Why is 02000 evaluated to 1024

Question

Out of curiosity, I wrote a program like this:

auto number1 = 100;
auto number2 = 02000;
auto number3 = 2;

auto result = (number1 + number2) / number3;

std::cout << result;

Interestingly, the program outputs 562. So, in visual studio, i hovered over the variable "number2" and it showed (int) 1024. I didn't understand why this happened. So, I tried its equivalent in php which was this:

$number1 = 100;
$number2 = 02000;
$number3 = 2;

$result = ($number1 + $number2) / $number3;

echo $result;

The result was the same 562. What is it that I am missing because if I remove the zero in front of number2 to make it '2000', it shows 1050 as expected

Leads to an interesting side note: In C++ 0 is explicitly stated to be octal, but in PHP it's decimal. — user4581301, Jun 28 '18 at 05:43
@smith, should I delete this question because it is sure a duplicate? — Hemil, Jun 28 '18 at 05:59

score 4 · Answer 1 · answered Jun 28 '18 at 05:29

It treats 02000 as octal number and converts to decimal. Anythings that starts with preceding 0 is considered octal number.

0 2 0 0 0 = 0*8*8*8*8 + 2*8*8*8 + 0*8*8 + 0*8 + 0
          = 0 + 2*512 + 0 + 0 + 0
          = 1024

Now 1024 is number2.

result = (100 + 1024)/2
       = 1124/2
       = 562

score 2 · Answer 2 · edited Jun 28 '18 at 06:55

2

~~As auto is used,~~ Number2 is considered as an octal number as it started with 0.
So number2=1024, number1=100, and number3=2.
thats why result is 562.

Edit:

learned a new thing ! Auto has nothing to do here ! It is just the preceding 0 .

edited Jun 28 '18 at 06:55

ArchLinuxTux

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answered Jun 28 '18 at 05:30

mahbubcseju

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Note, `auto` has nothing to do with it – valiano Jun 28 '18 at 05:40
@mahbubcseju what if auto is not used? Will it not be treated as octal? – Jun 28 '18 at 05:40
OK ! I got it just now ! It is just for preceding 0 ! Thanks . learned a new thing :) – mahbubcseju Jun 28 '18 at 05:47

score 0 · Accepted Answer · answered Jun 28 '18 at 05:33

Hey Hemil in PLs number system convention if you use 'zero' as a prefix, number will be considered as octal number but compiler will convert it into decimal number system and gives output in decimal. 02000 is in octal and its decimal value will be 1024.

Why is 02000 evaluated to 1024

3 Answers3