CODEWITHSUNDEEP
c++stringfile-iofile
Mark Lalor
Mark Lalor

Reputation: 7887

C++ append to string and write to file

Why does the following code not work

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

int main(){
    string data;
    int i=0;

    while(i <= 5){
      i++;
      data += i;
      data += "\n";
    }

    ofstream myfile;
    myfile.open ("data.txt");
    myfile << data;
    myfile.close();
}

It should append a number then newline and write it to a file (that doesn't yet exist).

The file should look like this...

1
2
3
4
5

What is wrong with the code?

Upvotes: 3

Views: 12076

Answers (5)

John Dibling
John Dibling

Reputation: 101456

Multiple issues with your code. First of all, you're #include-ing several deprecated headers, including <stdio.h> which should be <iostream>, <string.h> which should be <string>, and <stdlib.h> which should be <cstdlib>.

As for your specific question, it is doing exactly what you asked it to do. Problem is, you didn't ask it to do what you wanted it to do. In your code data += i; you are saying "append the binary value i to this string" which your code dutifully does. If you open your resulting text file in a binary-capable text editor, you'll find it is the binary data you inserted.

What you wanted to do was convert the integers to their string representations, and append that to the text file. A simple way do this in a C++ish way is to use stringstream, as so:

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

int main(){
    int i=0;

        stringstream ss;
    while(i <= 5){
      i++;
            ss << i << endl;
    }

    ofstream myfile;
    myfile.open ("data.txt");
    myfile << ss.str();
    myfile.close();
}

Upvotes: 2

codaddict
codaddict

Reputation: 454960

Alternatively you can also use sprintf as:

    char temp[10]; // assuming your string rep of the number won't take >9 char.

    while(i <= MAX){
            i++;
            sprintf(temp,"%d",i);
            data += temp;
            data += "\n";
    }

Upvotes: 1

Kirill V. Lyadvinsky
Kirill V. Lyadvinsky

Reputation: 99555

Why do you not use operator<<?

ofstream myfile;
myfile.open ("data.txt");
for ( int i = 1; i <= 5; ++i )
  myfile << i << "\n";
myfile.close();

Upvotes: 11

James Curran
James Curran

Reputation: 103485

I don't believe that operator+=(int) is defined for std::string, so the line data += i; would, if it compiles at all, be translates as:

 data += (char) i;

Let's use actual chars here:

char i='0';   

while(i <= '5'){   
  i++;   
  data += i;   
  data += "\n";   
}

Furthermore, you are including <string.h> which is the C run-time library for strings (string here being arrays of tiny integers); What you actually need to include is <string> which is the C++ library for std::string. As far as I can tell, you do not need stdio.h, stdlib.h or string.h at all.

Upvotes: 1

James
James

Reputation: 2474

The main problem that i see is that you are building off of the string data by adding integers to it.
What you actually want is the character version of the integer which would require something like this:

while(i <= 5){
  i++;
  data += char(i+48);
  data += "\n";
}

What this does is add the offset (48) from decimal representation to the ASCII representation of the number.
EDIT: Tested this, depending if you want to print up to 6 or not you're also gonna wanna replace the while(i <= 5) with a while(i < 5) due to the way you set up the inside of the while loop.

Upvotes: 1

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