Julia: Floats sum is wrong?

Question

How can I make sure that by adding 0.2 at every iteration I get the correct result?

some = 0.0
for i in 1:10
    some += 0.2
    println(some)
end

the code above gives me

0.2
0.4
0.6000000000000001
0.8
1.0
1.2
1.4
1.5999999999999999
1.7999999999999998
1.9999999999999998

Answer 1

Floats are only approximatively correct and if adding up to infinity the error will become infinite, but you can still calculate with it pretty precisely. If you need to evaluate the result and look if it is correct you can use isapprox(a,b) or a ≈ b.

I.e.

some = 0.
for i in 1:1000000
    some += 0.2
end
isapprox(some, 1000000 * 0.2)
# true

Otherwise, you can add integer numbers in the for loop and then divide by 10.

some = 0.
for i in 1:10
    some += 2.
    println(some/10.)
end
#0.2
#0.4
#0.6
#0.8
#1.0
#1.2
#1.4
#1.6
#1.8
#2.0

More info about counting with floats: https://en.wikipedia.org/wiki/Floating-point_arithmetic