Question

I’m doing a simulation of a few Ising models (lattice with 1 or -1) and I don’t want to use files (don’t gonna get into details here, I guess :D). So, the thing is, I’m using stringstreams to format the data & data’s filename to then separate them with bash. Show you an example code

#include <stdio.h>
#include <stdlib.h>
#include <string.h>    
#include <iostream>
#include <sstream>
#include <string>    
using namespace std;
void print2bash(string file, string data);

int main(int argc, char *argv[]){

    int i, j, n = atoi(argv[1]);
    // I want to put this outside main() {
    stringstream ssbash;    ssbash.clear(); ssbash.str(""); 
    stringstream ssfile;    ssfile.clear(); ssfile.str("");
    //} 
    //  So I don't have to do ss.clear and ss.str("") every time before using them

    ssfile << "this_" << n << "_is_the_filename.dat" << endl;
    for(i = 0; i < n ; i++){
        for(j = 0; j < n; j++)
            ssbash << i*n+j << "\t";
        ssbash << endl;
    }

    print2bash( ssfile.str(), ssbash.str() );

    return 0;
}
// print2bash(); prints string data; to string file; 
//               This will be used to separate data 
//               afterwards ( i.e. ./Ising.o <arguments> | bash )  
void print2bash(string file, string data){

    stringstream ssbash;    ssbash.clear(); ssbash.str(data);       
    stringstream output;    output.clear(); output.str("");
    string line;

    output << "for((i=0;i<1;i++)) do "<< endl; // Buffer all to bash at once
    while( getline(ssbash,line) )
        output << "    echo \"" << line << "\";" << endl;

    output << "done >> " << file; // Appending
    cout << output.str() << endl;

    ssbash.clear(); ssbash.str("");

    return ;
}

A regular output of this “sstreams.o” program would be

~work/Ising$ ./sstreams.o 5
for((i=0;i<1;i++)) do 
    echo "0 1 2 3 4 ";
    echo "5 6 7 8 9 ";
    echo "10 11 12 13 14 ";
    echo "15 16 17 18 19 ";
    echo "20 21 22 23 24 ";
done >> this_5_is_the_filename.dat

You get the idea right? From this thinking flow, what I actually write is ./sstreams.o 10 | bash so one gets nicely separated files. But the problem is:

I’m getting tired of writing this lines each time I want to use print2bash() (oh the irony!)

stringstream ssbash;    ssbash.clear(); ssbash.str(""); 
stringstream ssfile;    ssfile.clear(); ssfile.str("");

Because of that, I want to put the ss as global variables (don’t know if this is that kind of awful so the compiler doesn’t let me to do it: I’m just a Physicist). So, even if I write std::stringstream for the ss variables after the #includes (and using namespace std), I still get the error

error: ‘ssbash’ does not name a type
error: ‘ssbash’ does not name a type
error: ‘ssfile’ does not name a type
error: ‘ssfile’ does not name a type

from the compiler. Hope you can help meee.

---

EDIT: [SOLVED]

The problem was that if I wanted to declare a stringstream out of main() it failed because of the members clear & str where out of context so they can’t be used. Besides that, this use was unnecessary because stringstreams are created with empty contents, as @jpalecek and @umlaeute pointed out.

I’ve declared the stringstreams out of main and include a clear() + str("") on print2bash() to give the desired result:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>    
#include <iostream>
#include <sstream>
#include <string> 
#include <vector>

using namespace std;

stringstream ssbash, ssfile;

void print2bash(string file, string data);

int main(int argc, char *argv[]){

    int i, j, n = atoi(argv[1]), p, np = atoi(argv[2]);
    vector< vector<int> > chain;    chain.resize(n*n);

    for( p = 0; p < np; p++){

        ssfile << "chain_p="<< p << "_filename.dat" << endl;
        for(i = 0; i < n ; i++){
            for(j = 0; j < n; j++){

                chain[p].push_back( i*n + j );
                ssbash << i*n+j << "\t";
            }
            ssbash << endl;
       }
       print2bash( ssfile.str(), ssbash.str() );
    }

    return 0;
}
void print2bash(string file, string data){

    ssbash.str(data);
    stringstream output;
    string line;

    output << "for((i=0;i<1;i++)) do "<< endl; // Buffer all to bash at once
    while( getline(ssbash,line) )
       output << "    echo \"" << line << "\";" << endl;

    output << "done >> " << file; // Appending 
    cout << output.str() << endl;

    ssfile.clear(); ssfile.str("");
    ssbash.clear(); ssbash.str("");

    return ;
}

a regular output of ./sstreams.o

~/work/Ising$ ./sstreams.o 4 2
for((i=0;i<1;i++)) do 
    echo "0 1   2   3   ";
    echo "4 5   6   7   ";
    echo "8 9   10  11  ";
    echo "12    13  14  15  ";
done >> chain_p=0_filename.dat

for((i=0;i<1;i++)) do 
    echo "0 1   2   3   ";
    echo "4 5   6   7   ";
    echo "8 9   10  11  ";
    echo "12    13  14  15  ";
done >> chain_p=1_filename.dat

Thanks for everything

Answer 1

You needn’t call

ssbash.clear(); ssbash.str("");

because stringstream objects are automatically created with empty contents. That leaves you with

stringstream ssbash;
stringstream ssfile;

That could be a global declaration, but don’t do it, because it won’t help you. You would have to clear it each time you use it.

If you don’t like repeating the declaration over and over, you can substitute them with a macro

#define DECLARE_STREAMS \
  stringstream ssbash; \
  stringstream ssfile;

or better

#define DECLARE_STREAMS(a, b) \
  stringstream a; \
  stringstream b;

used like this: DECLARE_STREAMS(ssbash, ssfile).

But the best (and cleanest) solution would be to refactor code that formats the data into a single function. That way, you wouldn’t have to worry about repeating the stream declaration.

BTW why do you instead of writing to a file with C++ emit script that does the writing is beyond me.