Comparing Pointer types in C

[ladesidude]

Hi,

I have this program in C, and have commented each line as per my understanding. At the end, I have some questions which I havent been able to understand. Any help would be appreciated.

#include <stdio.h>
 
 
#define Size1 (4)
#define Size2 (3)
#define Size3 (5)
 
int main() {
  int* a1 = malloc(4 * sizeof(int)); /*16 bytes*/
  int* a2 = malloc(3 * sizeof(int)); /*12 bytes*/
  int* a3 = malloc(5 * sizeof(int)); /*20 bytes*/
  a1[0] = 1; a1[1] = 2; a1[2] = 3; a1[3] = 4; /*filling array a1*/
  a2[0] = 9; a2[1] = 8; a2[3] = 7; /*filling array a2*/
  a3[0] = -1; a3[1] = -2; a3[2] = -3; a3[3] = -4; a3[4] = -5; /*filling array a3*/
 
  int sizes[ ] = {Size1, Size2, Size3, 0}; /*local array of sizes*/
 
  int** ptr2ptr = malloc(sizeof(int*) * 4); /*array of 16 bytes*/
  ptr2ptr[0] = a1; /*storing the pointer to a1 in first element*/
  ptr2ptr[1] = a2; /*storing the pointer to a2 in second element*/
  ptr2ptr[2] = a3; */storing the pointer to 03 in second element*/
  ptr2ptr[3] = 0;  /*this is the null pointer for termination*/
 
  int i = 0;
  /*here we are looping until we dont reach the null terminator which ptr2ptr[3]*/
  while (ptr2ptr[i]) {
    int j;
    for (j = 0; j < sizes[i]; j++) /* for each loop print out the contents of the pointer and increment i */
      printf("%i ", ptr2ptr[i][j]);
    printf("\n");
    i++; /*increment i*/
  }
  return 0;
 

Questions:

/*
Q1: Are a1, a2, and a3 of the same type?

Ans. I compared a1 and a2 using == it said not equal. Why? Is there a way I can compare these pointers?

Q2.Are a1 and ptr2ptr of the same type?

Ans: I tried to compare them and got the following: "warning: comparison of distinct pointer types lack a cast". Why?

Q3. If a1, a2, and a3 are considered to be arrays, are they of the same size?

Ans: I ran a sizeof() on them, but I think that is not right as here by size we mean "Length", so how do I do this?

Q4. If ptr2ptr is considered to be an array, is it the same size as any of a1, a2, or a3?

Ans: I guess if I know Q3 I will know Q4.


*/

Thanks for the assistance.

[Banfa]

TBH this looks very much like homework anyway here are my answers.

Q1. The question is not do they have the same value but are they the same type, so for example can you answer this question

int a;
int b;
 
a = 7;
b = 4;
 

Are a and b the same type? If so the answer to question 1 should be obvious.

Q2. See my answer to Q1, it is talking of type not value. Answer the question and the reason you got the warning will become apparent.

Q3. Since they are actually pointers using sizeof will not help. You need to find another method.

Q4. Your guess is correct.


In general you seem to have tried to get the answer to your questions by executing code. However none of the questions can be answered in this manor. They must all be answered by examining the code and deducing the answers.