Lua for loop does not do all iterations

Question

I am new to lua, and i am using it to automate some tasks in the simulation program femm. In my script i have this type of for loop:

for i=0.1,0.3,0.1
do
  print(i)
end

The problem is it only iterates from 0.1 to 0.2(it does not enter i=0.3).I tried with other values (for example from 0.1 to 0.4) and it works properly. Why does this strange behaviour happen? Is this a floating point number problem?

Is this because it is not declared as an integer(i have not used lua before)? Should i use only integer values for the loop variable? — user19955
– user19955, Commented Sep 11, 2017 at 11:16
it sounds like you may need a primer in floating point operations — Mike Corcoran
– Mike Corcoran, Commented Sep 11, 2017 at 14:16

Sergey Kalinichenko · Accepted Answer · 2017-09-11 11:24:34Z

3

This happens because adding 0.1 to 0.1 three times produces a number that is slightly greater than 0.3. Hence the loop stops before reaching your target end number.

This is the danger of using floating point values for loop iteration. Rewrite the loop in integers instead, and perform a division to get your required number:

for j = 1,3
do
    i = j/10
    print(i)
end

Demo.

answered Sep 11, 2017 at 11:24

Sergey Kalinichenko

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