Copying structure elements and 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 a simple assignment. e1 = e2;
But this simple assignment doesn't work when copying array elements. Can anyone suggest an explanation?
Thank.
source to share
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 does not support structured arguments and structured return values. Then perhaps imagine Dennis realized that any return value that didn’t fit into case was too much work for the compiler at this point."
source to share
An array is not a mutable lvalue ( "what is the case (in memory)" ). This means that although it is an lvalue, it cannot be the left operand of an assignment operator =
.
In the case of a structure other than an assignment, * C does not perform operations on entire structures. You can not use the operators ==
, !=
to check whether the two structures are equal or not.
You can create dummy structures to add arrays to be copied later:
struct
{
int arr[5];
} arr1, arr2;
The latter you can assign
arr1 = arr2;
* The operator =
can only be used with compatible structure types.
source to share
The array name is an alias for 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 Why is the address of an array equal to its value in C?
So when you wrote arr1 = arr2
, the compiler thinks that you are just referencing the address of the array, not the whole array. When you write the name of a structure, the compiler knows that you are referring to the structure as a whole, not just the first member.
source to share