# external

Scilab Object, external function or routine

### Description

External function or routine for use with specific commands.

An "external" is a function or routine which is used as an argument
of some high-level primitives (such as `ode`

, `optim`

, `schur`

...).

The syntax of the external (function or routine) is imposed by the high-level primitive which sets the arguments of the external.

For example the external function `costfunc`

is an argument of the `optim`

primitive. Its syntax must be: `[f,g,ind]=costfunc(x,ind)`

and
`optim`

(the high-level optimization primitive) is invoked as follows:

optim(costfunc,...)

Here `costfunc`

(the cost function to be minimized by the primitive `optim`

)
evaluates `f=f(x)`

and `g=`

gradient of `f`

at `x`

(`ind`

is an integer. Its use is precised in the `optim`

help).

If other values are needed by the external function these variables can be defined in its environment. Also, they can be put in a list. For example, the external function

[f,g,ind]=costfunc(x,ind,a,b,c)

is valid for `optim`

if the external is `list(costfunc,a,b,c)`

and
the call to `optim`

is then:

optim(list(costfunc,a1,b1,c1),....

An external can also be a Fortran or C routine : this is convenient to speed up the computations.

The name of the routine is given to the high-level primitive as a character string. The syntax of the routine is also imposed.

External Fortran or C routines can also be dynamically linked (see `link`

).

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