I have a 1D array A[3] & a 2D array B[4][3]. I want to assign the array A[3] to one of the rows of array B[4][3]. How to do it correctly?
#include<stdio.h>
void main()
{
int A[3]={1,2,3};
int B[4][3]={0};
int row_select=2;
B[row_select][] = A;
}
But this assignment doesn't work. I don't want to assign element by element using a for loop. I want to do it in one statement.
memcpy could be a good option, although it's very likely it uses a loop internally.
memcpy(B[row_select], A, sizeof(A));
Don't Do This: Use memcpy
There is a way to do the assignment with a single statement, as long as you are willing to do some preliminary setup to render your code illegible. Your can use the fact that structures can (1) be assigned to each other in one step, and (2) contain fixed-size arrays. The compiler will probably run memcpy under the hood anyway, but it's a fun exercise in ridiculousness:
#include<stdio.h>
#define SZ 3 // this is just for convenience
// a pointer to an anonymous structure containing our array
typedef struct {
int x[SZ];
} *pthrowaway;
int main(void)
{
int A[SZ]={1,2,3};
int B[4][SZ]={0};
int row_select=2;
pthrowaway a = (pthrowaway)&A;
pthrowaway b = (pthrowaway)&B[row_select];
*b = *a; // magic
return 0;
}
The variables a and b are unnecessary. You can actually assign the arrays in a single statement:
*(pthrowaway)&B[row_select] = *(pthrowaway)&A;
Here is an IDEOne link showing the C99 version: https://ideone.com/IQ6ttg And here is a regular C one: https://ideone.com/pH1hS2
int[3] and pthrowaway are compatible types. Indeed, there seems to be no way an array and a struct can ever be compatible types.
memcpywould be a good option.memmove()ormemcpy(). The functions probably won't do byte copying, but that's how they're specified. (You can assign arrays indirectly if they're part of a structure; that wouldn't help you in this case, though.)Binto an appropriate structure, or union. Can you clarify?*b = *a;in your examples to assign the structures without getting the assignment errors, but I think you're running into strict aliassing problems instead. It's certainly devious, and not the sort of thing to show to someone asking this question.