Assign multiple values to array in C

问题

Is there any way to do this in a condensed form?

GLfloat coordinates[8];
...
coordinates[0] = 1.0f;
coordinates[1] = 0.0f;
coordinates[2] = 1.0f;
coordinates[3] = 1.0f;
coordinates[4] = 0.0f;
coordinates[5] = 1.0f;
coordinates[6] = 0.0f;
coordinates[7] = 0.0f;
return coordinates;

Something like coordinates = {1.0f, ...};?

回答1:

If you really to assign values (as opposed to initialize), you can do it like this:

 GLfloat coordinates[8]; 
 static const GLfloat coordinates_defaults[8] = {1.0f, 0.0f, 1.0f ....};
 ... 
 memcpy(coordinates, coordinates_defaults, sizeof(coordinates_defaults));

 return coordinates;

回答2:

The old-school way:

GLfloat coordinates[8];
...

GLfloat *p = coordinates;
*p++ = 1.0f; *p++ = 0.0f; *p++ = 1.0f; *p++ = 1.0f;
*p++ = 0.0f; *p++ = 1.0f; *p++ = 0.0f; *p++ = 0.0f;

return coordinates;

回答3:

There's a trick to wrap the array into a struct (which can be initialized after declaration).

ie.

struct foo {
  GLfloat arr[10];
};
...
struct foo foo;
foo = (struct foo) { .arr = {1.0, ... } };

回答4:

Exactly, you nearly got it:

GLfloat coordinates[8] = {1.0f, ..., 0.0f};

回答5:

You can use:

GLfloat coordinates[8] = {1.0f, ..., 0.0f};

but this is a compile-time initialisation - you can't use that method in the current standard to re-initialise (although I think there are ways to do it in the upcoming standard, which may not immediately help you).

The other two ways that spring to mind are to blat the contents if they're fixed:

GLfloat base_coordinates[8] = {1.0f, ..., 0.0f};
GLfloat coordinates[8];
:
memcpy (coordinates, base_coordinates, sizeof (coordinates));

or provide a function that looks like your initialisation code anyway:

void setCoords (float *p0, float p1, ..., float p8) {
    p0[0] = p1; p0[1] = p2; p0[2] = p3; p0[3] = p4;
    p0[4] = p5; p0[5] = p6; p0[6] = p7; p0[7] = p8;
}
:
setCoords (coordinates, 1.0f, ..., 0.0f);

keeping in mind those ellipses (...) are placeholders, not things to literally insert in the code.

回答6:

If you are doing these same assignments a lot in your program and want a shortcut, the most straightforward solution might be to just add a function

static inline void set_coordinates(
        GLfloat coordinates[static 8],
        GLfloat c0, GLfloat c1, GLfloat c2, GLfloat c3,
        GLfloat c4, GLfloat c5, GLfloat c6, GLfloat c7)
{
    coordinates[0] = c0;
    coordinates[1] = c1;
    coordinates[2] = c2;
    coordinates[3] = c3;
    coordinates[4] = c4;
    coordinates[5] = c5;
    coordinates[6] = c6;
    coordinates[7] = c7;
}

and then simply call

GLfloat coordinates[8];
// ...
set_coordinates(coordinates, 1.0f, 0.0f, 1.0f, 1.0f, 0.0f, 1.0f, 0.0f, 0.0f);

回答7:

With code like this:

const int node_ct = 8;
const int expected[node_ct] = { 1, 3, 4, 2, 5, 6, 7, 8 };

And in the configure.ac

AC_PROG_CC_C99

The compiler on my dev box was happy. The compiler on the server complained with:

error: variable-sized object may not be initialized
   const int expected[node_ct] = { 1, 3, 4, 2, 5, 6, 7, 8 };

and

warning: excess elements in array initializer
   const int expected[node_ct] = { 1, 3, 4, 2, 5, 6, 7, 8 };

for each element

It doesn't complain at all about, for example:

int expected[] = { 1, 2, 3, 4, 5 };

however, I decided that I like the check on size.

Rather than fighting, I went with a varargs initializer:

#include <stdarg.h>

void int_array_init(int *a, const int ct, ...) {
  va_list args;
  va_start(args, ct);
  for(int i = 0; i < ct; ++i) {
    a[i] = va_arg(args, int);
  }
  va_end(args);
}

called like,

const int node_ct = 8;
int expected[node_ct];
int_array_init(expected, node_ct, 1, 3, 4, 2, 5, 6, 7, 8);

As such, the varargs support is more robust than the support for the array initializer.

Someone might be able to do something like this in a macro.

Find PR with sample code at https://github.com/wbreeze/davenport/pull/15/files

Regarding https://stackoverflow.com/a/3535455/608359 from @paxdiablo, I liked it; but, felt insecure about having the number of times the initializaion pointer advances synchronized with the number of elements allocated to the array. Worst case, the initializing pointer moves beyond the allocated length. As such, the diff in the PR contains,

  int expected[node_ct];
- int *p = expected;
- *p++ = 1; *p++ = 2; *p++ = 3; *p++ = 4;
+ int_array_init(expected, node_ct, 1, 2, 3, 4);

The int_array_init method will safely assign junk if the number of arguments is fewer than the node_ct. The junk assignment ought to be easier to catch and debug.

回答8:

typedef struct{
  char array[4];
}my_array;

my_array array = { .array = {1,1,1,1} }; // initialisation

void assign(my_array a)
{
  array.array[0] = a.array[0];
  array.array[1] = a.array[1];
  array.array[2] = a.array[2];
  array.array[3] = a.array[3]; 
}

char num = 5;
char ber = 6;

int main(void)
{
  printf("%d\n", array.array[0]);
// ...

  // this works even after initialisation
  assign((my_array){ .array = {num,ber,num,ber} });

  printf("%d\n", array.array[0]);
// ....
  return 0;
}

来源：https://stackoverflow.com/questions/3535410/assign-multiple-values-to-array-in-c

标签

arrays

initialization

declaration