C++ Issue between .txt document to array

Question

I'm working on an assignment to take data out of a list of names in a .txt document (one on each line, each ending with a comma).

What I want is:

Count how many lines there were
Use the count to define the size of an array to hold the words
Pick two random words from the list and print them.

This is my code:

#include <fstream>
#include <iostream>
#include <string>

using namespace std;

int main() {    
    srand(time(NULL));
    int j;
    int rand1 = rand() % 5;
    int rand2 = rand() % 5;

    ifstream myfile ("NAMES.txt");

    if (myfile.is_open()) {
        myfile.unsetf(ios_base::skipws);

        unsigned line_count = count(
            istream_iterator<char>(myfile),
            istream_iterator<char>(), 
            '\n');
        j = line_count;

        cout << j << endl;      

        string *MN1 = new string[j];

        for(int i = 0; i <= j; i++) {
            getline(myfile, MN1[i], ',');
        }
        cout << rand1 << endl;
        cout << MN1[rand1] << " " << MN1[rand2] << endl;
    }
    else {
        cout << "Unable to open file" << endl;
    }
}

However, there seems to be something going wrong between when the code reads the number of lines, uses that as the size of the array, then prints the random words.

wt is the exact issue ? as you can see, is line_count ok ? — ANjaNA
– ANjaNA, Commented Jun 7, 2015 at 2:23
Please be more specific when writing questions. Consider: "There seems to be something going wrong" could mean anything. Did the program crash? Infinite loop? Print out the lyrics to "Friday" by Rebecca Black instead of the output you were expecting? What output DID you expect? Notice that the only way for others to have a shot at seeing exactly what you saw would require something you don't provide here... e.g. the file content of "NAMES.TXT". Read up on MCVE and sscce...the bugs you find may be your own! — HostileFork says dont trust SE
– HostileFork says dont trust SE, Commented Jun 7, 2015 at 23:40

HostileFork says dont trust SE · Accepted Answer · 2015-06-07 23:45:54Z

2

It isn't necessary to parse the file twice, and you can use myfile.ignore to ignore the trailing endline which will otherwise pollute your output. It's also not necessary to allocate your strings on the heap, which is generally best to avoid. Here's an example solution using this technique.

#include <fstream>
#include <iostream>
#include <string>
#include <time.h>

using namespace std;

int main()
{
    srand(time(NULL));
    int j;
    int rand1 = rand() % 5;
    int rand2 = rand() % 5;

    ifstream myfile("NAMES.txt");

    if (myfile.is_open()) {
        myfile.unsetf(ios_base::skipws);

        string str;
        for (int i = 0; !myfile.eof(); i++)
        {
            getline(myfile, str, ',');
            myfile.ignore(1);

            if (i == rand1 || i == rand2) {
                cout << str << endl;
            }
        }


    }
    else {
        cout << "Unable to open file" << endl;
    }
}

edited Jun 7, 2015 at 23:45

HostileFork says dont trust SE

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answered Jun 7, 2015 at 3:38

caustik

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Comments

Havenard · Accepted Answer · 2015-06-07 02:25:10Z

1

The first iterator reads the file fully. You have to rewind the file after that so you can read it again.

myfile.clear();  // reset file states
myfile.seekg(0); // go back to the the beginning
for(int i = 0; i<=j; i++)
....

answered Jun 7, 2015 at 2:25

Havenard

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2 Comments

Havenard Over a year ago

Note: To avoid having to read the file twice it would be best to not do this line counting thingy. Resizing buffers is time costly, but reading a file twice costs far more time.

ANjaNA Over a year ago

you are correct. My suggestion is to use a stl::vector insted.

BufferOverflow · Accepted Answer · 2015-06-07 03:13:11Z

0

I would recommend you to use vector <std::string> instead of array of strings, this way you can read all the lines into memory from the file, and then count the number of lines.

std::vector <std::string> MN1;

Reading lines:

for ( std::string line; std::getline ( myfile, line );)
{
    MN1.push_back ( line );
}

Count the number of lines:

j = MN1.size ( );

And i will recommend you to avoid using of keyword new unless it's necessary, and if you allocate memory with keyword new you also have to clean it up when it's not used anymore, this is done with keyword delete.

answered Jun 7, 2015 at 3:13

BufferOverflow

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