Problem with reading from file causing infinite loop

问题

Ok this program I am working on seems to be all ok except there is a problem. Here is the code

#include <iostream>
#include <fstream>

using namespace std;

/*
Function Name: CalculateBinary
CalculateBinary takes a number from the main function and finds its binary form.
*/

void CalculateBinary( long InputNum)
{   
    //Takes InputNum and divides it down to "1" or "0" so that it can be put in binary form.
    if ( InputNum != 1 && InputNum != 0)
        CalculateBinary(InputNum/2);

    // If the number has no remainder it outputs a "0". Otherwise it outputs a "1". 
    if (InputNum % 2 == 0)
        cout << "0";
    else
        cout << "1";
}


void main()
{
    // Where the current number will be stored
      long InputNum;

    //Opens the text file and inputs first number into InputNum. 
    ifstream fin("binin.txt");
    fin >> InputNum;

    // While Input number is not 0 the loop will continue to evaluate, getting a new number each time.
    while (InputNum >= 0)
    {
        if(InputNum > 1000000000)
            cout << "Number too large for this program ....";
        else
            CalculateBinary(InputNum);

        cout << endl;
        fin >> InputNum;        
    }
}

Here is the text file I am reading in

12
8764
 2147483648
2
-1

When I get to 8764, it just keeps reading in this number over and over again. It ignores the 2147483648. I know I can solve this by declaring InputNum as a long long. But I want to know why is it doing this?

回答1:

That is the usual problem with such loops which you've written.

The correct and the idiomatic loop is this:

ifstream fin("binin.txt");
long InputNum;
while (fin >> InputNum && InputNum >= 0)
{
   //now construct the logic accordingly!
    if(InputNum > 1000000000)
         cout << "Number too large for this program ....";
    else
         CalculateBinary(InputNum);
    cout << endl;
}

回答2:

That number is too large for a long to store, so fin >> InputNum; does nothing. You should always read as while(fin >> InputNum) { ... }, as that will terminate the loop immediately on failure, or at least check the stream state.

回答3:

It would appear that the long type on your platform is 32 bits wide. The number 2147483648 (0x80000000) is simply too large to be represented as a signed 32-bit integer. You either need an unsigned type (which obviously won't work with negative numbers) or a 64-bit integer.

Also, you should check whether the read is successful:

  ...
  cout << endl;
  if (!(fin >> InputNum)) break; // break or otherwise handle the error condition
}

回答4:

You don't check for EOF, thus being trapped in a loop forever. fin >> InputNum expression returns true if succeeded, false otherwise, so changing you code to something like this will solve the problem:

while ((fin >> InputNum) && InputNum >= 0)
{
  // ...
}

来源：https://stackoverflow.com/questions/7405608/problem-with-reading-from-file-causing-infinite-loop