Initializing struct containing arrays

Question

I have a struct in C, which members are float arrays. I want to initialize it during compile time like this:

typedef struct curve {                                                                                         
    float *xs;                                                                                             
    float *ys;                                                                                             
    int    n;                                                                                              
} curve;                                                                                                       

curve mycurve1 = {                                                                                             
    {1, 2, 3},                                                                                                     
    {4, 2, 9},                                                                                                     
    3                                                                                                              
};

curve mycurve2 = {
    {1, 2, 3, 4},
    {0, 0.3, 0.9, 1.5},
    4
};                                                                                                              

But I get compile errors.

One possible solution might be to use arrays and not pointers in the struct. This is the accepted answer of a very similar question from here: https://stackoverflow.com/a/17250527/1291302, but the problem with that approach is that I don't know the array size at typedef time. Not only that, I might want to initialize another, bigger curve.

Another approach might be with malloc, but I find that overkill, because I know the array size at compile time and I don't need it to change during run-time.

I don't know another approaches, which might be useful. Maybe casting array to pointer?? - I don't really know how I would approach that.

Answer 1

You may not initialize a scalar object like a pointer with a braced list that contains several initializers.

But you can use compound literals.

Here is a demonstrative program.

#include <stdio.h>

typedef struct curve {                                                                                         
    float *xs;                                                                                             
    float *ys;                                                                                             
    int    n;                                                                                              
} curve;                                                                                                       

int main(void) 
{
    curve mycurve1 = 
    {                                                                                             
        ( float[] ){ 1,  2, 3 },                                                                                                     
        ( float[] ){ 4,  2, 9 },                                                                                                     
        3
    };

    curve mycurve2 = 
    {
        ( float[] ){ 1, 2, 3, 4 },
        ( float[] ){ 0, 0.3, 0.9, 1.5 },
        4
    };

    for ( int i = 0; i < mycurve1.n; i++ )
    {
        printf( "%.1f ", mycurve1.xs[i] );
    }
    putchar( '\n' );

    for ( int i = 0; i < mycurve2.n; i++ )
    {
        printf( "%.1f ", mycurve2.ys[i] );
    }
    putchar( '\n' );

    return 0;
}

Its output is

1.0 2.0 3.0 
0.0 0.3 0.9 1.5 

Answer 2

A suggested take on @Vlad from Moscow good answer.

Use const when constant

because I know the array size at compile time and I don't need it to change during run-time.

Consider const curve mycurve1 = .... This allows for select optimizations, identifies mis-use and allows passing &mycurve1 to bar(const curve *). Also with const float [... allows passing mycurve1.xs to foo(const float *).

Avoid magic numbers

#define CURVE1_N 3
const curve mycurve1 = {
  ( const float[CURVE1_N] ){ 1,  2, 3 },
  ( const float[CURVE1_N] ){ 4,  2, 9 },  
  CURVE1_N
};