My while loop exits prematurely - c++

Question

thanks for reading this.
I am writing a code to read a big data file. And I try to use a while loop to read it one piece at a time.
But when I write
while(TimeStep++)
it will exit at the first loop.
if I write,
while(TimeStep+=1)
it will be just fine.
Also, if I initialize
int TimeStep=-1;
it will exit at the first loop. But if I initialize
int TimeStep=0;
it will be fine. The magic of while() confuse me. Please help me understand while loop.
Here is all my code.
//get diffusion of all the particles in the 256Coordinate.txt file and diffusion of a single particle.
using namespace std;
typedef vector<double> vec;
int ReadStructure(vec & Coordinate,int size,ifstream & TrajectoryFile){
double a;
for(int i=0;i<size*3;i++){
if(!(TrajectoryFile.eof())){
TrajectoryFile>>a;
Coordinate[i]=a;
}
}
//cout<<Coordinate[1]<<endl;
if(TrajectoryFile.eof()){
return 1;
} else {
return 0;
}
}
int main(){
int ContinueFlag=0,i,j,k;
double a,b,c;
vec Coordinate;
string filename= ("../256Coordinate.txt"); // a file that contains 256*5000*3 numbers
int size=256;
Coordinate.resize(size*3);
int TimeStep=0;
ifstream TrajectoryFile(filename.c_str());//open the .txt file and begin the read data
//TrajectoryFile>>a;
//cout<<a<<endl;
while(TimeStep+=1){//keep looping untils breaks.
ContinueFlag=ReadStructure(Coordinate,size,TrajectoryFile);//read the .txt file and store the values in the vector Coordinate[3*256]. Read 3
*256 numbers at a time.
// cout<<"ContinueFlag= "<<ContinueFlag<<endl;
if(ContinueFlag==1) break;//if we reach the end of the file, exit.
// cout<<Coordinate[1]<<endl;
}
cout<<"total number of timesteps= "<<TimeStep-1<<endl;
}

Answer 1

the body of while loop will execute when the loop condition under
while(loop condition)
is true.
So if you set TimeStep =0 to start with. It will test whether TimeStep ==0 before executing the while loop. Any non-zero value is treated as True. If it is 0, loop body will not execute.
If you initialize as int TimeStep=-1;, TimeStep+=1 will set TimeStep =0, which is equivalent to false, so loop body will not execute.
If you do not know the loop termination condition beforehand, simply use
while (true)
is better than using such a TimeStep variable.
Try:
while(true){
ContinueFlag=ReadStructure(Coordinate,size,TrajectoryFile);
if(ContinueFlag==1)
break;
}

Answer 2

In C++ the integer value 0 is False, any other value including negative integer is True. While loop exits when false.

Answer 3

I think your main problem is not understanding the while loop, it's understanding the increment operator ++.
Let's work with an example:
int x = 5;
int y = x++;
Here, x will have a value of 6 (because you made ++), but which value will y have? Actually, it will be 5. This is a so-called 'postincrement' operator: see, you assign first, and increment later.
If you wrote this
int x = 5;
int y = (x += 1);
Then you would have x = 6 as before, but this time y = 6 also, so you first increment x and only then assign it to y.
This should make your while loop misunderstanding go away:
int TimeStep = 0;
while(TimeStep++)
Here, TimeStep will get the value of 1, but only after it was used by while to test for exit, but while will see the old value (as y in the example above), and the old value is 0, so while exits immediately.
int TimeStep = 0;
while(TimeStep+=1)
In this case the loop goes on because you first increment the TimeStep and then let while test if it's nonzero.
I would really suggest you write a simple loop, why are you testing if TimeStep is nonzero anyway? Just do it like this:
while(true) { // Infinite cycle, until brake is encountered
TimeStep++;
}

Answer 4

The while loop expects a true/false value, according to that, TimeStep++ if TimeStep = -1 is false, because TimeStep++add 1 to TimeStep , so == 0, if TimeStep = 0and you add 1 then is ALWAYS true, because true is every value != 0...

Answer 5

I think you may need to get a better understanding of boolean algebra.
Here's a link to a tutorial http://www.electronics-tutorials.ws/boolean/bool_7.html.
A while loop is based around a boolean expression. If the expression within the while loop parentheses is true it will enter the loop and stop until that expression evaluates to false.
It works when the integer that you are using is set to 0 or 1 because 0 represents false and 1 represents true. You can't use an integer as a boolean expression if it is not 0 or 1.
It looks like you want the loop to break when ContinueFlag==1. So just use that as the while loop parameter. An alternative way would be to just change that code to while (true).
Since you want ContinueFlag to be set at least once (so you know when to break) I would suggest using a do while loop which executes at least once and then repeats if the expression is true.
USE THIS:
do {
ContinueFlag=ReadStructure(Coordinate,size,TrajectoryFile);
TimeStep++; //This allows for TimeStep to increment
} while (ContinueFlag!=1); //It will loop while ContinueFlag!=1 which will cause
//the loop to end when ContinueFlag==1
This is a better way of writing your code (as opposed to while (true)). This allows you to easily see what the purpose of the loop is.