Strange behavior of fmod()

[Schizoid Man]

I have the following code snippet where I am validating time in minutes
and hours entered in as a double variable.

If I enter the value 10.59, the value printed out is 10:58. For this
input, the correct value is being passed to 'temp', so I just can't
figure out why fmod() is misbehaving.

Any ideas?


double InputTime, temp;
int Hours, Minutes;

cout << "Enter the start time: ";
cin >> InputTime;

Hours = static_cast<int>(InputTime);
temp = InputTime - Hours;
temp = temp * 100.0;
Minutes = fmod(temp, 100);
cout << "Time is " << Hours << ":" << Minutes;

 

[Schizoid Man]

If I enter the value 10.59, the value printed out is 10:58. For this
input, the correct value is being passed to 'temp', so I just can't
figure out why fmod() is misbehaving.

Taking a closer look, it seems that this code works properly till 10.50.

For 10.50 the time is 10:50, so far so good.
For 10.51 the time is 10:50, which is very odd indeed.

I really would appreciate some help.

 

[Schizoid Man]

Taking a closer look, it seems that this code works properly till 10.50.

For 10.50 the time is 10:50, so far so good.
For 10.51 the time is 10:50, which is very odd indeed.

On further analysis, it seems that this problem only occurs if the input
is between 10.51 and 10.71.

This is very strange indeed. I really would appreciate any help at all.

Thanks.

 

[peter koch]

I have the following code snippet where I am validating time in minutes
and hours entered in as a double variable.

If I enter the value 10.59, the value printed out is 10:58. For this
input, the correct value is being passed to 'temp', so I just can't
figure out why fmod() is misbehaving.

Any ideas?


double InputTime, temp;
int Hours, Minutes;

cout << "Enter the start time: ";
cin >> InputTime;

Hours = static_cast<int>(InputTime);
temp = InputTime - Hours;
temp = temp * 100.0;
Minutes = fmod(temp, 100);
cout << "Time is " << Hours << ":" << Minutes;

You need to look-up floating-point precision.

/Peter

 

[Gernot Frisch]

double InputTime, temp;
int Hours, Minutes;

cout << "Enter the start time: ";
cin >> InputTime;

Hours = static_cast<int>(InputTime);
temp = InputTime - Hours;
temp = temp * 100.0;

Floating point representation problem.
temp = 0.5899999999999;

Minutes = fmod(temp, 100);

now Minutes = 58, which is correct.

Try:
Minutes = fmod(temp+.1, 100);

to get rid of the problem.