Question

I think this has happened to me before. This isA3.txt:

%INSERT
MARK 29 
DAVID 21
JOHN 44
JOHN 51
LARRY 39
MARK 21
DAVID 18
JOHN 28
MARK 35
DONALD 41
PHIL 26

Even though I use sourcefile >> reader at the end of the loop, the program keeps outputting "reader: MARK", meaning the sourcefile >> reader; statement isn’t working (i.e., it keeps getting the same input over and over again, or it’s not getting any input).

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

using namespace std;  

struct data
{
 string name;
 int id;
 data* link;
};

data* start;
data* node;
ifstream sourcefile;

int main()
{
 data* start = new data;

 start -> link = NULL;

 string input;
 string reader;
 sourcefile.open("A3.txt");

 bool firstnode = true;

 sourcefile >> input;

 node = start;

 cout << "Recieved command: " << input << endl;

 if(input == "%INSERT")
 {
  // unlike the other ones, this function keeps reading until it hits another command
  sourcefile >> reader;

  cout << "Insert name: " << reader << endl;


  // iterates through the link list until it hits the final node
  while(node -> link != NULL)
    node = node -> link;


  while(reader[0] != '%')
  {
   if(firstnode)
    start -> link = new data;
   else
    node -> link = new data;


   sourcefile >> node -> name;
   sourcefile >> node -> id;
   node -> link = NULL;

   sourcefile >> reader;
   cout << "reader: " << reader << endl;
  }
 }
 else
  return 0;

}

Also… offtopic. The compiler said that switch statements can’t be used with strings, is that really true, or was I doing something else wrong?

Answer 1

sourcefile >> node -> id; fails, and after that none of the input operations from sourcefile succeed, as failbit becomes set in the sourcefile stream. sourcefile >> node -> id; fails because it tries to read an integer but encounters “DAVID” in the stream. That happens because sourcefile >> reader; consumes “MARK”, sourcefile >> node -> name; consumes “29”, so sourcefile >> node -> id; is then left with “DAVID”. Try replacing sourcefile >> node -> name; with node -> name = reader.

And yes, you can’t use strings in switch, only integral and enumeration expressions.

On another offtopic note, you don’t seem to release the memory allocated for nodes (easy solution: just use std::list).

EDIT: Here’s how your program might look like when if you used std::list:

#include <iostream>
#include <fstream>
#include <string>
#include <list>

using namespace std;  

struct data
{
 string name;
 int id;
};

ifstream sourcefile;

int main()
{
 list< data > datalist;

 string input;
 string reader;
 sourcefile.open("A3.txt");

 sourcefile >> input;

 cout << "Received command: " << input << endl;

 if(input == "%INSERT")
 {
  // unlike the other ones, this function keeps reading until it hits another command
  sourcefile >> reader;

  cout << "Insert name: " << reader << endl;

  while(reader[0] != '%')
  {
   data d;
   d.name = reader;
   sourcefile >> d.id;
   datalist.push_back( d );

   sourcefile >> reader;
   cout << "reader: " << reader << endl;
  }
 }
}