Reading Large Text File in C++

maheshrajn · 02-26-2006, 04:37 PM

Hi all,
I am facing one problem in my research project.

I have a large file containing text as:

1 4000000.55555+ 2 4 50000000.0002+ 2.....
......
......
9 3.000000+ 2 5 2.000111+ 3......

I need to read this file reading each tab seperated string and storing them as vector element in some vector in C++.

Ho do I reading each line in the file identifying each string and storing them simultaneously?

Please suggest me the solution to this problem.
I would appreciate all input in this.

I am in need of urgent solution.
regard
RAJ

xhi · 02-26-2006, 05:09 PM

search for fstream and vector, they have everything you need..

maheshrajn · 02-26-2006, 11:50 PM

Thanks for the insight,
But I am seeking more help in this regards.
How do I keep track of space delimited strings in a single and need to check the whole file while storing string in some vector?
Please help me out.
Raj

maheshrajn · 02-27-2006, 09:34 PM

Hi I have one problem,
I want to store each numerical value in a line of a text file as string in the vector.
If I have the line like this:
1.00000- 5 0.00000+0 1.03912+4 0.00000+0 1.03912+4 1.48000+16457 3 1
14454.4000 1.26000+1 18764.3000 1.13615+1 21379.6000 1.10428+16457 3 1
23453.5000 1.08531+1 30000.0000 1.03848+1 35272.2000 1.01028+16457 3 1
40000.0000 9.92998+0 45365.5000 9.75999+0 57500.0000 9.47555+06457 3 1
65000.0000 9.27821+0 71033.3000 9.08284+0 85448.7000 8.76920+06457 3 1
98085.2000 8.55090+0 113163.000 8.35730+0 137582.000 8.12755+06457 3 1
How do I store each element in a vector of a string?
Please provide me insight in it.
Thanks in advance.
Mahesh

Flesym · 02-28-2006, 11:29 AM

First, I won't write the code for you! But I can give you some hints, so that you (hopefully) will be able to write it yourself:

Begin by going through this tutorial about file-streams (until you understand everything!):
http://www.cplusplus.com/doc/tutorial/files.html
A detailed documentation of the "iostream library" with all classes and members you will find here:
http://www.cplusplus.com/ref/iostream/

A basic introduction to STL ('vector' and 'string' is part of this) is located here:
http://www.msoe.edu/eecs/ce/courseinfo/stl/index.htm
A complete list of all STL-classes (including vector and string/basic_string) and members is here:
http://hea-www.harvard.edu/MST/simul..._contents.html

After reading through these sites you will see, that your problem isn't very hard to solve:

Code:

1. declare/open a file-stream for reading (std::ifstream sampleFile("sample.txt");)
2. declare a vector of strings (std::vector<std::string> vec2str;)
3. if you haven't already reached the end of file, then do the following, else goto 8:
  4. declare a temporary string that will hold the next line (std::string tmpStr;).
  5. fill this temporary string with the current line of your file (std::getline(sampleFile, tmpStr);)
  6. add the string to your string-vector (vec2str.push_back(tmpStr);)
7. go back to 3
8. close the file
9. finish

maheshrajn · 03-01-2006, 02:52 PM

Hi
Thanks for helpful hint.
I have one more question:
Now I am able to store alternate value into the corresponding vector, I want to change them in to the floating point values based on +/- . lets suppose I have string of 1.0000-5 then I want to change it to 0.0000001 and store in new vector.
Can you help me out in this?
Mahesh

Flesym · 03-04-2006, 08:54 AM

I don't know, how you get from "1.0000-5" to "0.0000001"? This notation of floats is no common one, at least as far as I know. However, I guess "1.0000-5" should be the exponential notation? If that is true, then the usual way to write this would be "1.0000e-5". And this is equal to "0.00001". So you first have to format the string properly, so that there is an "e" right before the "+/-". After that you will use "istringstream" (part of the iostream library) to convert it into a 'float'. Here I wrote you a little sample, of how this can be accomplished:

Code:

#include <iostream>
#include <sstream>

using namespace std;
int main(){
  string sf = "1.0000-5";
  /*
    Now find the position of '+' OR '-', beginning at the second(!) char, 
    because the first one may also be a sign!
  */
  int ePos = sf.find_first_of("+-", 1); 
  
  if (ePos != string::npos) //if a sign was found, then insert an "e" before it
    sf.insert(ePos, "e");

  //Now the string has the right format to be used with 'istringstream'
  istringstream istr(sf);
  float f;
  istr >> f; //... and do with 'f' what ever you want ;-)
}