Question

I’m confused with this snippet of code:

t=0
while  t<5:  #currently loop runs for 10 seconds
    print "in loop",t
    if (t<5):
        print "true"
    t=t+0.01

prints in the last run through of the loop:

in loop 5.0 
true

Now if it is true that t = 5.0 on the last pass through the loop, shouldn’t the condition that t < 5 in the if statement not be met? And furthermore, shouldn’t it not even be running through the loop for t=5 since it should have also failed the while condition?

Answer 1

5 isn’t necessarily 5:

t=0
while  t<5:  #currently loop runs for 10 seconds
    print "in loop",t, repr(t)
    if (t<5):
        print "true"
    t=t+0.1

produces

in loop 0 0
true
in loop 0.1 0.1
true
in loop 0.2 0.2
true
in loop 0.3 0.30000000000000004

[…]

in loop 4.8 4.799999999999999
true
in loop 4.9 4.899999999999999
true
in loop 5.0 4.999999999999998
true

0.1 can’t be represented exactly in binary.

[Ah, I just noticed that I used 0.1 instead of 0.01, like you did. Well, it’s the same issue.]

Two “how floating point works” references: classical and gentler.