# NAG Library Routine Document

## 1Purpose

f06klf determines the index of the first negligible element of a real vector.

## 2Specification

Fortran Interface
 Function f06klf ( n, x, incx, tol)
 Integer :: f06klf Integer, Intent (In) :: n, incx Real (Kind=nag_wp), Intent (In) :: x(*), tol
#include nagmk26.h
 Integer f06klf_ (const Integer *n, const double x[], const Integer *incx, const double *tol)

## 3Description

f06klf finds the first element of the $n$-element real vector $x$ for which
 $xk+1≤tolmaxx1,…,xk$
and returns the index $k$ via the function name. If no such $k$ exists, then the value $n$ is returned. If a negative value of $\mathit{tol}$ is supplied, the value of machine precision is used in place of $\mathit{tol}$.

None.

## 5Arguments

1:     $\mathbf{n}$ – IntegerInput
On entry: $n$, the number of elements in $x$.
2:     $\mathbf{x}\left(*\right)$ – Real (Kind=nag_wp) arrayInput
Note: the dimension of the array x must be at least $\mathrm{max}\phantom{\rule{0.125em}{0ex}}\left(1,1+\left({\mathbf{n}}-1\right)×{\mathbf{incx}}\right)$.
On entry: the $n$-element vector $x$. ${x}_{\mathit{i}}$ must be stored in ${\mathbf{x}}\left(1+\left(\mathit{i}-1\right)×{\mathbf{incx}}\right)$, for $\mathit{i}=1,2,\dots ,{\mathbf{n}}$.
Intermediate elements of x are not referenced.
3:     $\mathbf{incx}$ – IntegerInput
On entry: the increment in the subscripts of x between successive elements of $x$.
Constraint: ${\mathbf{incx}}>0$.
4:     $\mathbf{tol}$ – Real (Kind=nag_wp)Input
On entry: the value $\mathit{tol}$.

None.

Not applicable.

## 8Parallelism and Performance

f06klf is not threaded in any implementation.