Output to file c++

Question

very simple program, not sure why it isn't working:

#include <iostream>
#include <fstream>
#include <stdio.h>

using namespace std;

int main ()

{
ofstream myfile ("test.txt");
if (myfile.is_open()) 
{
    for(  int i = 1;  i < 65535;  i++  )

     {
     myfile << ( "<connection> remote 208.211.39.160 %d udp </connection>\n", i );
     }
    myfile.close();
}
  return 0;
}

Basically it should print that sentence 65535 times, and then save it to a txt file. But the txt file just has a list of numbers from 1 to 65535, no words or formatting. Any ideas? Thanks for help.

Read here how comma operator is used. http://stackoverflow.com/questions/12824378/how-is-the-comma-operator-being-used-here/12824426#12824426 — Coding Mash, Dec 14 '12 at 08:49
Thanks it works now, however i made a mistake in that i can only have up to 64 numbers, and they need to be random between 10001 and 65535. Anyone know? — user1448260, Dec 14 '12 at 09:02

Bartek Banachewicz · Accepted Answer · 2012-12-14T08:50:00.287

5

If you want to concatenate the output, just pipe your data into two << operators, as such:

myfile << "<connection> remote 208.211.39.160 %d udp </connection>\n" << i;

Note that interpolation does not work in that case, so if you want to put your i variable into the middle of the string, you have to either split it by hand:

myfile << "<connection> remote 208.211.39.160 " << i << " udp </connection>\n"

Or apply some sort of other interpolation formatting before outputting it.

The problem

The problem exists in your code because in C++, (a, b) (the comma operator) returns b. So, in your code it meant you just wrote i to a file.

edited Dec 14 '12 at 08:50

answered Dec 14 '12 at 08:44

Bartek Banachewicz

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does `<<` work like printf? cuz i believe he is using printf syntax – Karthik T Dec 14 '12 at 08:44
Nope, it of course doesn't. I've added a bit to clarify that. – Bartek Banachewicz Dec 14 '12 at 08:50

score 1 · Answer 2 · answered Dec 14 '12 at 08:45

1

change

myfile << ( "<connection> remote 208.211.39.160 %d udp </connection>\n", i );

to

myfile << "<connection> remote 208.211.39.160 " << i << " udp </connection>\n";

answered Dec 14 '12 at 08:45

billz

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werupokz · Answer 3 · 2012-12-14T08:52:07.160

1

Try the following:

myfile << "<connection> remote 208.211.39.160 %d udp </connection>\n" << i;

Basically, myfile << (str , i) means "evaluate (str , i) and write the result of evaluation to ostream myfile".

The result of ( "<connection> remote 208.211.39.160 %d udp </connection>\n", i ) evaluation is equal to i

Take a look to comma operator description: http://en.wikipedia.org/wiki/Comma_operator

edited Dec 14 '12 at 08:52

answered Dec 14 '12 at 08:46

werupokz

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4

score 0 · Answer 4 · answered Dec 14 '12 at 08:46

0

You are using printf syntax to write using ofstream. Others have explained why it doesnt work. To fix it do the following

myfile << "<connection> remote 208.211.39.160"<<i<<"udp </connection>\n";

or if you want to go C style

printf( "<connection> remote 208.211.39.160 %d udp </connection>\n", i ); //fprintf to write to file

answered Dec 14 '12 at 08:46

Karthik T

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How he is supposed to use `fprintf` if he opened the `fstream`? I'd understand `sprintf` suggestion. – Bartek Banachewicz Dec 14 '12 at 09:12
@BartekBanachewicz sorry for not explaining better, C style would mean `fopen`/`fclose` as well. – Karthik T Dec 14 '12 at 09:14

score 0 · Answer 5 · answered Dec 14 '12 at 08:47

0

Looks like you are trying to "printf" and stream...

I think this is more like what you want:

myfile << "<connection> remote 208.211.39.160 " << i << " udp </connection>"<<std::endl;

answered Dec 14 '12 at 08:47

Russ Freeman

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Output to file c++

5 Answers5

The problem