Difference between for and while loop for ifstream;

[Silversonic]

Hey, I'm having a litte bit of an issue when counting the amount of array entries when using ifstream to input values in to two double arrays.

#include <iostream>
#include <fstream>
#include <cstdlib>
#include <cmath>
#include <iomanip>

using namespace std;
const int arraymax = 1000;

// prototype function declaration(s)

int main(){

	char infilename[50];
	char outfilename[50];

	// read the input and output filenames

	cout << "Please enter the input filename: " << endl;
	cin >> infilename;
	cout << "Please enter the output filename: " << endl;
	cin >> outfilename;
	
	// declare arrays, open input file and read numbers into arrays
	
	double xarray[arraymax];
	double yarray[arraymax];
	int n = 0;

	ifstream input(infilename);

	if(!input){
		cerr << "Failed to open input file " << infilename << endl;
        exit(1);
        } // error message appears if the input file fails to open, it then quits execution. 

	for(int i = 0; input && i < arraymax; ++i, n++){
		input >> *(xarray + i);
		input >> *(yarray + i);
	}
}

The variable int n is what I use to count the array size. Everytime the for loop inputs values in to my arrays, it adds one to n and then carries out the for loop until input no longer holds true or the local variable i reaches the maximum allowed size of my array (1000).

The problem I'm having, is that my arraysize n seems to overcount by one. This forces me to set int n = -1, but this causes problems when I have an input file which contains 1000 values (the maximum size of my array) that have to be placed in to each array.

However, I notice that;

int n = 0
int j = 0;
      while(input){                          // in is true until "end-of-file"
            if(j >= arraymax) break;
            // word[i] is a string. Input into a string reads up to
            // the next space, or newline
            if(input >> *(xarray+j) >> *(yarray+j)){
	    j++;
	    n++;
	    }       
      }

Solves all my problems.

So I'm trying to deduce the difference here, and hoping you could help me. My input file looks like this;

2 1.69
3 3.25
4 2.44
5 3.45
6 6.29
7 3.94
8 5.23
9 5.78
10 6.31
11 8.25
12 7.84

What I can gather, is that in my first for loop. Input will STILL test true once it has reached the last part of my input file (past 7.84). However, once it tries to read in to the double arrays after having reached the last part, it will then test false - this means that int n will over count by one.

In the second while loop, it tests whether input is still valid. If it does, the if statement checks if anything has actually been added in to the arrays, if anything has then it adds one to int n. So, even though input still tests true once it reaches the last part of my input file, it will only add one to int n if anything is actually input in to the arrays.

Is this correct? (I'm still very new to this).

[vlad from moscow]

The conditions in these two statements

while(input){ // in is true until "end-of-file"
if(j >= arraymax) break;

can be combined as

input && ! ( j >= arraymax )

that is equivalent to

input && j < arraymax

that corresponds to the condition in the first loop. So I do not see any difference between these two loops.

EDIT:
The difference is in the additional condirion in the second loop

            if(input >> *(xarray+j) >> *(yarray+j)){
	    j++;
	    n++;
	    }  

That is you are incrementing j only if input was successful.

In the first loop you are incrementing i in any case irrespective of whether the input was successful.

[Silversonic]

That is true Moscow, however surely if you put the condition you've just said in to the while statement, they would still be different.

Becase the for loop contains;

input >> *(xarray + i);
input >> *(yarray + i);

as the statement to be executed, whereas the while loop would have;

if(input >> *(xarray+j) >> *(yarray+j)){
j++;
n++;
}

as the statement to be executed. These two statements are different surely, the first inputs stuff in to the arrays and then adds one to n, whereas the second checks that someone has actually been added to the arrays and THEN adds n. The first doesn't check whether anything has actually been added?

I think it should work if I had a for loop as

for(int i = 0; input && i < arraymax; ++i){
		if(input >> *(xarray + i) >> *(yarray + i)){
                n++;
                }
	   } 

[ne555]

int n=0;
while(input>>xarray[n]>>yarray[n]) n++;