Page 416 - Excel for Scientists and Engineers: Numerical Methods

P. 416

APPENDIX 3 CUSTOM FUNCTIONS HELP FILE 393

InterpC2
Performs cubic interpolation in a 2-way table of x-, y- and z-values. x and y are the
independent variable, z is the dependent variable. Returns the interpolated z-value
corresponding to a specified x-value.

Syntax:
InterpC2 (x-lookup, y-lookup, known-x Is, known-y's, known-2's)

x-lookup the x-value for which you want to find the interpolated z-value
y-lookup the y-value for which you want to find the interpolated z-value
known-x's the set of x-values in the table (independent values)
known-y 's the set of y-values in the table (independent values)
kno wn-z's the set of z-values in the table (dependent values)

Remarks
x-lookup and y-lookup can be either numbers or references to a cell that contains a
number.
The function cannot handle implicit references; that is, a name or range reference
cannot be used for a range of lookup values.
The values in the table of x- , y- and z-values must be numbers.
The table must be arranged in ascending order of both x-values and y-values.
The function cannot be used for extrapolation. An x-lookup value that is either
greater than or less than the range of x-values, or a y-lookup value that is either
greater than or less than the range of y-values returns #REF!.
The function uses the LaGrange 4th-order polynomial. See InterpC for details.

Example
= InterpC2(K7,L7,$A$4:$A$29,$6$3:$1$3,$6$4:$1$29) where K7 is a reference to the
x-lookup value, L7 is a reference to the y-lookup value, $A$4:$A$29 is the range
containing the independent x-values, $6$3:$1$3 is the range containing the independent
y-values and $6$4:$1$29 is the range containing the dependent or z-values.