Is there an alternative to Range Initialization of arrays in C?

Question

I believe it's a simple question: Is there a way to initialize an array with values that are nonzero without using loops?

One way I know is to use Range Initialization to initialize the array with the values I want. For example:

int main()
{
   /* Using Designated Range Initialization*/
   int my_array[10] = {[0 ... 3] = 5, [4 ... 7] = 15, [8 ... 9] = 30};
   /* Using Explicit initialization for each element */
   int other_array[10] = {5, 5, 5, 5, 15, 15, 15, 15, 30, 30};

   return 0;
}

However, this method is just an extension of the GCC compiler, and not part of ISO C. So given this possible non-portability between systems, is there a way to do an array initialization in a similar way? Of course, without using loops.

Also, the method I'm looking for is just beyond the explicit initialization of each element of the array.

Does just listing out all the values suffice: `int my_array[10] = {5,5,5,5,15,15,15,15,15,30,30};`? — kaylum, Aug 13 '21 at 22:27
Thanks for the answer! But this can be a tedious task if it is a very large array. — onlyMinimum, Aug 13 '21 at 22:31
True. But you haven't stated what your exact requirements are other than "an alternative". So if you have other specific requirements then suggest updating your question to say so. — kaylum, Aug 13 '21 at 22:32
Indeed! I will update the question trying to be more restrictive. — onlyMinimum, Aug 13 '21 at 22:37

score 4 · Answer 1 · answered Aug 13 '21 at 22:27

4

The only portable alternative is to list them all explicitly.

int my_array[] = {
    5, 5, 5, 5,
    15, 15, 15, 15,
    30, 30
};

This can of course be cumbersome if the array is large. In that case, you can use a simple script in bash or awk to create the declaration. It can write it to a header file that you merge in with #include.

answered Aug 13 '21 at 22:27

Barmar

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I was not aware of using awk or bash scripts for this purpose. Thank you in advance for the answer. – onlyMinimum Aug 13 '21 at 22:33
@onlyMinimum The idea of using of `awk/bash` to generate a `.h` is a good one. I would have suggested it (also, using `perl/python`). You could even write the generator in C, although a scripting/macro language is usually more convenient. This would have been called macro programming (in the past for full fledged macro processors (e.g. `m4`--another choice)), but the modern term is _metaprogramming_ – Craig Estey Aug 13 '21 at 23:04
@CraigEstey, haven't reached that level of programming knowledge yet, but that's what I'm trying to achieve. – onlyMinimum Aug 13 '21 at 23:11

chux - Reinstate Monica · Accepted Answer · 2021-08-13T22:54:46.550

4

Code needs to explicitly initialize the non-zero array elements.

Could use macro magic if there is a pattern:

#define ONE(x) (x), (x)+1, (x)+2, (x)+3, (x)+4, (x)+5, (x)+6, (x)+7, (x)+8, (x)+9
#define TEN(x) ONE(x),ONE((x)+10),ONE((x)+20),ONE((x)+30),ONE((x)+40), \
    ONE((x)+50),ONE((x)+60),ONE((x)+70),ONE((x)+80),ONE((x)+90)

int main() {
  int count[100] = { TEN(1) };
  printf("%d %d\n", count[0], count[99] );
}

Output

1 100

Example for OP

#define X2(x) (x), (x)
// Note the nested macro
#define X4(x) X2(x), X2(x)
#define X8(x) X4(x), X4(x)
#define X10(x) X8(x), X2(x)
#define X16(x) X8(x), X8(x)
// ....


int main()
{
   int my_array[10] = { X4(5), X4(15), X2(30)};
   printf("%d %d\n", my_array[0], my_array[9] );
   return 0;
}

Output

5 30

edited Aug 13 '21 at 22:54

answered Aug 13 '21 at 22:41

chux - Reinstate Monica

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How would you use this for the example in the question? – Barmar Aug 13 '21 at 22:43
How interesting! It didn't cross my mind to use this way, but I would ask the same question as the user above. – onlyMinimum Aug 13 '21 at 22:45
1

That's a nice way to do it. – Barmar Aug 13 '21 at 22:50
The first set of macro names are not very clear. Maybe they should be named `TEN_FROM()` and `TEN_BY_TEN_FROM()` – Barmar Aug 13 '21 at 22:52
This is an example of creativity. It's a good way to implement it. – onlyMinimum Aug 13 '21 at 22:53
But the second set of macros is nice for repeated groups as in the question. – Barmar Aug 13 '21 at 22:53
@onlyMinimum I suspect one could make macros like `INIT(from, to, value)` as long as `to - from` is not too insane. _Macro magic_ can get carried away though - so best used in moderation. – chux - Reinstate Monica Aug 13 '21 at 22:59
@chux-ReinstateMonica, no doubt, abuse of Macro Magic can lead to polluted code. – onlyMinimum Aug 13 '21 at 23:10

Craig Estey · Answer 3 · 2021-08-14T00:36:00.977

From comments under Barmar's answer ...

@CraigEstey, haven't reached that level of programming knowledge yet, but that's what I'm trying to achieve. – onlyMinimum

I guessed as much--that's why I suggested writing the program in C.

Not to worry ... Metaprogramming is a fancy/sexy term, it can be really quite simple to implement/use.

To get you started or give you the idea, here is a generator sample program (e.g. gen.c):

#include <stdio.h>

void
dorange(int lo,int hi,int val)
{

    for (;  lo <= hi;  ++lo)
        printf("%d, ",val);
}

int
main(void)
{
    printf("int myarray[10] = {\n");

    dorange(0,3,5);
    dorange(4,7,15);
    dorange(8,9,30);

    printf("\n");
    printf("};\n");

    return 0;
}

Here is the output of that program:

int myarray[10] = {
5, 5, 5, 5, 15, 15, 15, 15, 30, 30,
};

So, if you were going to put the output into myarray.h using a Makefile:

all: myprogram

# create the target program [after creating the generated file]
myprogram: myprogram.c myarray.h
    cc -o myprogram myprogram.c

# create the generated file
myarray.h: gen
    ./gen > myarray.h

# create the generator program
gen: gen.c
    cc -o gen gen.c

# clean up files ...
clean:
    rm -f gen myarray.h myprogram

Here is myprogram.c:

#include <stdio.h>
#include "myarray.h"

int
main(void)
{

    for (int idx = 0;  idx < sizeof(myarray) / sizeof(myarray[0]);  ++idx)
        printf(" %d",myarray[idx]);

    printf("\n");

    return 0;
}

Here is the output of myprogram:

 5 5 5 5 15 15 15 15 30 30

I liked the implementation! And even more using the shell redirection operator, which I have a little more experience with. — onlyMinimum, Aug 14 '21 at 00:19

Is there an alternative to Range Initialization of arrays in C?

3 Answers3