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Tip of the Day
Home » Tip Bank » C++
Language: C++
Expertise: Beginner
May 6, 1997

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Two-dimensional array

Question:
How can I dynamically declare a 2D array and pass it as a parameter to a function? Here's an example of what I'm trying to do:
#include 
#include 
#include 

const SIZE = 50;

void display (float myArray[][])
{
    for (int i = 0; i < 50; i++)
	for (int j = 0; j< 50; j++)
	    cout << myArray[i][j] << ' ';
}

void main()
{
    float *myArray;

    myArray = new float[SIZE][SIZE];

    for (int i = 0; i < 50; i++)
	for (int j = 0; j < 50; j++)
	    myArray[i][j] = 9.9;

    display(myArray);
    
}
Here are the error codes I get:
cd /a/home/ce/ugrad/stinson/cs/354/temp/
g++ -g array.cc -o array -lm
array.cc:8: pointer or reference to array of unknown bound in parm type
array.cc: In function `void display(float (*)[])':
array.cc:11: invalid use of array with unspecified bounds
array.cc: In function `int main(...)':
array.cc:18: assignment to `float *' from `float (*)[50]'
array.cc:22: invalid types `float[int]' for array subscript
array.cc:24: passing `float *' as argument 1 of `display(float (*)[])'

Compilation exited abnormally with code 1 at Thu Apr 17 10:13:41

Answer:
When using the array syntax, the compiler must always know the size of the array that is being used; hence the error. The way to solve it is to define the display method as

void display (float myArray[][SIZE]);
or
void display (float **myArray);
Also, remember the display function must somehow be able to know how many items were actually allocated in both cases.

In main() you are also assigining the return value from new float [SIZE][SIZE] to a float * this is an error you should make the type of myArray to float ** to correct this error.

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