Thread: Reading Strings from a text file?

Now I doubt you need to actually read this code (or if it is even possible). I don't really know C++, which is apparent, but I do know Matlab so I'm attempting this mini-project in C++ for my work place. I've been trying to locate the answer for a while, but I feel like it might be a standard function.

Basically I need to check an id from a scanning system and compare it to a list of ids to see if a person has clearance. I have it set up right now so the scanning system is ok, and the checking system is ok for only 1 id. Now I need to get it so it checks a text file for a large quantity of id numbers (all in the same form as id2).

So essentially, how is it possible to extract a single string from a text file, compare to see if it is acceptable, and repeat this process until a match is found or the person does not have clearance? As of now I have a runtime error which closes the program and it becomes unresponsive.

Code:

#include <iostream>
    #include <vector>
    #include <string>
    #include <stdio.h>
    #include <string.h>
    #include <stdlib.h>
#include <fstream>
using namespace std;

int main()
{
 
    string id1="%130360765";
  string id2="U30360765";
  string id="aaaaaaaaaaaa";
  string id3="aaaaaaaaaaaa";
   
    cout << "Enter User ID"<<endl;
   cin >> id1;
   
   id=id1.substr(2,8);
   cout<<id<<endl;
   
   
    int x=0;

  ifstream in("C:\\Users\\Mark\\Desktop\\towel.txt");

  if(!in) {
    cout << "Cannot open input file.\n";
    return 1;
  }

  char str[255];

  while(in) {
    in.getline(str, 255);  // delim defaults to '\n'
    string id2=string(str);
    id3=id2.substr(1,8);
   cout<<id3<<endl;

  if(id.compare(id3)==0)
  {
                        cout<<"Patron has service"<<endl;
                        x=1;                 
  }

 
}
 in.close(); 
if (x==0)
   {
     cout<<"Patron does not have towel service"<<endl;
     }
     
system("PAUSE");

  return 0;
  


}

you need to use a loop like for or while and use an ifstream object to read your file, this will also give you the getline() member function to collect your string.

Code:

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

using namespace std;
int main()
{
	ifstream in("c.cpp");//file path
	if(!in.good())
	{
		cout << "File may not be good" << endl;
		return;
	}

	string id;
	cout << "Enter id:" <<endl;
	getline(cin,id); 
	id = id.substr(2,8);

	string line;
	bool cleared=false;
	while(getline(in,line))
	{
		string id_line = line.substr(1,8); 
		// these are the bounds you supplied , starting at 1 .. ending 8 characters after index 1
		if(id_line == id)
		{
			cleared = true;
			break;
		}
	}
	cout << (cleared ? "Patron has service" : "Patron does not have towel service") << endl;
}

you need to post your code for this one, the file handling in c++ is pretty straight forward so i can only imagine you did something really wrong, are you maybe repeatedly opening the stream in a loop? before any other instance is close()..d

I've had a look at your code and I've noticed these two lines:

Code:

id = id.substr(2,8);

and

Code:

string id_line = line.substr(1,8);

Just out of curiousity, why are you 'clipping' these strings?

This is a minor note but instead of doing this:

Code:

if(id_line == id)

it may be better if you did something like this:

Code:

if( !id.compare(id_line) )

Also, in your if statement you set the value of 'cleared' to true and then print out a statement based on the value of 'cleared'. Why not just print the statement from your if statement?

Edit: I just realised that I was reading rodrigorules's code instead, but your program works in a very similar way.

Why do you print out a statement saying a patron has towel service in the while loop but then you have this if statement afterwards which seems a bit out of place. The only way this if statement will be true is if no one has towel service because in your while loop you change the value of x to '1' if someone has it, but you don't take it back down afterwards, and even if you did set it to '0' when someone didn't have it, it wouldn't print out the message because your if statement is after the loop.

Code:

if (x==0)
   {
     cout<<"Patron does not have towel service"<<endl;
     }

Also, change this line:

Code:

while(in)

to:

Code:

while(in.good())

I'm also not sure what the point of this is:

Code:

string id1="%130360765";
  string id2="U30360765";
  string id="aaaaaaaaaaaa";
  string id3="aaaaaaaaaaaa";

Why do you initially set them to some stuff if you don't do anything with them? It really makes very little difference to the program, so maybe I'm just being pedantic. Are they relics from your initial version of the code?