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Getting specific lines in an input file.

Posted on 2007-11-29
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Last Modified: 2010-04-01
Ok my question is how would I grab certain lines in a input file? lets say the input file has these sentences:

input.txt

Showing line 1
Showing line 2
showing line 3
etc...

now how would I get a specific line?

I know how to get first line by using

getline(file,str) //assuming string str; and ifstream file;
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Question by:jschmuff
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7 Comments
 
LVL 53

Expert Comment

by:Infinity08
ID: 20377625
If the lines don't have a fixed length, then you'll have to read one line at a time until you reach the line you need (discarding all lines you read, except the last).

If the linesDO have a fixed length, you can set the file pointer to the correct location, and then read the line.
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LVL 86

Expert Comment

by:jkr
ID: 20377719
For a general approach, you can

#include <fstream>
#include <string>
#include <iostream>
using namespace std;

ifstream is("input.txt");

vector<string> vLines;

while(!is.is_eof()){

  string sLine;

  getline(is,sLine);

  vLines.push_back(sLine);
}

// show 3rd line
cout << vLines[2] << eof; // 2 because indices are zero-based
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Author Comment

by:jschmuff
ID: 20377826
so there is no way set a spot to start after?
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Author Comment

by:jschmuff
ID: 20377864
jkr>> what is the bracket for after vLine.push_back(sline);
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Accepted Solution

by:
Infinity08 earned 350 total points
ID: 20377866
>> so there is no way set a spot to start after?

You can save a position in a file that you can go back to later by using tellg and seekg :

        http://www.cplusplus.com/reference/iostream/istream/tellg.html
        http://www.cplusplus.com/reference/iostream/istream/seekg.html

but of course you'd still have to get to the position you want first before you can use tellg.
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Author Comment

by:jschmuff
ID: 20377867
Oh haha never mind I didnt see the beginning bracket for the while
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LVL 86

Expert Comment

by:jkr
ID: 20377877
>>what is the bracket for after vLine.push_back(sline);

Um, what bracket? The one that ends the 'while' loop?
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