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I am trying to load a tab and or whitespace delimited text file into a two dimensional array. The file looks something like this:

1 -3 4
4 -3 7
8 -1 10

I have access to a piece of code that suggests it is permissible to do something such as the following:

int nums[][] = {
    #include "matrix.txt"
};

However, whenever I try to compile this code I am obtaining the error:

$ gcc hangserver.c 
hangserver.c:10:5: error: array type has incomplete element type
In file included from hangserver.c:11:0:
matrix.txt:1:5: error: expected ‘}’ before numeric constant
$

I know there are less-elegant ways to load this file into an array, however out of pure curiosity I would like to know if it is possible to implement the methodology shown above. Thank you so much for taking the time to answer my question.

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5
  • 1
    int nums[][] isn't valid. The maximum of flexibility you can have is int nums[][<some integer>]. Commented Oct 27, 2013 at 15:12
  • If you're sure not needing to read the file at runtime, and the file's contents is correctly formatted you should be able to do so (don't try to solve an assignment in a clever way) ... Commented Oct 27, 2013 at 15:16
  • The #include directive simply places the referenced file into that source file when it's compiled. You could cut+paste the contents of matrix.txt into hangserver.c and figure the syntax errors out from there. Commented Oct 27, 2013 at 15:33
  • 'I know there are less-elegant ways to load this file into an array' Elaborate on that, if you want to receive a serious answer! Commented Oct 27, 2013 at 17:33
  • I find alk's comment to be true. Commented Oct 27, 2013 at 17:58

3 Answers 3

Reset to default
4

There must be a comma after every number and every row has to be inside a {}:

{ 1, -3, 4 },
{ 4, -3, 7 },
{ 8, -1, 10 }
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1 Comment

Thank you everybody for your responses. I also needed to bound the array as described by alk.
2

There is a conceptual problem in your approach.

If for example you had 

1, 2, 3, 4, 5, 6,

How should the compiler know you want a 3x2 or 2x3 or a 1x6 or a 6x1 array?

So it needs to know the number of columns in advance.

For the example above, this

int matrix [][3] = {
#  include "data.txt"
};

would do, as well a this:

int matrix [][2] = {
#  include "data.txt"
};

and this:

int matrix [][1] = {
#  include "data.txt"
};

and this:

int matrix [][6] = {
#  include "data.txt"
};

Although you get a compiler warning about missing braces, as (for the 1st case) above data.txt really should look like:

{1, 2, 3,},{4, 5, 6,},

(The trailing ,s are optional.)

To fully steer this via files from the outside do:

int matrix[][
#  include "colums.txt"
] = {
#  include "data.txt"
};

Here the content of columns.txt would just be an integer describing the intended number of columns the data from data.txt shall be broken down to.

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4 Comments

Ouch, that doesn't feel right to give such recommendation! Seriously! (though its technically correct of course) ...
@g-makulik: The OP asked for, so let's discuss it.
Let's discuss as soon the OP's wakes up to clarify a bit more about his real intend ;). It might be OK if a solution's input is somehow generated in this way in a bigger system, or just for testing purposes.
@g-makulik: I love automated configuration, builds and testing.
1

The line expands into:

int nums[][] = {
    1 -3 4
    4 -3 7
    8 -1 10
     };

which is not acceptable C and C++ syntax. Try with changing the matrix.txt file into

{1, -3, 4},
{4, -3, 7},
{8, -1, 10}
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