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The equivalence of pointers and arrays in C

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• Notable observation: a pointer variable look exactly like an array name
  • If you look carefully at the last program where we used the [ ] operator to access array elements:

    #include <stdio.h> int main(int argc, char *argv[]) { int a[10] = { 11, 22, 33, 44, 55, 66, 77, 88, 99, 777 }; int* p; p = &a[0]; // p points to variable a[0] printf("p[0] = %d, a[0] = %d\n", p[0], a[0] ); printf("p[1] = %d, a[1] = %d\n", p[1], a[1] ); printf("p[2] = %d, a[2] = %d\n", p[2], a[2] ); printf("p[3] = %d, a[3] = %d\n", p[3], a[3] ); }

    You will notice that:

    The expression used to access the array element a[i] is p[i] (for any value i = 0, 1, 2, 3....)

    So:

    There is a lot of similarity between An array name (a)           and        A pointer variable (p)

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  • The C community calls this property in the C programming language:

    The ``equivalence of arrays and pointers in C.'' When we speak of this ``equivalence'' we do not mean that arrays and pointers are the same thing (they are in fact quite different), but rather that they can be used in related ways, and they share many operations between them. Quote above taken from: click here (and modified a bit by me)

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• Why does a pointer variable behave like an array name in C ?
  • The answer is very simple:

    The name of an array in C ≡ the address of the array element 0 in the array: int a[10]; Then: a ≡ &(a[0])

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  • So if we make p point to the base of the array:

    p = &(a[0]); // Or: p = a

    Then

    p[i] and a[i] will both access the same array element !!! (and will behave very similarly in many operations)

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• Note: a pointer variable is not the same as an array name
  • Facts:

    An array name is a constant (= a fixed location in memory A pointer variable can point to any where in the memory It's more flexible !!!

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  • Example: we can access a "shifted" array using pointer variables:

    #include <stdio.h> int main(int argc, char *argv[]) { int a[10] = { 11, 22, 33, 44, 55, 66, 77, 88, 99, 777 }; int* p; p = &a[1]; // p points to variable a[1] !!! printf("p[0] = %d, a[1] = %d\n", p[0], a[1] ); printf("p[1] = %d, a[2] = %d\n", p[1], a[2] ); printf("p[2] = %d, a[3] = %d\n", p[2], a[3] ); printf("p[3] = %d, a[4] = %d\n", p[3], a[4] ); }

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set2/pointer-arithm2b.c

    How to run the program:

    To compile:   gcc pointer-arithm2b.c          To run: ./a.out