Cannot assign an array to another

Question

I tried through different ways to copy an array pointer to another one, without any success. Here are my attempts, with the associated error message.

typedef long int coordinate;
typedef coordinate coordinates[3];

void test(coordinates coord) {
    coordinates coord2 = coord; // error: invalid initializer
    coordinates coord3;
    coord3 = coord; // error: incompatible types when assigning to type ‘coordinates’ from type ‘long int *’
    coord3 = (coordinates) coord; // error: cast specifies array type
    coord3 = (coordinate[]) coord; // error: cast specifies array type
    coord3 = (long int*) coord; // error: incompatible types when assigning to type ‘coordinates’ from type ‘long int *’
}

I know I could use typedef coordinate* coordinates; instead, but it does not look very explicit to me.

ouah · Accepted Answer · 2012-06-02 13:10:40Z

5

You cannot assign arrays in C. Use memcpy to copy an array into another.

coordinates coord2;

memcpy(coord2, coord, sizeof coord2);

answered Jun 2, 2012 at 13:10

ouah

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2 Comments

Valentin Lorentz Over a year ago

At first, I was looking for a way to keep the same data, but only copy the pointer. However, it now looks clearly better to copy data. Thank you for the tip and the code.

ouah Over a year ago

@valentinLorentz of course if you don't need to copy the data, working with pointers would be more efficient.

Community · Accepted Answer · 2017-05-23 12:04:53Z

1

When arrays are passed by value, they decay to pointers. The common trick to work around that is wrapping your fixed-size array in a struct, like this:

struct X {
    int val[5];
};

struct X a = {{1,2,3,4,5}};
struct X b;
b = a;
for(i=0;i!=5;i++)
    printf("%d\n",b.val[i]);

Now you can pass your wrapped arrays by value to functions, assign them, and so on.

edited May 23, 2017 at 12:04

CommunityBot

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answered Jun 2, 2012 at 13:22

Sergey Kalinichenko

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Comments

Rishi Diwan · Accepted Answer · 2012-06-02 13:11:50Z

0

Your coordinates need to be initialized as pointers

coordinates *coords2, *coords3;

Try that out and then assign.

answered Jun 2, 2012 at 13:11

Rishi Diwan

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2 Comments

robert Over a year ago

But that won't actually copy the data.

Rishi Diwan Over a year ago

true, it would be a pointer assigned to the same array @ouah has the better answer on this one

Adrian Mole · Accepted Answer · 2020-03-03 14:29:53Z

0

You cannot assign one array to another using the assignment operator, e.g. a = b, because an array doesn't know the size of itself.

This is the reason why we cannot assign one array to another using the assignment operator.

edited Mar 3, 2020 at 14:29

Adrian Mole

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answered Mar 3, 2020 at 14:00

Raman Sharma

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