hello, can somebody explain why do I receive this error, when I do:
int boardAux[length][length] = {{0}};
thanks in advance
views:
50answers:
3
A:
You cannot do it. C compiler cannot do such a complex thing on stack.
You have to use heap and dynamic allocation.
What you really need to do:
- compute size (n*m*sizeof(element)) of the memory you need
- call malloc(size) to allocate the memory
- create an accessor: int* access(ptr,x,y,rowSize) { return ptr + y*rowSize + x; }
Use *access(boardAux, x, y, size) = 42 to interact with the matrix.
Pavel Radzivilovsky
2010-06-21 07:55:41
one more question, why do I receive an error invalid use of array with unspecified bounds?printf("%d", board[i][j]);
helloWorld
2010-06-21 08:00:51
-1 C99 allows for dynamic allocation in the stack as the user code (excluding the initialization). There is no need to perform dynamic allocations.
David Rodríguez - dribeas
2010-06-21 08:02:15
@helloWorld because array dimensions have to be known.
Pavel Radzivilovsky
2010-06-21 08:55:39
+3
A:
I am assuming that you are using a C99 compiler (with support for dynamically sized arrays). The problem in your code is that at the time when the compilers sees your variable declaration it cannot know how many elements there are in the array (I am also assuming here, from the compiler error that length is not a compile time constant).
You must manually initialize that array:
int boardAux[length][length];
memset( boardAux, 0, length*length*sizeof(int) );
David Rodríguez - dribeas
2010-06-21 08:06:17
@David Rodríguez - dribeas: I can use for this purpose malloc as well, what about the second question, I wrote it after Pavel's reply
helloWorld
2010-06-21 08:10:28
@helloWorld: With stack allocated arrays, `printf( "%d", boardAux[1][2] )` compiles fine. The compiler knows the sizes and knows in what position in memory the (1,2)-th element is. If you use dynamic allocation the array is uni-dimensional and you must perform the math yourself: `printf("%d", boardAux[ 1*length + 2 ])`
David Rodríguez - dribeas
2010-06-21 08:39:12
@AndreyT: Thanks for pointing the error in the `memset` call out. I have just corrected it.
David Rodríguez - dribeas
2010-06-21 08:39:45
+2
A:
You receive this error because in C language you are not allowed to use initializers with variable length arrays. The error message you are getting basically says it all.
6.7.8 Initialization
...
3 The type of the entity to be initialized shall be an array of unknown size or an object type that is not a variable length array type.
AndreyT
2010-06-21 08:09:30
@helloWorld: This is from the language standard (C99). You can get a "working" copy with TC3 updates here http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf
AndreyT
2010-06-21 08:13:10
There are subjects for which some will always disbelieve you if you only provide the informal explanation. Variable length arrays are one of these topics. +1 for quoting the standard.
Pascal Cuoq
2010-06-21 08:30:25