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## 4.8 Predicates for Numeric Objects

Since the type of a variable may change during the execution of a
program, it can be necessary to do type checking at run-time. Doing this
also allows you to change the behavior of a function depending on the
type of the input. As an example, this naive implementation of `abs`

returns the absolute value of the input if it is a real number, and the
length of the input if it is a complex number.

function a = abs (x) if (isreal (x)) a = sign (x) .* x; elseif (iscomplex (x)) a = sqrt (real(x).^2 + imag(x).^2); endif endfunction |

The following functions are available for determining the type of a variable.

__Function File:__**issquare***(*`x`)If

`x`is a square matrix, then return the dimension of`x`. Otherwise, return 0.**See also:**size, rows, columns, length, ismatrix, isscalar, isvector.

__Function File:__**issymmetric***(*`x`,`tol`)If

`x`is symmetric within the tolerance specified by`tol`, then return the dimension of`x`. Otherwise, return 0. If`tol`is omitted, use a tolerance equal to the machine precision. Matrix`x`is considered symmetric if`norm (`

.`x`-`x`.', inf) / norm (`x`, inf) <`tol`**See also:**size, rows, columns, length, ismatrix, isscalar, issquare, isvector.

__Function File:__**isdefinite***(*`x`,`tol`)Return 1 if

`x`is symmetric positive definite within the tolerance specified by`tol`or 0 if`x`is symmetric positive semidefinite. Otherwise, return -1. If`tol`is omitted, use a tolerance equal to 100 times the machine precision.**See also:**issymmetric.

__Function File:__**isprime***(*`n`)Return true if

`n`is a prime number, false otherwise.Something like the following is much faster if you need to test a lot of small numbers:

`t`= ismember (`n`, primes (max (`n`(:))));If max(n) is very large, then you should be using special purpose factorization code.

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