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how to move to the second line in C++ .txt file reading?

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how to move to the second line in C++ .txt file reading?

Nov 4th, 2006

if I have three words in there lines in a .txt file, and I want to read them to three string, how to do this using c++ ifstream? how to read every line(word)?

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Re: how to move to the second line in C++ .txt file reading?

Nov 4th, 2006

Reading from a file works the same way as reading from any other istream, such as cin:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
//read up to newline
std::string str;
std::getline(cin, str);
//read up to newline
std::string str;
std::getline(cin, str);

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
//read up to whitespace (after a word or character)
std::string str;
cin >> str;
//read up to whitespace (after a word or character)
std::string str;
cin >> str;

With files, replace cin with the name of your ifstream

Last edited by Bench; Nov 4th, 2006 at 7:31 am.

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Re: how to move to the second line in C++ .txt file reading?

Nov 4th, 2006

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Originally Posted by Bench

Reading from a file works the same way as reading from any other istream, such as cin:
Help with Code Tags

C++ Syntax (Toggle Plain Text)
//read up to newline
std::string str;
std::getline(cin, str);
//read up to newline std::string str; std::getline(cin, str);
Help with Code Tags

C++ Syntax (Toggle Plain Text)
//read up to whitespace (after a word or character)
std::string str;
cin >> str;
//read up to whitespace (after a word or character) std::string str; cin >> str;
With files, replace cin with the name of your ifstream

and just repeat that three times.

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Re: how to move to the second line in C++ .txt file reading?

Nov 4th, 2006

yes,but that is read one line.
how to jump to the second line?

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Re: how to move to the second line in C++ .txt file reading?

Nov 4th, 2006

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Originally Posted by linq

yes,but that is read one line.
how to jump to the second line?

you can't "jump", file access is serial. if you need a certain position, read the first line(s) and discard it/them until you reach the position in the file that you need.

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Re: how to move to the second line in C++ .txt file reading?

Nov 4th, 2006

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Originally Posted by Bench

you can't "jump", file access is serial. if you need a certain position, read the first line(s) and discard it/them until you reach the position in the file that you need.

I just want to ask how to reach the position which I want

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Re: how to move to the second line in C++ .txt file reading?

Nov 4th, 2006

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Originally Posted by linq

I just want to ask how to reach the position which I want

Just repeat the code Bench posted until you get to the desired line. You will have to use a loop and a counter integer to keep track of the line number that was just read. If you are confused about loops -- which is understandable -- you should read about them in your text book.

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Re: how to move to the second line in C++ .txt file reading?

Nov 4th, 2006

but how to use pointer to move the reading postion? point to what?

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Re: how to move to the second line in C++ .txt file reading?

Nov 4th, 2006

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Originally Posted by linq

but how to use pointer to move the reading postion? point to what?

Looks like you need some code right? Otherwise you're just going to be going around in circles?

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Re: how to move to the second line in C++ .txt file reading?

#10

Nov 4th, 2006

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Originally Posted by linq

but how to use pointer to move the reading postion? point to what?

Imagine file access is like an old fashioned mono-directional cassette tape player (ie, no rewinding!) - the only way to find the position of a chunk of data is to play through the tape from the start, and read each chunk of data sequentially.

The chunk of data you read is always at the "head" position - The tape moves forward, rather than the head (and since you can't rewind, the data which has already passed the head is gone from the stream). You can choose to either store the data at the head or discard it. The tape rolls on once the head has read whatever's at that position. Whatever you do, you *must* read it in order to move the tape on.

here's a quick example of reading the 3rd word in a stringstream (Which is just another 'flavour' of streams - fstreams work in exactly the same way)

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
#include <sstream>
 
int main()
{
    std::stringstream ss("the quick brown fox jumped over the lazy dog");
    std::string str;
    const int position(3);
 
    for(int i(0); i!=position; ++i)
        ss >> str;
 
    std::cout << str;
}
#include <iostream>
#include <sstream>

int main()
{
    std::stringstream ss("the quick brown fox jumped over the lazy dog");
    std::string str;
    const int position(3);

    for(int i(0); i!=position; ++i)
        ss >> str;

    std::cout << str;
}

This example outputs the word "brown" - the first 2 words, "the" and "quick" were also read, but discarded, since the only word I was interested in was the 3rd one.