Reading and writing to text files.

[JayRyGeo]

I have this bit of code that will write two randomly generated vectors to two files:

#include <iostream>
#include <string>
#include <fstream>
#include <stdlib.h>
using namespace std;

int main()
 {
	int n = 10;

	ofstream outFile( "vectors.txt", ios::out);

	//exit program if unable to create file
	if(!outFile)
	{
		cerr << "File could not be opened" << endl;
		exit( 1 );
	}

	//write out the size of vector
	outFile << "Size: " << n << endl;

	//create vector1 with N random elements
	outFile << "Vector1: " ;
	for (int i = 0 ; i < n ; i++)  outFile << rand()%n << ' ';
	outFile << endl;

	//create vector2 with N random elements
	outFile << "Vector2: " ;
	for (int i = 0 ; i < n ; i++)	 outFile << rand()%n << ' ';
	outFile << endl;

	return 0;
}

This I have this code that will display the vectors. However, I keep getting sig faults:

#include <iostream>
#include <string>
#include <fstream>
using namespace std;

int dotproduct(int *Vector1,int *Vector2,int N);

int main()
{
	string info;
	int n, *vector1, *vector2;

	//create input file to read the file vectors.txt
	ifstream inFile("vectors.txt",ios::in);
	if (!inFile)
	{	cerr << "File could not be openned" << endl;
	}

	//read vector size from the input file and print out to screen
	inFile >> info; cout << info;
	inFile >> n;    cout << n << endl;
    
	// read vector 1 from the vectors.txt file
	inFile >> info ; cout << info;
	for (int i = 0 ; i > 10 ; i++)
	{
		inFile >> vector1[i] ;	cout << vector1[i] << ' ';
	}
	cout << endl;

	//Read vector 2
    inFile >> info ; cout << info;
	for (int i = 0 ; i > 10 ; i++)
	{
		inFile >> vector2[i] ;	cout << vector2[i] << ' ';
	}
	cout << endl;


	return 0;
}

[Chervil]

Why do you have i > 10 at lines 25 and 33?
for (int i = 0 ; i > 10 ; i++)
Since i starts at 0, it is definitely not bigger than 10, the condition is false and the loop doesn't execute.

Also, vector1 and vector2 are uninitialised pointers. They may not be dereferenced. Hence vector1[i] and vector2[i] are errors.

[JayRyGeo]

I switched around the i > 10 and forgot to switch it back. How would I deference them? Our prof gave us the first file and then the first part of the second and told us to finish it. So really, I guess I don't know why you have to use pointers in the first place here and I don't know how to deference them.

[Chervil]

In the first program there is this:

    //write out the size of vector
    outFile << "Size: " << n << endl;

In the second program you need to read and make use of this information.
inFile >> n;
Now you know how many elements there are. So that can be used in two important ways.
1. allocate enough storage to hold the data you will be reading.

    vector1 = new int[n];

2. use that value to control the loop

    for (int i = 0 ; i < n; i++)

Since vector1 is now a valid pointer it can be used as you originally planned, to store each value read in from the file.

[JayRyGeo]

Okay that worked, but how does the in file know that n is the size? I realize that n is initialized as 10 in the output file, but when it is written to the text there is no reference to n at all, just a solid 10.

[Chervil]

but how does the in file know that n is the size?

It doesn't "know". It is up to you as the programmer to match the code that is written to interpret the contents of the file according to your knowledge about the structure of the data in the file.

Here's an example of the file vectors.txt, it looks like this:
Size: 10
Vector1: 1 7 4 0 9 4 8 8 2 4
Vector2: 5 5 1 7 1 1 5 2 7 6

The first line has the word "Size:" followed by some integer value. So you design the second program in such a way as to make use of your knowledge about the meaning of the data contained in the file.