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# WITHIN

*n*--

_{1}| u_{1}n_{2}| u_{2}n_{3}| u_{3}*flag*)

Perform a comparison of a test value *n _{1} | u_{1}* with a
lower limit

*n*and an upper limit

_{2}| u_{2}*n*, returning

_{3}| u_{3}*true*if either (

*n*<

_{2}| u_{2}*n*and (

_{3}| u_{3}*n*<=

_{2}| u_{2}*n*and

_{1}| u_{1}*n*<

_{1}| u_{1}*n*)) or (

_{3}| u_{3}*n*>

_{2}| u_{2}*n*and (

_{3}| u_{3}*n*<=

_{2}| u_{2}*n*or

_{1}| u_{1}*n*<

_{1}| u_{1}*n*)) is true, returning

_{3}| u_{3}*false*otherwise. An ambiguous condition exists

*n*,

_{1}| u_{1}*n*, and

_{2}| u_{2}*n*are not all the same type.

_{3}| u_{3}

## See:

## Rationale:

**WITHIN**without mentioning circular number spaces (an undefined term) or providing the code. Here is a number line with the overflow point (

*o*) at the far right and the underflow point (

*u*) at the far left:

*u*---------------

*o*

*n*range straddles the overflow/underflow points or it does not. Lets examine the non-straddle case first:

_{2}| u_{2}... n_{3}| u_{3}*u*-----[.....)-----

*o*

*n*, the ) denotes

_{2}| u_{2}*n*, and the dots and [ are numbers

_{3}| u_{3}**WITHIN**the range.

*n*is greater than

_{3}| u_{3}*n*, so the following tests will determine if

_{2}| u_{2}*n*is

_{1}| u_{1}**WITHIN**

*n*and

_{2}| u_{2}*n*:

_{3}| u_{3}*n*

_{2}| u_{2}*<=*

*n*and

_{1}| u_{1}*n*<

_{1}| u_{1}*n*.

_{3}| u_{3}*u*.....)-----[.....

*o*

*n*is less than

_{3}| u_{3}*n*and the following tests will determine if

_{2}| u_{2}*n*is

_{1}| u_{1}**WITHIN**

*n*and

_{2}| u_{2}*n*:

_{3}| u_{3}*n*

_{2}| u_{2}*<=*

*n*or

_{1}| u_{1}*n*<

_{1}| u_{1}*n*.

_{3}| u_{3}**WITHIN**must work for both signed and unsigned arguments. One obvious implementation does not work: Assume two's-complement arithmetic on a 16-bit machine, and consider the following test:

`33000 32000 34000 WITHIN`

The above implementation returns *false* for that test,
even though the unsigned number 33000 is clearly within the
range {{32000 ... 34000}}.

The problem is that, in the incorrect implementation, the signed comparison < gives the wrong answer when 32000 is compared to 33000, because when those numbers are treated as signed numbers, 33000 is treated as negative 32536, while 32000 remains positive.

Replacing < with U< in the above implementation makes it work with unsigned numbers, but causes problems with certain signed number ranges; in particular, the test:

For two's-complement machines that ignore arithmetic overflow (most machines), the following implementation works in all cases:

## ContributeContributions

## Max sample implementation that can also be interpretedSuggested reference implementation2020-02-05 20:49:51

The current sample implementation uses `>R`

and `R>`

, which are compile-only (technically "Interpretation semantics for these words are undefined").

Instead `WITHIN`

is expected to work also when interpreting.

The following implementation fixes that, and is correct under the same condition "For two's-complement machines that ignore arithmetic overflow" as the current sample implementation:

```
: WITHIN ( test low high -- flag ) OVER - ROT ROT - U> ;
```