Question

I have a simple for loop in Perl

for ($i=0; $i <= 360; $i += 0.01)
{
print "$i ";
}

Why is it that when I run this code I get the following output, where as soon as it gets to 0.81 it suddenly starts to add in a load more decimal places? I know I could simply round up to avoid this issue but I was wondering why it happens. An increment of 0.01 does not seem at all crazy to do.

 0.77
 0.78
 0.79
 0.8
 0.81
 0.820000000000001
 0.830000000000001
 0.840000000000001
 0.850000000000001
 0.860000000000001
 0.870000000000001

Answer 1

Computers use binary representations. Not all decimal floating point numbers have exact representations in binary notation, so some error can occur (its actually a rounding difference). This is the same reason why you shouldn’t use floating point numbers for monetary values:

(Picture taken from dailywtf)

Most elegant way to get around this issue is using integers for calculations, dividing them to the correct number of decimal places and using sprintf to limit the number of decimal places printed. This will both make sure:

• There’s always to correct result printed
• The rounding error doesn’t accumulate

Try this code:

#!/usr/bin/perl
for ($i=0; $i <= 360*100; $i += 1) {
  printf "%.2f \n", $i/100;
}