CODEWITHSUNDEEP
carrayspointers
user3421751
user3421751

Reputation: 209

Accessing arrays in C

A couple questions about 2D arrays in C.

  1. If I create an two dimensional array:

    int c[5][25];

    Then create a pointer to the beginning of the array:

    int *p = c;

    Edit: Changing the second line to:

    int (*p)[25] = c;

    worked. Will this still let me access the array in the following questions?

  2. Would *(c+26) access the array at c[1][1]?

    I know in a one dimensional array like the following:

    double *p;
double balance[10];

p = balance;

    *(balance + 4) is the same as balance[4], but wasn't sure if the memory is assigned "back to back" for a 2D array.

  3. Can you access c[1][1] by doing something like c[2][-24]?

    Would be an odd way to do it, but I couldn't find anything addressing that specific scenario.

  4. Would you be able to access c[1][1] by the statement p[1][1]?

    My guess would be no, since p is a pointer array.

  5. Saw these examples a while ago and wrote them down, but don't remember if they are correct or not:

    1[c[1]];
26[p];

    Would either of those be equivalent to c[1][1]?

I know a lot of this is very basic, but I'm having trouble finding sources that address these specific instances online.

Thanks!

Upvotes: 3

Views: 394

Answers (1)

R Sahu
R Sahu

Reputation: 206717

Q: 1.) Did I declare the second line correctly?

A: No, you did not. The correct way would be:

int (*p)[25] = c;

Q: 2.) Would *(c+26) access the array at c[1][1]?

A: No. *(c+26) is same as c[26]. In your case, it will lead to undefined behavior since the array index, 26 is not valid for your array.

Q: 3.) Can you access c[1][1] by doing something like c[2][-24]?

A: No, you cannot. Use of array indices that are out of bounds will lead to undefined behavior.

EDIT Even though -24 appears to be an out of bound array index, using c[2][-24] appears to be same as using c[1][1]. I retract my first answer.

Q: 4.) Would you be able to access c[1][1] by the statement p[1][1]?

A: Yes, you can if you change the declaration of p to the way I have done it.

Q: 1[c[1]]; and 26[p];

Would either of those be equivalent to c[1][1]?

A 1[c[1]] is equivalent to c[1][1]. 26[p] is equivalent to p[26] but it is not equivalent to c[1][1] regardless of whether p is defined your way or my way.

Upvotes: 2

Related Questions