clearing a member variable

[Gary Wessle]

hi

I have a design problem with this code, it is suppose to take a space
delimited data file
file_info x( f ); //f is a text file provided below
and return the column of choice
vector<string> bb = x.get_col(1);
but if I tried it the second time
vector<string> bb = x.get_col(2);
it does not clear the member variable from the old data of col(1). how
can I do this. I can pop the elements out from vec.begin() to
vec.end() but there may be a better way for this task all together.

**************** test.txt ****************
2 0
3 1
4 2
5 3
6 4
7 5
8 6
7 5
6 4
5 3
4 2
3 1
6 4
9 7
11 9
10 8
9 7
8 6
7 5
5 3





**************** main.cpp ****************
#include <iostream>
using std::cout;
using std::endl;

#include <vector>
using std::vector;

#include<string>
using std::string;

#include "useful.h"

int main() {
string f = "test_data.txt";
file_info x( f ); // space delimited
vector<string> bb = x.get_col(1);
vector<string> aa = x.get_col(2);
vector<double> b, a;

for (unsigned i=0; i<bb.size(); i++) //aa.size() give segmentation fault.
cout << bb << '\t'
<< aa << '\n';
}


**************** useful.h ****************
#include <iostream>
using std::cout;

#include <string>
using std::string;

#include <vector>
using std::vector;

#include <utility>
using std:air;

#include "useful.h"


#ifndef USEFUL_H
#define USEFUL_H
#include <vector>
using std::vector;

class file_info
{
int nRows, nCol;
string file_name;
vector<string> all_cols;
vector<string> col;
void stock_take();
void set_col(int);

public:

file_info(string const& fileName);
~file_info();

/** returns a pair of values, the first is the
* number of rows in a space delimited data file
* and the second is the number of columns.
**/
pair<int,int> get_counts();

/** takes a number n and returns a vector
* containing the nth column of the data file
**/
vector<string> get_col(int);

};


#endif

**************** useful.cpp ****************
#include <utility>
using std:air;
using std::make_pair;

#include <string>
using std::string;

#include <fstream>
using std::ifstream;

#include <sstream>
using std::stringstream;

#include "useful.h"


file_info::file_info( string const& fileName )
: file_name( fileName ){
nCol = 0;
nRows = 1;
stock_take();
}
void file_info::stock_take() {
ifstream in(file_name.c_str());
string line;
getline(in, line);
stringstream input( line.c_str() );

string word;
while(input >> word) {
nCol++; // init'd by constructor
all_cols.push_back(word);
}

while (getline(in, line)){
nRows++; // init'd by constructor
stringstream input( line.c_str() );
string word;
while(input >> word)
all_cols.push_back(word);
}

}
file_info::~file_info() {}

pair<int,int> file_info::get_counts()
{
return make_pair(nRows, nCol);
}

void file_info::set_col(int x){
for(int i = (x-1); static_cast<unsigned> (i) < all_cols.size(); i = (i+nCol) )
col.push_back(all_cols);
}

vector<string> file_info::get_col(int y){
set_col(y);
return col;
}

 

[Stuart Redmann]

hi

I have a design problem with this code, it is suppose to take a space
delimited data file
file_info x( f ); //f is a text file provided below
and return the column of choice
vector<string> bb = x.get_col(1);
but if I tried it the second time
vector<string> bb = x.get_col(2);
it does not clear the member variable from the old data of col(1). how
can I do this. I can pop the elements out from vec.begin() to
vec.end() but there may be a better way for this task all together.

Use "vec.clear ();". Just one question: Why do use a member variable for
creating the vector that you return by value? Why don't you just
declare a local variable inside get_col like the following method:

vector<string> file_info::get_col(int)
{
vector RetVal;
for(int i = (x-1); static_cast<unsigned> (i) < all_cols.size();
i = (i+nCol) )
RetVal.push_back(all_cols);
return RetVal;
}

Since you copy the whole vector anyway, I don't see a reason why you
should keep the vector col around (except for the case that you want to
optimize for the case that the same column gets queries consequtively).

Regards,
Stuart