Allocate memory to char *** in C

Question

So, I\'m having trouble in allocating memory for a char *** type variable. My objective is to create a matrix of strings and the code I currently have for memor

Answer 1

As you have said, you were able to successfully create an array of strings using your below code:

char **list;
list = calloc(n, sizeof(char *));
for (j = 0; j < n; j++){
list[j] = calloc(MAX_STR, sizeof(char));
}

Now, you need an array of array of strings, so it should be:

char ***matrix;

matrix = calloc(n, sizeof(char**)); //This creates space for storing the address of 'n' array of strings 
for(z = 0; z < n; z++) { //This loop creates the 'n' array of strings.
    matrix[z] = calloc(n, sizeof(char*));
    for(i = 0; i < n; i++) {
        matrix[z][i] = calloc(MAX_STR, sizeof(char));
    }
}

So, basically, in the second code, you are just creating space for storing 'n' lists. Hope this makes it clear.

Answer 2

Assuming you want to allocate storage for n arrays, each with n strings, each up to MAX_STR long, there are a couple of mistakes in the code

matrix = calloc(n*MAX_STR, sizeof(char**));

should be

matrix = calloc(n, sizeof(char**));

and

for(i = 0; i < MAX_STR; i++) {

should be

for(i = 0; i < n; i++) {

In a little more detail,

matrix = calloc(n*MAX_STR, sizeof(char**));
for(z = 0; z < n; z++) {

seems wrong. You allocate n*MAX_STR elements but only use n of them

matrix[z] = calloc(n, sizeof(char*));
for(i = 0; i < MAX_STR; i++) {

is also questionable and is wrong for n<MAX_STR. (You allocate n elements then write to MAX_STR of them.)

Finally, depending on whether you consider MAX_STR to include space for a null terminator, you may need to change

matrix[z][i] = calloc(MAX_STR, sizeof(char));

to

matrix[z][i] = calloc(MAX_STR+1, 1);

Answer 3

Matrix of String or/ 3D char array:

Suppose you need N matrices, each matrix can store R strings of length MAX_STR-1 then you should allocated memory your loop as follows, like:

char ***matrix;

matrix = calloc(N, sizeof(char**)); 
for(z = 0; z < N; z++) { 
    matrix[z] = calloc(R, sizeof(char*));
    for(i = 0; i < R; i++) {
        matrix[z][i] = calloc(MAX_STR, sizeof(char));
    }
}

Its will create matrix like:

matrix
+-------------------+------------------+-----------------------+
| 0                 | 1                | 2                     |
+-------------------+------------------+-----------------------+        
 |                           |                        |
 ▼                           ▼                        ▼
+--+    +----------+       +--+    +----------+     +--+    +----------+
|0 +---►| MAX_STR  |       |0 +---►| MAX_STR  |     |0 +---►| MAX_STR  |
+--+    +----------+       +--+    +----------+     +--+    +----------+
|1 +---►| MAX_STR  |       |1 +---►| MAX_STR  |     |1 +---►| MAX_STR  |
+--+    +----------+       +--+    +----------+     +--+    +----------+
|2 +---►| MAX_STR  |       |2 +---►| MAX_STR  |*    |2 +---►| MAX_STR  |
+--+    +----------+       +--+    +----------+     +--+    +----------+
|3 +---►| MAX_STR  |       |3 +---►| MAX_STR  |     |3 +---►| MAX_STR  |
+--+    +----------+       +--+    +----------+     +--+    +----------+
|4 +---►| MAX_STR  |       |4 +---►| MAX_STR  |     |4 +---►| MAX_STR  |
+--+    +----------+       +--+    +----------+     +--+    +----------+
|5 +---►| MAX_STR  |       |5 +---►| MAX_STR  |     |5 +---►| MAX_STR  |
+--+    +----------+       +--+    +----------+     +--+    +----------+
|6 +---►| MAX_STR  |       |6 +---►| MAX_STR  |     |6 +---►| MAX_STR  |
+--+    +----------+       +--+    +----------+     +--+    +----------+
|7 +---►| MAX_STR  |       |7 +---►| MAX_STR  |     |7 +---►| MAX_STR  |
+--+    +----------+       +--+    +----------+     +--+    +----------+
 ^        ^ 
 |        |
 |        matrix[z][i]
 matrix[z]

Here N = 3, and
R = 8

Its char 3D array of size matrix[N][R][MAX_STR]

Suppose, if someone wants to printf string I marked * in diagram, that is third string in second array, then he/she need to index like

printf("%s",matrix[1][2]);

_{Although answer is accepted I am updating my answer so onw can find it helpful in future}