pass 2d array to function

[bharat64]

How to Pass 2d Array in to function of c++

I have two 2d array of 3 X 3 named a and b in void main function

Passing both to mult function and return back to main function with the product of a and b ...How?

[Meetee]

How to Pass 2d Array in to function of c++

Hi,

You can directly pass it in function like normal parameter.

[CODE=CPP]void functionarray (int arg[][], int length)
{
...function body...
}
[/CODE]

Regards

[weaknessforcats]

void functionarray (int arg[][], int length)
{
...function body...
}

Actually, that won't compile. C++ has only one-dimensional arrays and the number of elements is specified between brackets.

Read this:

First, there are only one-dimensional arrays in C or C++. The number of elements in put between brackets:
[code=c]
int array[5];
[/code]

That is an array of 5 elements each of which is an int.

[code=c]
int array[];
[/code]

won't compile. You need to declare the number of elements.

Second, this array:
[code=c]
int array[5][10];
[/code]

is still an array of 5 elements. Each element is an array of 10 int.

[code=c]
int array[5][10][15];
[/code]

is still an array of 5 elements. Each element is an array of 10 elements where each element is an array of 15 int.


[code=c]
int array[][10];
[/code]

won't compile. You need to declare the number of elements.

Third, the name of an array is the address of element 0
[code=c]
int array[5];
[/code]

Here array is the address of array[0]. Since array[0] is an int, array is the address of an int. You can assign the name array to an int*.

[code=c]
int array[5][10];
[/code]

Here array is the address of array[0]. Since array[0] is an array of 10 int, array is the address of an array of 10 int. You can assign the name array to a pointer to an array of 10 int:
[code=c]
int array[5][10];

int (*ptr)[10] = array;
[/code]

Fourth, when the number of elements is not known at compile time, you create the array dynamically:

[code=c]
int* array = new int[value];
int (*ptr)[10] = new int[value][10];
int (*ptr)[10][15] = new int[value][10][15];
[/code]

In each case value is the number of elements. Any other brackets only describe the elements.

Using an int** for an array of arrays is incorrect and produces wrong answers using pointer arithmetic. The compiler knows this so it won't compile this code:

[code=c]
int** ptr = new int[value][10]; //ERROR
[/code]

new returns the address of an array of 10 int and that isn't the same as an int**.

Likewise:
[code=c]
int*** ptr = new int[value][10][15]; //ERROR
[/code]

new returns the address of an array of 10 elements where each element is an array of 15 int and that isn't the same as an int***.

With the above in mind you call a funcion by passing in the address of element 0.

This array:
[code=cpp]
int array[5];
[/code]
is a passed as:
[code=cpp]
functionarray(a rray, 5);
[/code]
where:
[code=cpp]
void functionarray (int arg* int length)
{
...function body...
}
[/code]

But this array:
[code=cpp]
int array[5][3];
[/code]
is passed as:
[code=cpp]
functionarray(a rray, 5);
[/code]
where
[code=cpp]
void functionarray (int (*arg)[3] int length)
{
...function body...
}
[/code]

[smalpani]

[CODE = c]

void func(int array[][], int row,int col)\\
{
/*this is just a proof*/
}

int main()
{

int a[5][5];
func(a,5,5);
return 0;

}

[/CODE]

This compiles without any problem for me

[Meetee]

Actually, that won't compile. C++ has only one-dimensional arrays and the number of elements is specified between brackets.

Read this:


With the above in mind you call a funcion by passing in the address of element 0.

This array:
[code=cpp]
int array[5];
[/code]
is a passed as:
[code=cpp]
functionarray(a rray, 5);
[/code]
where:
[code=cpp]
void functionarray (int arg*, int length)
{
...function body...
}
[/code]

But this array:
[code=cpp]
int array[5][3];
[/code]
is passed as:
[code=cpp]
functionarray(a rray, 5);
[/code]
where
[code=cpp]
void functionarray (int (*arg)[3] int length)
{
...function body...
}
[/code]

But I tried this code for two dimentions. It works correctly.

[CODE=CPP]
void printarray(int args[][2],int rows,int cols)
{
for(int i=0;i<rows;i++)
{
for(int j=0;j<cols;j++)
cout<<args[i][j]<<endl;
}
}
int main()
{
int firstarray[][2] = {{1,2},{3,4}};
printarray(firs tarray,2,2);
}
[/CODE]

Regards

[smalpani]

Even this will work, i did not understand moderator's post

[CODE=CPP]
void printarray(int args[][],int rows,int cols)
{
for(int i=0;i<rows;i++)
{
for(int j=0;j<cols;j++)
cout<<args[i][j]<<endl;
}
}
int main()
{
int firstarray[][2] = {{1,2},{3,4}};
printarray(firs tarray,2,2);
}
[/CODE]

[weaknessforcats]

Even this will work, i did not understand moderator's post


Code: ( cpp )
void printarray(int args[][],int rows,int cols)
{
for(int i=0;i<rows;i++)
{
for(int j=0;j<cols;j++)
cout<<args[i][j]<<endl;
}
}
int main()
{
int firstarray[][2] = {{1,2},{3,4}};
printarray(firs tarray,2,2);
}

This won't even compile. Here is what Visual Studio.NET had to say:
[code=c]
c:\scratch\inst ructor\question 2007-10-17-01.cpp(18) : error C2087: 'args' : missing subscript
c:\scratch\inst ructor\question 2007-10-17-01.cpp(29) : error C2664: 'printarray' : cannot convert parameter 1 from 'int [2][2]' to 'int [][1]'
Types pointed to are unrelated; conversion requires reinterpret_cas t, C-style cast or function-style cast
[/code]

How can you say this works?

[Meetee]

This won't even compile. Here is what Visual Studio.NET had to say:
[code=c]
c:\scratch\inst ructor\question 2007-10-17-01.cpp(18) : error C2087: 'args' : missing subscript
c:\scratch\inst ructor\question 2007-10-17-01.cpp(29) : error C2664: 'printarray' : cannot convert parameter 1 from 'int [2][2]' to 'int [][1]'
Types pointed to are unrelated; conversion requires reinterpret_cas t, C-style cast or function-style cast
[/code]

How can you say this works?

I tried it on linux. and compiles there!! Can it be a platform dependent? Kindly clearify.

[weaknessforcats]

I tried it on linux. and compiles there!! Can it be a platform dependent? Kindly clearify.

This has nothing to do with Linux.

What compiler are you using??

Are you using the -pedantic switch to disable non ANSI C extensions?

I mean, this is just fundamental array handling in C. It's been documented since 1972.