Compute a piecewise straight line through its arguments that passes through a set of ordered control points.

For variables less than the first (greater than the last) point, the value of the first (last) point is used.

\[ \frac{y_{i+1}-y_i}{x_{i+1}-x_i}(s-x_i)+y_i ; if x_i<s<x_{i+1} \]

\[ y_N ; if x>x_{N-1} \]

\[ y_1 ; if x<x_0 \]

Control points are passed using the POINT0=... POINT1=... syntax as in the example below

If one argument is supplied, it results in a scalar quantity. If multiple arguments are supplied, it results in a vector of arguments.

Compulsory keywords

ARG

the input for this action is the output from one or more other actions. The particular output that you used is referenced using that action of interests label. If the label appears on its own then the value of the relevant Action is taken. If * or *.* appears the information from all arguments is taken. Some actions have multi-component outputs, each component of the output has a specific label so for instance an action labelled dist may have three componets x, y and z. To take just the x component you should use dist.x, if you wish to take all three components then use dist.*

POINT

This keyword is used to specify the various points in the function above. You can use multiple instances of this keyword i.e. POINT1, POINT2, POINT3...

Options

NUMERICAL_DERIVATIVES

( default=off ) calculate the derivatives for these quantities numerically

Examples

dist1: DISTANCE ATOMS=1,10
dist2: DISTANCE ATOMS=2,11

pw: PIECEWISE POINT0=1,10 POINT1=1,PI POINT2=3,10 ARG=dist1
ppww: PIECEWISE POINT0=1,10 POINT1=1,PI POINT2=3,10 ARG=dist1,dist2
PRINT ARG=pw,ppww.1,ppww.2

(See also PRINT and DISTANCE).