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Comment by joshumax | Hacker News Reader

Cyberferrit said new programmers to the project, not specifically new to the language or programming itself.

Working on emulation software I commonly run across snippets such as `res = !!iRegisterVal` which results in some C newbies scratching their heads as to why a useless double negation would be there. But for more seasoned developers its intent can pretty easily recognized...

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Why is !! useful? Asking as a ruby programmer.

It is a shorthand method of casting to a boolean value - C didn't have a true bool type until "recently" (but did have the concept of truthiness, where true is `is not 0`), so many projects and libraries use `#define TRUE 1`, and #define FALSE 0` or some similar variant.

So rather than write `foo ? TRUE : FALSE` or being more explicit `(foo != 0) ? 1 : 0`, you can just do `!!foo`.

It's a quick cast to a boolean.

Because Ruby doesn't have a to_boolean method. !! is used to convert "truthy" or "falsy" values [0] into true boolean true or false values.

For example,

    def user_signed_in?
      !!current_user
    end

This returns "true" or "false", instead of a (truthy) User object or a (falsy) nil.

[0] https://gist.github.com/jfarmer/2647362

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