Incorrect output using array_diff() when integer values are left padded with zeros

Question

The following code works fine with the supplied array.

$arr1 = array(1,2,4,6,7,9,13,15); 

$arr2 = range(1,max($arr1));                                                    

$missing = array_diff($arr2,$arr1); 

foreach ($missing as $key => $val) {
    echo $val;
    echo ",";
}

But if I change $arr1 to contain 001,002,004,006,007,009,013,015, it gives this output: 003,005,008,009,010,012,

I format the output by putting this in the foreach loop:

$val = str_pad($val, 3, "0", STR_PAD_LEFT);

I cannot figure out why it declares 009 to be missing; why it doesn't declare 011 and 014 to be missing; and why it concludes with a comma.

Is there a way to rewrite the code so it processes the array correctly?

Is there a way to rewrite the code so it processes the array correctly? — Benjamin
– Benjamin, Commented Oct 12, 2016 at 15:11

jakub wrona · Accepted Answer · 2016-10-12 19:31:09Z

2

This is because numbers starting with 0 are octal numbers:

http://php.net/manual/pl/language.types.integer.php

To solve this you may store your values in the original array as strings:

$arr = ['001', '002', '005', '011', '007'];

then

echo max($arr); //011

still finds the right one, so now just:

for ($i=1; $i<=ltrim(max($arr), '0'); $i++) {
    $newArr[] = str_pad($i, 3, '0', STR_PAD_LEFT);
}

And then the diff is OK:

print_r(array_diff($newArr, $arr));
Array
(
    [2] => 003
    [3] => 004
    [5] => 006
    [7] => 008
    [8] => 009
    [9] => 010
)

TESTED ON PHP7

Reference:

edited Oct 12, 2016 at 19:31

answered Oct 12, 2016 at 15:08

jakub wrona

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1 Comment

Benjamin Over a year ago

I tried using ' ' marks around the values but it didn't work. I'll try that again.

LordWilmore · Accepted Answer · 2016-10-12 16:12:37Z

0

Your leading zeroes have made the numbers octal.

answered Oct 12, 2016 at 16:12

LordWilmore

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6 Comments

Benjamin Over a year ago

Is there a way to handle octals?

LordWilmore Over a year ago

Octal, decimal, hexadecimal, etc, are just ways of presenting a number, just the same as binary. We present hexadecimal numbers by putting 0x on the front, and we present octal numbers by putting a zero on the front. So when you have a value 011 in your code you are explicitly declaring a value equivalent to the decimal value 9. Why do you have to put the leading zeroes? If you need that for some tabulated results then I suggest adding leading zeroes to the string output, but leave the input as decimal.

Benjamin Over a year ago

The leading zeroes are coming from the database. They are part of an exam number which has the structure ALPHA-ALPHA-NUMERIC-NUMERIC-NUMERIC-ALPHA-ALPHA

Benjamin Over a year ago

So, for example GC005ABX. Candidates are uniquely identified from 001 - 999; the other information relates to exam centre, level and year. I parse the exam numbers and loop through 001 to 999 and generate correspondence and certificates for each candidate. If a candidate pulls out - say GC035ABX - I want to be able to detect that gap in the sequence and omit them from further actions. Sequencing from 1,2,3 .... 999 won't reference the actual exam numbers in the system.

LordWilmore Over a year ago

If you're converting from a string then consider using the intval function (php.net/manual/en/function.intval.php), which takes the base as an optional parameter. The default value is 10, i.e. decimal, but it won't hurt to specify that explicitly.

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