can't open a file by reading its name from another file in c++

Question

Hi I am trying to read a file name from another file and then read it. But I can only read the first file which contains the name of the second file I want to open. Here is how I am doing it..

int main()
{
freopen("input1.txt","r",stdin);
while(cin>>fileName>>source>>destination)
{
    //reads perfectly
    char file[100];

    for(int i=0;i<(int)fileName.size();i++)
        file[i] = fileName[i];

    file[(int)fileName.size()] = NULL;        

    freopen(file,"r",stdin);

    mp.clear();
    mp1.clear();

    for(int i=0;i<cityNumber;i++)
        adj[i].clear();
    cityNumber = 0;

    while(cin>>city1>>city2>>distanc)
    {
       //doesn't read
    }
}

Am i the only one who doesn't know what *mp.clear();mp1.clear();* are doing? — SChepurin, Apr 23 '13 at 13:36
I know I should have deleted them before posting! But its obvious they don't have anything to do with taking input. — Tahlil, Apr 23 '13 at 20:15

Angew is no longer proud of SO · Accepted Answer · 2013-04-23T13:30:22.570

4

Your code uses overly complicated constructs. Why not just do it the straightforward C++ way:

#include <fstream>

int main()
{
std::ifstream input1("input1.txt");
while(input1 >> fileName >> source >> destination)
{
    std::ifstream file(fileName.c_str());

    mp.clear();
    mp1.clear();

    for(int i=0;i<cityNumber;i++)
        adj[i].clear();
    cityNumber = 0;

    while(file >> city1 >> city2 >> distanc)
    {
       //work with values
    }
}

edited Apr 23 '13 at 13:30

answered Apr 23 '13 at 13:17

Angew is no longer proud of SO

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Yes yours are much simpler. Thanks. – Tahlil Apr 23 '13 at 13:24
But it shows an error `variable 'std::ifstream input1' has initializer but incomplete type` – Tahlil Apr 23 '13 at 13:28
@kalkin I've edited the answer to include the necessary header. – Angew is no longer proud of SO Apr 23 '13 at 13:31

score 1 · Answer 2 · answered Apr 23 '13 at 13:11

The man page for freopen says:

The freopen() function opens the file whose name is the string pointed to by path and associates the stream pointed to by stream with it. The original stream (if it exists) is closed.

Hence,

freopen("input1.txt","r",stdin);

closes the stdin stream, and

freopen(file,"r",stdin);

finds no open stream to associate to file.

You should probably plain fopen the file input.txt and read from it to leave stdin for the final target.

score 1 · Answer 3 · edited Jun 20 '20 at 09:12

A few things to consider:

do not use freopen/fopen family of files in C++ (unless it's a really special case)
do not mix std::iostreams with fopen family of files (freopen should not be used here)

these two points will fix your particular error (as @Angew pointed out)
prefer std::string with std::getline instead of char file[100]; This avoids buffer overflow if reading over 100 chars (I believe you don't check for errors) and simplifies your code.
prefer iterators over iterating by index.

That means instead of:

for(int i=0;i < cityNumber;i++)
    adj[i].clear();

you could/should write:

// C++11
for(auto& city: adj)
    city.clear();

// C++98
for(<type of adj>::iterator city = adj.begin(); city != adj.end(); ++city)
    city->clear();

can't open a file by reading its name from another file in c++

3 Answers3