a function that store data from file to an array

[almo]

hi guys, I have question about writing a function that will store some data from a file in an array, then when that function is call in main, I can use that array in the main() for calculation. Here is what I have for that function:

void input(double Array[], int n)
{
int t;
//double * Array;
cout << "enter dimension of Array" << endl;
cin >> n;
Array = new double [n];
ifstream inputData ("c:\\test.txt" );
if(inputData.is _open())
{
//inputData.open( "c:\test.tx t" );
for(t=0;t<n;t++ )
{
inputData >> Array[t];
cout << Array[t] << endl;
}
inputData.close ();
}else
{
cout << "unable to open file.";
}
}

now if I put what isnt the void into the main code it will work fine, I am not sure how can I return this function so that when I call out this function in main, it will take the data in test.txt and store in the array I assigned in main.

when I call out this function in main like this:
void input(&Array, n);

complier will have a error saying 'input' illegal use of type void and too many initializers.

I am not sure how to correctly declare this function or call this function, would be nice if someone can help me clear things up =)

thanks~

[Laharl]

Well...first off, do your file I/O in input() if you're going to use it. To pass in an array that needs to be changed for other use, either a) use a vector& since you're working in C++, or b) pass a pointer to the array, eg int*[] or int** due to array decay. In the Howtos section, weaknessforcats has written a very good tutorial on arrays in C/C++ that you should go read before he c/ps it in this thread.

[almo]

thanks Laharl, I gone through the examples and stuff and clean up the code a little bit... so right now I have the file I/O in a separate function and it gets call in the the main function.

I am declaring the input function like this:

void input(double *Array[], int n)
{
int t;
*Array = new double [n];
ifstream inputData ("c:\\test.txt" );
if(inputData.is _open())
{
for(t=0;t<n;t++ )
{
inputData >> *Array[t];
cout << Array[t] << endl;
}
inputData.close ();
}else
{
cout << "unable to open file.";
}
}

but when I call it in main like this:

void input(&Data, n);

the compiler will return error C2182: 'input' : illegal use of type 'void'
and C2078: too many initializers

I am not sure what I am doing wrong in here, should I use double or something else instead of void when I declare "input" ? I tried using double instead of void and return 0 at the end of the function and that C2182 error will be gone, but I still have the too many initializer error

any hint?

[weaknessforcats]

void input(&Data, n);

You are calling the function. The void only appears in the function prototype and function definition.

You should:

input(&Data, n);

In the Howtos section, weaknessforcats has written a very good tutorial on arrays in C/C++ that you should go read before he c/ps it in this thread.

Actually, I have not written an article so I will need to c/ps it in this thread anyway. Here it is:

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 this array:
[code=cpp]
int array[10] = {0,1,2,3,4,5,6, 7,8,9};
[/code]
has a memory layout of

0 1 2 3 4 5 6 7 8 9

Wheras this array:
[code=cpp]
int array[5][2] = {0,1,2,3,4,5,6, 7,8,9};
[/code]
has a memory layout of

0 1 2 3 4 5 6 7 8 9

Kinda the same, right?

So if your disc file contains

0 1 2 3 4 5 6 7 8 9

Does it make a difference wheher you read into a one-dimensional array or a two-dimensional array? No.

Therefore, when you do your read use the address of array[0][0] and read as though you have a
one-dimensional array and the values will be in the correct locations.