string usage with textfiles c/c++ - c++

Question

I got a problem using strings. So I had the idea of writing a program, that multiplicates two parenthesis, since I had some with 10 variables each. I put a parenthesis in a .txt file and wanted to read it and just print into another .txt file. I am not sure if it has problems with the specific signs.
So here is my txt that I read
2*x_P*x_N - x_P^2 + d_P - 2*x_N*x_Q + x_Q^2 - d_Q
and here is what it actually prints
2*x_--x_P^++d_P-2*x_++x_Q^--
as You can see it is completely wrong. In addition I get an error after executing, but it still prints it into the .txt. So here is my code:
#include <stdio.h>
#include <string>
using namespace std;
int main()
{
int i;
const int size = 11;
string array[ size ];
FILE * file_read;
file_read = fopen( "alt.txt", "r" );
for( i = 0; i < size; i++ ) //Read
{
fscanf( file_read, "%s", &array[ i ] );
}
fclose( file_read );
FILE * file_write;
file_write = fopen( "neu.txt", "w" );
for( i = 0; i < size; i++ ) //Write
{
fprintf( file_write, "%s", &array[ i ] );
}
fclose( file_write ); printf("test");
return 1;
}
Thanks for suggestions. You can put suggestions made with iostream as well.

Answer 1

You are mixing C++ and C forms of file input:
When you write:
fscanf( file_read, "%s", &array[ i ] );
the C standard library expects that you provide a pointer to a buffer in which the string read in the file will be stored in form of a C string, that is an array of null terminated characters.
Unfortunately, you provide a pointer to a C++ string. So this will result in undefined behaviour (most probably memory corruption).
Solution 1
If you want to keep using the C standard library file i/o, you have to use an interim buffer:
char mystring[1024]; //for storing the C string
...
fscanf( file_read, "%1023s", mystring );
array[ i ] = string(mystring); // now make it a C++ string
Please note that the format is slightly changed, in order to avoid risks of buffer overflow in case the file contains a string that is larger than your buffer.
Solution 2
If you learn C++ (looking at your C++ tag and the string header), I'd strongly suggest that you have a look at fstream in the C++ library. It's designed to work very well with strings.
Here how it could look like:
#include <iostream>
#include <string>
#include <fstream>
using namespace std;
int main()
{
const int size = 11;
string array[ size ];
ifstream file_read( "alt.txt");
for(int i = 0; i < size && file_read >> array[ i ]; i++ ) //Read
;
file_read.close();
ofstream file_write("neu.txt");
for(int i = 0; i < size; i++ ) //Write
file_write << array[ i ] <<" "; // with space separator
file_write.close();
cout << "test"<<endl;
return 0;
}
And of course, the next thing you should consider, would be to replace classic arrays with vectors (you don't have to define their size in advance).