isnan check for "Not a Number" entries

## Syntax

    r=isnan(x)

## Arguments

x

  real or complex vector or matrix r : boolean vector or matrix

## Description

`isnan(x)` returns a boolean vector or matrix which contains true entries corresponding with "Not a Number" `x` entries and false entries corresponding with regular `x` entries.

In this paragraph, we give the reason of the existence of this function. Consider the floating point number `x` and assume that we want to know if this number is a Not A Number. The condition `(x==%nan)` is false if `x` is equal to `%nan`. This is why the function `isnan` is helpful: it allows to see if a floating point number is a NaN, without actually comparing its value against `%nan`.

## Examples

    isnan([1 0.01 -%nan %inf-%inf])

In the following example, we see that `%nan` has very particular comparison properties and compare the result with the `isnan` function.

        %nan == %nan
        %nan <> %nan
        isnan(%nan)

In the following example, we extract the non-NaNs values `y` from the array `x`.

    x = [1 2 %nan 3 %nan 4]
    k = find(~isnan(x))
    y = x(k)

## See also

- isinf