Writing a C++ vector to an output file

Question

ofstream outputFile ("output.txt");

if (outputFile.is_open())
{
     outputFile << "GLfloat vector[]={" <<  copy(vector.begin(), vector.end(), ostream_iterator<float>(cout, ", ")); << "}" << endl;
}
else cout << "Unable to open output file";

How do I output a vector to a file, with each float separated by commas? I would also like to avoid printing square brackets if possible.

Solution 1

First, you shouldn't call your variable vector. Give it a name which is not the name of a class from the Standard Library.

Secondly, ostream_iterator will append a ',' even after the last element of the vector, which may not be what you want (a separator should be a separator, and there's nothing to separate the last value of the vector from a further value).

In C++11, you could use a simple range-based for loop:

outputFile << "GLfloat vector[]={";
auto first = true;
for (float f : v) 
{ 
    if (!first) { outputFile << ","; } 
    first = false; 
    outputFile << f; 
}
outputFile << "}" << endl;

In C++03, it is going to be just a bit more verbose:

outputFile << "GLfloat vector[]={";
auto first = true;
for (vector<float>::iterator i = v.begin(); i != end(); ++i) 
{ 
    if (!first) { outputFile << ","; c++; } 
    first = false;
    outputFile << *i;
}
outputFile << "}" << endl;

Solution 2

You've taken the solution and attempted to stick it into the stream insertion. That's not how it works. It should be a separate line:

outputFile << "GLfloat vector[]={";
copy(vector.begin(), vector.end(), ostream_iterator<float>(outputFile, ", "));
outputFile << "}" << endl;

The copy algorithm simply copies elements from one range to another. ostream_iterator is a special iterator that will actually insert (with <<) into the given stream when you do *it = item_to_insert;.

Comments

[user2136754]

It still says copy_n is not declared, despite including all the following: #include <iostream> #include <fstream> #include <string> #include <vector> #include <iterator> #include <algorithm> #include <stdlib.h>

[David G]

Think you got that for loop mixed up.

[David G]

No problem. Is it okay if I ask: Why haven't you been answering questions lately?

[Andy Prowl]

@0x499602D2: I have a lot of work to do and some health issues that forced me to re-prioritize my activity on SO, but I plan to get back soon :)

[David G]

Best of luck. I hope everything gets better for you. :)

[Andy Prowl]

@0x499602D2: Thank you, I appreciate it :)

[aatish]

@AndyProwl Shouldn't the 'c++' be outside of the if block? If not, c would never increase from 0. Also, perhaps a bool would be a better option than using an int.

[Andy Prowl]

@user19448: Indeed, that was an oversight. I edited the answer, thank you.