Solved

Getting specific lines in an input file.

Posted on 2007-11-29
7
210 Views
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;
0
Comment
Question by:jschmuff
  • 3
  • 2
  • 2
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.
0
 
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
0
 

Author Comment

by:jschmuff
ID: 20377826
so there is no way set a spot to start after?
0
Free Tool: IP Lookup

Get more info about an IP address or domain name, such as organization, abuse contacts and geolocation.

One of a set of tools we are providing to everyone as a way of saying thank you for being a part of the community.

 

Author Comment

by:jschmuff
ID: 20377864
jkr>> what is the bracket for after vLine.push_back(sline);
0
 
LVL 53

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.
0
 

Author Comment

by:jschmuff
ID: 20377867
Oh haha never mind I didnt see the beginning bracket for the while
0
 
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?
0

Featured Post

Networking for the Cloud Era

Join Microsoft and Riverbed for a discussion and demonstration of enhancements to SteelConnect:
-One-click orchestration and cloud connectivity in Azure environments
-Tight integration of SD-WAN and WAN optimization capabilities
-Scalability and resiliency equal to a data center

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

C++ Properties One feature missing from standard C++ that you will find in many other Object Oriented Programming languages is something called a Property (http://www.experts-exchange.com/Programming/Languages/CPP/A_3912-Object-Properties-in-C.ht…
Basic understanding on "OO- Object Orientation" is needed for designing a logical solution to solve a problem. Basic OOAD is a prerequisite for a coder to ensure that they follow the basic design of OO. This would help developers to understand the b…
The goal of the video will be to teach the user the difference and consequence of passing data by value vs passing data by reference in C++. An example of passing data by value as well as an example of passing data by reference will be be given. Bot…
The viewer will be introduced to the member functions push_back and pop_back of the vector class. The video will teach the difference between the two as well as how to use each one along with its functionality.

790 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question