pointers to 2D arrays

Question

I have a 2d array like this...

char a[][20]={"Hi","Hello","Bye","Good Morning"};

Now I need a char pointer to refer this array , modify one of the string in the array through pointer and print both modified array and array through pointer...

Modifications to array:

    strcpy(p+1,"Come");//copy string "Come" to array

    a[1][1]='O';//copy char 'O' index [1][1] 

ie.. Expected output:

Array: Hi COme Bye Good Morning

Pointer: Hi COme Bye Good Morning

I have a problem in assigning pointer to this array and printing array through pointer Please help...

Answer 1

If you want to turn your 2D array into something that is an array of pointer to char pointers, you are either going to have to allocate space for your char** on the stack in the local scope, or dynamically. If you go for the local scope approach, then you would need to simply create an array of char* , and then make each member of your array point to each string like so:

char a[][20]={"Hi","Hello","Bye","Good Morning", "" };

int total_strings = 0;
int index = 0;
while(strlen(a[index++])) total_strings++;

char* ptrs[total_strings];

for (int i = 0; i < total_strings; i++)
{
    ptrs[i] = a[i];
}

Now you can use the array of pointers like ptr[i][j] (where j is less than the length of the string at index i ), and if you need to pass it to a function by reference or assign it to another variable, the ptrs array would decay into a pointer, so you could pass ptrs as a char** to a function, or to another char** variable.

The next approach would be dynamic allocation of the pointer array ... that would look like the following:

char a[][20]={"Hi","Hello","Bye","Good Morning", "" };

int total_strings = 0;
int index = 0;
while(strlen(a[index++])) total_strings++;

//dynamic allocation of string pointer array 
char** ptrs = calloc(total_strings, sizeof(char*));

for (int i = 0; i < total_strings; i++)
{
    ptrs[i] = a[i];
}

//...more code
free(ptrs);

BTW, you can't do something as simple as char** ptr = a ... that is because of the way that C expects the memory for a char** to be laid out. In other words a char array[][] is actually a linear array in memory, even though it can be indexed as a two dimensional array. On the other hand a char** ptr is a variable that contains a memory address that is pointing to an array of pointers. So both ptr has to contain an address that points to a proper array of pointers, and every pointer ptr[i] needs to contain an address pointing to an array of char . Doing something as simple as char** ptr = a doesn't create the proper memory layout for that to happen.