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interp proto



interp -- compute an interpolated value from a table of x,y pairs


interp tbl_file


Text file containing X,Y pairs comprising the table. The pairs must be in either ascending or descending order.
curve_gen = no
If set to no, x-values are read from the file(s) specified by the parameter "input". If set to yes, the parameters x1, x2, and dx are used to create a list of new x,y pairs interpolated at x1, x1+dx, ... x2.
input = STDIN
File(s) containing x-values for the interpolation
int_mode = 'linear'
The interpolation mode may be either linear or spline.
The starting x-value for generating a series of new x,y pairs.
The ending x-value of the generated series of pairs.
The difference by which the x-values are incremented during the series generation.


The pairs of X,Y values are read from the tbl_file. There must be at least 1 pair in the file. The table is then used to interpolate or extrapolate new y-values for given x-values. The x-values may come from a file including STDIN (if curve_gen=no), or they may be internally generated (if curve_gen=yes) to produce a finely sampled version of the table. This may be useful for plotting a smooth curve through a series of points.

The table X,Y values must be in a monotonic order, either ascending or descending. No restriction is made on spacing.

If only one point is present in the table, all returned interpolated values will have the value at that point. If only two points are present, linear interpolation (or extrapolation) will be used. If additional points are present, an obscure but reliable algorithm is used to interpolate (or extrapolate).


1. The following command reads the X,Y table from file testdata and waits for x-values from the terminal.

    cl> interp testdata STDIN

2. The following command generates points to plot (by piping to graph) in the range from x=10 to x=20 at intervals of 0.1 (10.0, 10.1 ... 19.9, 20.0).

    cl> interp testdata curve_gen=yes x1=10 x2=20 dx=.1 | graph

3. The curve will be displayed and the original points from the table may be overlaid by:

    cl> graph testdata pointmode=yes append=yes


If a blank (null) table filename is entered, a floating divide error occurs.

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