How to initialize multidimentional array in C Programming

Question

I am getting error when i am running this code

int row1=2,col1=2;
int mat1[row1][col1]=
{
 {1,5},
 {4,6}
};

What is wrong with this code??

IDE: CodeBlocks

error: variable-sized object may not be initialized|

• user2371524: The error tells it all.
• Christian Gibbons: The error says it all. You are trying to make a two-dimensional Variable-Length Array and send it an initializer. Since it is a VLA, it's dimensions are unknown at compile-time, so how can you give it an initializer of fixed dimensions?
• Philip Couling: I'm confused why you need a variable sized array with a fixed size initialiser. That doesn't really make sense. Do you want arrays of different sizes at runtime?
• user2371524: I wonder how this gets upvoted? You really can't be any more specific in an error message ...
• Daniel H: @Lundin What does the memcpy buy you, especially if the dimensions might actually need to change?

Answer 1

What you have here is a variable length array. Such an array cannot be initialized. You can only initialize an array if the dimensions are constants (i.e. numeric constants, not variables declared as const):

int mat1[2][2]=
{
 {1,5},
 {4,6}
};

• John Bollinger: Quite so, but I suspect that the OP does not appreciate the distinction between integer literals and integer constants expressed via macros. Absent that understanding, this answer would appear to be over-restrictive.
• user2371524: With a fixed initializer, what do macros for the dimensions buy you except the risk for bugs when changing them?
• Daniel H: @FelixPalmen Not much, especially with multidimensional arrays, but if you want zeros at the end it can make sense.

Answer 2

As per C specs, an array defined like

int mat1[row1][col1]=
{
 {1,5},
 {4,6}
};

is a VLA (Variable Length Array) and cannot be initialized.

Quoting C11, chapter §6.7.6.2/P4,

[...] If the size is an integer constant expression and the element type has a known constant size, the array type is not a variable length array type; otherwise, the array type is a variable length array type.

and chapter §6.7.9

The type of the entity to be initialized shall be an array of unknown size or a complete object type that is not a variable length array type.

You need to use compile time constant expressions as array dimensions to be able to use brace enclosed initializers.

You can use #define MACROs for this, like

#define ROW 2 //compile time constant expression
#define COL 2 //compile time constant expression
int mat1[ROW][COL]=
{
 {1,5},
 {4,6}
};

• user8965395: Baaam. It worked. that means i need to make the variable as final in MACROs.
• Sourav Ghosh: @Enam It has to be compile time contants, that's all.
• user8965395: But i need to initialize the size of array at run time.now how can i do that so???
• Sourav Ghosh: @Enam You need to understand, initialization happens in the compile time, you cannot have a VLA, (which determines the size at run time) to be initialized at compile time. Maybe you need pointers and memory allocator functions for that.

Answer 3

You are trying to initialize a variable-sized object. You could try assigning the values later somewhere else or simply use numbers instead of variables.