Question

why does this function work
```
function is_composite(n){
return "1".repeat(n).match(/^1?$|^(11+?)\1+$/);
}
```

https://jsfiddle.net/dy8t4vou/

Answer 1

@ParthKohli

Answer 2

or anyone

Answer 3

woah lol what is that so cool

Answer 4

what language?

Answer 5

javascript, try it in ur browser console

Answer 6

should be `\( \)`

Answer 7

don't really know what you are trying to make it look like if it is a latex question....

Answer 8

dang that looks so beautiful haha, i wish i knew what that syntax meant

Answer 9

i can't help. don't really get the result you desire to get.

Answer 10

screen shot or pic? (you can quite well draw it in colors and everything else with wio's extension for chrome which I am sure you have heard about...)

Answer 11

whatever. won't interrupt.

Answer 12

@SolomonZelman

Answer 13

how much of the syntax are u able to understand?

Answer 14

you want to make that picture

\(\normalsize\color{blue}{ 4 }\)
\(\normalsize\color{blue}{ ^\text{______________________________________} }\)
\(\normalsize\color{blue}{ 6 }\)
\(\normalsize\color{blue}{ ^\text{______________________________________} }\)
\(\normalsize\color{blue}{ 8 }\)
\(\normalsize\color{blue}{ ^\text{______________________________________} }\)
\(\normalsize\color{blue}{ 9 }\)
\(\normalsize\color{blue}{ ^\text{______________________________________} }\)
\(\normalsize\color{blue}{ 10 }\)
\(\normalsize\color{blue}{ ^\text{______________________________________} }\)
\(\normalsize\color{blue}{ 12 }\)
\(\normalsize\color{blue}{ ^\text{______________________________________} }\)

??

Answer 15

I'm able to follow it till "why does this function work"

Answer 16

(I am not very good at this)

Answer 17

lool

Answer 18

Haha typical programmer from math background

Answer 19

It's just a function that takes parameter "n" and returns "1" if the function is composite and "0" if it's not, i.e., it's prime.

Answer 20

You got it! you're ready to use the function !

Answer 21

Looks like it checks if it contains certain keywords in its binary representation.

Answer 22

thats very close, it actually converts the given number into unary number system

Answer 23

Or hex representation? I don't know.

Answer 24

Oh, unary...

Answer 25

5 = 11111
3 = 111

etc

Answer 26

"1".repeat(n) produces a string of "n" ones

Answer 27

All the percentages and pluses and question-marks... oh dear.

Answer 28

for simplicty, we can condence it further. below also works for n > 1 :

```
function is_composite(n){
return "1".repeat(n).match(/^(11+)\1+$/);
}
```

Answer 29

The problem is that I don't really know what that is.
What I think here is that you should first generate the number (parameter n) by repeating 1 'n' times, then start a number that is not 1 or the number itself, keep repeating it and reach the parameter. If such a number exists, then the thing is composite.

Answer 30

Like it should definitely be a loop, but I don't know where it mentions it being a loop. What I see is that the .match is attached to the .repeat so does the .repeat function tell it to repeat or what?

Answer 31

consider an example in unary system

`1111` is composite becuase it can be split as `11 11`

Answer 32

`11111` is not composite because it cannot be split as 2 ones or 3 ones or 4 ones groups

Answer 33

Yeah, makes sense. Where does it say that in the program? :|

Answer 34

match()

is a string matching function

Answer 35

it returns the match if it finds even groups of (11)+

otherwise it returns null

Answer 36

? looks like it's a conditional operator.

Answer 37

for example :

"parthkohli".match(/java/)

returns null

Answer 38

"parthkohli".match(/art/)

returns true

Answer 39

OK, I gotcha. Of course. This thing is looping over the size of the groups, right? So I'm sort of understanding this thing. I guess it says something like, "does it form even groups of this size? If not, proceed to the next size." Is that what (11)+ means?

Answer 40

`11+` I mean

Answer 41

Exactly!

Answer 42

\(a+\)

means 1 or more occurrences of \(a\)

Answer 43

`/^1?$|^(11+?)\1+$/`

Could you walk us through each and every symbol above? Also, please do it quickly, because my battery is gonna finish in 10 minutes...

Answer 44

lets look at the condensed expression
`/^(11+?)\1+$/`

Answer 45

`^` matches start of string

`$` matches end of string

Answer 46

`11+?` matches "two" or more occurrences of "1"

Answer 47

`\1` backreferences the previous matched stuff in a parenthesis : `(11+?)`

Answer 48

look up "regular expressions" for more details
their syntax mostly same in almost all programming languages

Answer 49

That is amazing. Wow.

Answer 50

OK, good night then. Laterz.

Answer 51

gnite!