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carrayspointers
ZeroVash
ZeroVash

Reputation: 558

C array of pointers on string

Well I don't work a lot with a pointers but now I get confused. I'm trying create array of pointers at strings (in this example just 2 pointers).

    char str[120];
    char* st[2];
    int i=1;
    while (fgets(str,120,f)) {

    printf("%s",str);
    st[i]=&str;

    i++;
    if (i==3)
        break;
}
for (i=1;i<=2;i++) {
    puts(st[i]);
}

warning: assignment from incompatible pointer type [enabled by default]

so can you help me understand all this situation with pointers on a strings. Please

Upvotes: 0

Views: 79

Answers (1)

Mrunmoy
Mrunmoy

Reputation: 111

Arrays always use 0-based index.

for example: 

int my_arr[] = { 10, 11, 12, 13, 14, 15 };

means that the array my_arr has 6 elements starting from index 0 till 6-1 (5). So, an N element array will have its indices from 0 to N-1. Also, note that my_arr by itself is also a pointer to the array.

define a int pointer:

int *iPtr = my_arr; // note that we do not use &my_arr here. my_arr is already a pointer to the array it defined.

or

int *iPtr;
iPtr= my_arr;

they are the same.

Array and Pointer in Memory

In the figure, the box represent the values and their addresses are written besides the boxes.

When you iterate over the array using variable i as index. You basically do the following indirectly:

i = 0
my_arr[i]

is same as 

my_arr[0]

is same as

*(my_arr + 0)

is same as (substitute the value of box named my_arr)

*(20014000+0)

The * operator gets you the value at the address 0x20014000 in this example which is 10.

The above operation is also same as

iPtr[0] or *(iPtr+0)

Look at the box named iPtr, it also contains the same value 0x20014000.

Now lets define an array of pointers:

int *myptr_arr[3];
int varA = 25;
int varB = 34;
int varC = 67;

myptr_arr[0] = &varA; // note that varA is not a pointer, to get its address use &
myptr_arr[1] = &varB; // note that varB is not a pointer, to get its address use &
myptr_arr[2] = &varC; // note that varC is not a pointer, to get its address use &

enter image description here

once again, when you write myptr_arr[0] it translates to 

*(myptr_arr + 0)

*(20152000 + 0)

which will get you 0x20162000 i.e. the value of box named [0] = the address of varA. Now to get the value at this address, you need to dereference it with * operator.

*myptr_arr[0]

which is same as * ( *(myptr_arr + 0) ) which is same as *(0x20162000) which is the value of box varA

Please see : Example usage of pointers

Upvotes: 2

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