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.txt" );
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~
3 4203
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.
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?
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:
That is an array of 5 elements each of which is an int.
won't compile. You need to declare the number of elements.
Second, this array:
is still an array of 5 elements. Each element is an array of 10 int.
is still an array of 5 elements. Each element is an array of 10 elements where each element is an array of 15 int.
won't compile. You need to declare the number of elements.
Third, the name of an array is the address of element 0
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*.
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: -
int array[5][10];
-
-
int (*ptr)[10] = array;
-
Fourth, when the number of elements is not known at compile time, you create the array dynamically: -
int* array = new int[value];
-
int (*ptr)[10] = new int[value][10];
-
int (*ptr)[10][15] = new int[value][10][15];
-
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: -
int** ptr = new int[value][10]; //ERROR
-
new returns the address of an array of 10 int and that isn't the same as an int**.
Likewise: -
int*** ptr = new int[value][10][15]; //ERROR
-
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: -
int array[10] = {0,1,2,3,4,5,6,7,8,9};
-
has a memory layout of
0 1 2 3 4 5 6 7 8 9
Wheras this array: -
int array[5][2] = {0,1,2,3,4,5,6,7,8,9};
-
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.
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