For loop assigns wrong array index

Question

I am trying to create an array with the following structure:

[10] => Array
    (
        [10] => test key
        ...
        [100] => test key
    )

[20] => Array
    (
        [10] => test key
        ...
        [100] => test key
    )

[30] => Array
    (
        [10] => test key
        ...
        [100] => test key
    )
...

This is the loop that I use in order to create such structure:

$array = array();
for ($x = 0.1; $x <= 1; $x+=0.1) {
    $index = $x*100;
    for ($z = 10; $z <= 100; $z+=10) {
        $array[(int)$index][$z] = 'test key';
    }
}

Unfortunately, the output that I receive looks like this once it is supposed to reach the 90 and 100 index:

[80] => Array
    (
        [10] => test key
         ...
        [100] => test key
    )

[89] => Array
    (
        [10] => test key
         ...
        [100] => test key
    )

[99] => Array
    (
        [10] => test key
         ...
        [100] => test key
    )

Why does it calculate the index to be 89 and 99? Shouldn't it be 90 and 100 respectively?

*edit I am using floats as that's the measurement of percentages that we use to make some calculations. I know I could go by 10 increments as well, but that would require me to eventually divide by 100 again in order to get the float

Read, learn and understand; then appreciate that PHP rounds down when casting from float to int — Mark Baker
– Mark Baker, Commented Feb 20, 2017 at 23:52
I am using floats as that's the measurement of percentages that we use to make some calculations. — Chris Palmer Breuer
– Chris Palmer Breuer, Commented Feb 20, 2017 at 23:59

dana · Accepted Answer · 2017-02-21 00:23:13Z

2

Not sure why you are using floats with $x... Just start at 10 and step by 10 until you hit 100.

$array = array();
for ($x = 10; $x <= 100; $x+=10) {
    $f = ((float)$x) / 100.0; # calculate float value here
    for ($z = 10; $z <= 100; $z+=10) {
        $array[$x][$z] = 'test key';
    }
}

As others have pointed out, what you are seeing is most likely due to floating-point arithmetic rounding.

edited Feb 21, 2017 at 0:23

answered Feb 20, 2017 at 23:53

dana

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2 Comments

Chris Palmer Breuer Over a year ago

I am using floats as that's the measurement of percentages that we use to make some calculations. I know I could go by 10 increments as well, but that would require me to eventually divide by 100 again in order to get the float

dana Over a year ago

Got it... In that case, convert from int to float when needed. See updated answer.

Alfonso Jaquez · Accepted Answer · 2017-02-20 23:56:48Z

0

Why does it calculate the index to be 89 and 99? Shouldn't it be 90 and 100 respectively?

I think that it occurs because you use a decimal for the x value, why you don't use an integer value?

$array = array();
for ($x = 10; $x <= 100; $x+=10) {
    for ($z = 10; $z <= 100; $z+=10) {
        $array[$x][$z] = 'test key';
    }
}

answered Feb 20, 2017 at 23:56

Alfonso Jaquez

10312 bronze badges

1 Comment

Chris Palmer Breuer Over a year ago

I am using floats as that's the measurement of percentages that we use to make some calculations. I know I could go by 10 increments as well, but that would require me to eventually divide by 100 again in order to get the float

junkfoodjunkie · Accepted Answer · 2017-02-20 23:56:57Z

Slightly more modern syntax:

<?php

$array = [];
for ($x = 10; $x <= 100; $x+=10) {
    for ($z = 10; $z <= 100; $z+=10) {
        $array[$x][$z] = 'test key';
    }
}

print_r($array);
?>

Outputs:

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