Read File in Loop C++

Question

I have a function:

void ReadInput(const char * name)
{
    ifstream file(name);
    cout << "read file " << name << endl;
    size_t A0, A1, A2;
    file >> A0 >> A1 >> A2;
}

Now i want to read two files: INPUT1.txt and INPUT2.txt in a loop, such as:

int main ()
{
    for (int i = 1; i < 3; i++){
        ReadInput(INPUT$i);
    }
    return 0;    
}

The question is how do i define the loop correctly.

Thanks for your time in advance.

Here is the whole code:

#include <iostream>
#include <string>



using namespace std;


void ReadInput(const string& _name){
    ifstream file(_name);
    size_t A0, A1, A2;
    file >> A0 >> A1 >> A2;
}


int main ()
{


    for (int i = 1; i < 3; ++i) {
        string file_name = "INPUT" + to_string(i) + ".txt";
        ReadInput(file_name);
    }

    return 0;
}

OK, all good, now i can compile in c++98 by converting string to const char and stringstream instead of to_string. My goal was to run an automated program with input files all in the same directory. The suggestions about possible duplicate of the question does not achieve that as i have to pass on the input file number as i execute, as i understand it, which is impractical for 3,000 something files.

Why not just do `ReadInput("Input1.txt")` then `ReadInput("Input2.txt")`? For only 2 files you don't need a loop really. — sjrowlinson, May 15 '16 at 21:42
@dato Well that's a different story, but doesn't seem to be the case here. — sjrowlinson, May 15 '16 at 21:47
So your question basically is how to generate the filename using a number? — πάντα ῥεῖ, May 15 '16 at 21:47
Yes, that was just an example. I have up to 4,000 files to read. — user147813, May 15 '16 at 21:54
So this has nothing to do with reading files, or with loops. It is a question about how to convert a number to a string, and there are millions of questions about that already. Did you perform any research? — Lightness Races in Orbit, May 15 '16 at 21:57
Yes, i did. I'm not sure how to do it with the specific format that i have. — user147813, May 15 '16 at 23:27
Possible duplicate of [What does int argc, char \*argv\[\] mean?](http://stackoverflow.com/questions/3024197/what-does-int-argc-char-argv-mean) — Superlokkus, May 16 '16 at 12:18

sjrowlinson · Answer 1 · 2016-05-16T13:43:04.910

1

If you have a number of files (up to n_max+1), with names of the form "INPUTn.txt", where a loop would be warranted, then the following would be a potential solution:

for (int i = 1; i < n_max; ++i) {
    std::string file_name = "INPUT" + std::to_string(i) + ".txt";
    ReadInput(file_name);
}

This requires altering ReadInput to:

void ReadInput(const std::string& _name);

instead of using const char*.

If you don't have C++11 access, then use this in-place of std::to_string:

#include <sstream>
//...
template<typename T> std::string to_string(const T& x) {
    return static_cast<std::ostringstream&>((std::ostringstream() << std::dec << x)).str();
}

edited May 16 '16 at 13:43

answered May 15 '16 at 21:51

sjrowlinson

3,297
1
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Thanks. However, when compiling i get two errors: 'name' was not declared in this scope and 'to_string' was not declared in this scope. – user147813 May 15 '16 at 22:04
@user147813 You probably need to `#include ` – sjrowlinson May 15 '16 at 22:10
I added it but the same error still persist. I added the code to my question. What am i doing wrong? – user147813 May 15 '16 at 22:45
1

@user147813 There's an underscore in argument `_name`, whereas there is no underscore in the `name` within the function body - either change argument to `name` or use `_name` in body of function – sjrowlinson May 15 '16 at 23:08

Andreas DM · Accepted Answer · 2016-05-16T00:47:23.750

1

Corrected a few things in your code below. Keep in mind, for std::to_string to work you need to at least compile with the flag -std=c++11

#include <iostream>
#include <string>
#include <fstream> // you need this for std::ifstream

using namespace std;

void ReadInput(const string& _name){
    // ifstream file(name); should be '_name'
    ifstream file(_name);
    // size_t A0, A1, A2 - what if your file contains characters?
    string A0, A1, A2; 
    file >> A0 >> A1 >> A2;

    // output
    cout << "File: " << _name << '\n';
    cout << A0 << " " << A1 << " " << A2 << '\n';
}

int main ()
{
    for (int i = 1; i < 3; ++i) {
        string file_name = "INPUT" + to_string(i) + ".txt";
        ReadInput(file_name);
    }

    return 0;
}

Or if the file is longer, you may want to read using std::getline

void ReadInput(const string& _name){
    ifstream file(_name);

    string line;
    cout << "File: " << _name << '\n';
    while (getline(file, line)) {
        cout << line << '\n';
    }
}

edited May 16 '16 at 00:47

answered May 16 '16 at 00:41

Andreas DM

10,685
6
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Thanks Andreas DM but i have trouble getting the right answer. Say i have a file named INPUT1.txt with 3 values: 50 50 50. The code as is doesn't real 50 50 50 but gives me 6422476 6422296 435480. Any ideas why? – user147813 May 16 '16 at 02:03
@user147813 works correctly for me. I am not sure what you do differently – Andreas DM May 16 '16 at 02:19
how would i can I do away with to_string in c++98? I know i have to use stringstream but not sure exactly how. – user147813 May 16 '16 at 12:31
I can compile and run with no problem. But on a cluster, i can't compile. This is my g++ version:g++ (GCC) 4.4.7 20120313 (Red Hat 4.4.7-4) Copyright (C) 2010 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. -std=c++11 or -std=c++0x results in recognized command line. – user147813 May 16 '16 at 13:03
@user147813 Check out the update to my answer for an alternative to `to_string` in pre-c++ 11. – sjrowlinson May 16 '16 at 13:43

score 0 · Answer 3 · edited May 23 '17 at 12:23

0

I guess what you want is:

int main (int argc, char* argv[])
{
    using namespace std;
    for (int i = 1; i < argc; ++i) {
        string file_name = string(argv[i]) + to_string(i) + ".txt";
        ReadInput(file_name);
    }

    return 0;
}

For further information see: https://stackoverflow.com/a/3024202/3537677

edited May 23 '17 at 12:23

Community

1
1

answered May 16 '16 at 12:15

Superlokkus

4,731
1
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Read File in Loop C++

3 Answers3