer. Semantically you wind up treating them the same. But the memory layout is different. In the first case you have a single pointer to a block of memory that represents some number of 100 byte arrays. In the second case you have a pointer to an array (of some size) of pointers. Each of the elements in the "second" array then points to individual arrays of 100 bytes. So in the first case a location is found with a pointer dereference, followed by a 2 calculations (find the block, then calculate where the desired array is, then calculate the position of the element). In the second case there are two dereferences followed and two calculation (find the second array, calculate which pointer you want, dereference that, then calculate where the element is).
LVL 16

Expert Comment

ID: 7047494
P.S. note that the difference in the memory layout at the root of why they have to be malloced differently. In the first case there is a single block of memory, so a single malloc or calloc can fill the requirements.

But in the second case there are separate memory blocks for each array, plus one for the pointer array. Therefore you have to do all the mallocs or callocs separately.

Author Comment

ID: 7047511
Thank You

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