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carrayscopystructure
sheetal_158
sheetal_158

Reputation: 7951

Copying structure elements vs copying array elements in C

As described in the answer to this question Copying one structure to another, We can copy the contents of a structure element to another by simple assignment. e1=e2;

But this simple assignment does not work when copying array elements. Can someone offer an explanation?

Thanks.

Upvotes: 3

Views: 1165

Answers (3)

schrödinbug
schrödinbug

Reputation: 863

The name of an array is an alias of the address of the first element of the array*:

#include <stdio.h>

int main()
{
    int foo[5] = {2, 3, 4, 5, 6};
    int *bar = foo; 
    // now bar == foo, i.e. bar == &foo[0], so changing bar[2] changes foo[2]
    // bar[2] works because of pointer arithmetic

    printf("    foo is: %p\n",foo);     // prints the address of the first element of foo
    printf("&foo[0] is: %p\n", &foo[0]); // also prints the address of the first element of foo
    printf("    bar is: %p\n", bar);
    printf("&bar[0] is: %p\n", &bar[0]);
    printf(" foo[2] is: %d\n", foo[3]);
    printf(" bar[2] is: %d\n", bar[3]);

    return 0;

}

*with some exceptions. Namely sizeof foo != &foo[0]. See How come an array's address is equal to its value in C?

So when you wrote arr1 = arr2 the compiler thinks you are just referring to the address of the array not the whole array. When you write the name of a struct, the compiler knows you are referring to the struct as a whole, not just the first member.

Upvotes: 0

haccks
haccks

Reputation: 106022

Array is not a modifiable lvalue ("something that has a location (in memory)"). This means that although it is a lvalue, it can't be a left operand of assignment operator =.

In case of structure, other than assignment*, C provides no operations on entire structures. One can't use ==, != operators to test whether two structures are equal or not.

You can create dummy structures to enclose arrays that will be copied later: 

struct
{
    int arr[5];
} arr1, arr2;

Latter you can assign 

arr1 = arr2;

*The = operator can be used only with compatible structure types.

Upvotes: 1

ouah
ouah

Reputation: 145839

Arrays are second-class citizens in C: you cannot assign an array to an array and you cannot return an array from a function.

Chris Torek offers this explanation in comp.lang.c:

"Note that V6 C also did not support struct-valued arguments and struct-valued return values. One might, then, imagine that Dennis figured that any return value that did not fit in a register was too much work to put into the compiler at that point."

Upvotes: 1

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