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arrayscpointerspointer-arithmetic

indexing array in function from the pointer of some element

Imagine some array

uint8_t var[5] = {1,2,3,4,5};

so var will be pointer to the first element of this array, and

uint 8_t* a=var;
b=a[3]

and

b=var[3]

will give the same result.

But will

a = &var[2];
b = a[1];

and

b=var[3];

be same?

Solution

  • After this assignment

    a = &var[2];

    that is the same as

    a = var + 2;

    due to the implicit conversion of the array designator to a pointer to its first element the pointer a points to the element var[2].

    So a[0] yields var[2] and a[1] yields var[3].

    Pay attention to that the subscript operator a[i] is evaluated like *( a + i ).

    So you have a[1] is equivalent to *( a + 1 ) that is in turn equivalent to *( var + 2 + 1 ) that is to *( var + 3 ).