Why for with float step dont do last iteration? - php

Question

Why does my for loop with a float step (0.1) don't do the last iteration in PHP - any version?
My test code:
$procents_list = [];
for($i = 0.5; $i <= 1.5; $i += 0.1)
{
$procents_list[] = $i;
}
print_r($procents_list);
Iteration from 5 to 15 with step 1 all ok.
Why that print:
Array
(
[0] => 0.5
[1] => 0.6
[2] => 0.7
[3] => 0.8
[4] => 0.9
[5] => 1
[6] => 1.1
[7] => 1.2
[8] => 1.3
[9] => 1.4
)

Answer 1

Computers use Base-2 (0,1) instead of Base-10 (0,1,2,...9) to represent numbers. In Base-10, some numbers cannot be represented exactly (like 1/3=0.33333). In Base-2, a different set of numbers cannot be represented exactly. In fact, 1/10, which is 0.1 in decimal, has no exact representation in binary. So, math with floats can do weird things and should never be taken as exact values.
I couldn't figure out how to get PHP to print out all the digits without rounding, but writing the same example in Java prints out the following values:
0.5
0.6
0.70000005
0.8000001
0.9000001
1.0000001
1.1000001
1.2000002
1.3000002
1.4000002
So, the value you were getting was slightly larger than 1.5 and the loop thus ended.
In this case, one alternative is to just use integers for the loop and divide by 10 when putting them in the array:
$procents_list = [];
for ($i = 5; $i <= 15; $i += 1)
{
$procents_list[] = $i / 10;
}
print_r($procents_list);