Single-variable linear level finding ("inverse" INTERP1): findX

Estimates the XI values at which the dependent variables Y reach a specified level YI.

Description

XI = findX(X,Y,YI) estimates the XI values at which the dependent
variables Y reach or cross a specified target level YI (scalar value). If there are multiple solutions, findX finds all of them. It may be seen as the "inverse" operation of the MATLAB's function
INTERP1, under 'linear' method.

X is a vector containing a sequence of monotone increasing values of the independent variable. Y is a vector containing the discrete values of the dependent variable (underlying function).

[XI,IDEXACT] = findX(X,Y,YI) returns in IDEXACT the indices where the original values of Y exactly reach the target level YI, i.e. Y(IDEXACT)=YI, so interpolation was not needed.

• MATLAB release: MATLAB 6.5 (R13)

[Darren Rowland]

The first few lines of this file are

% findX.m
%
% Antoni J. Canós, ITACA Institute - Technical University of Valencia
%
% Based on find_cross.m by
% Phillip M. Feldman, Northrop Grumman Space Technology
% Version 2.0 Nov. 9, 2005
% in http://www.mathworks.com/matlabcentral/fileexchange/8975
% (File ID: #8975)

Features a large number of other poor comments such as
% idx should be a very small array, so we can afford to use a loop

when no loop is present.

[Darren Rowland]

This file is now much improved, featuring a H1 line and useful descriptions. If I could suggest some areas for minor improvement:
- error checking to ensure inputs are, for instance, not complex,
- if only two inputs are given i.e. no YI, then YI defaults to zero.

[Phillip M. Feldman]

Your code is amazingly compact, and I like it better than mine.

I was actually trying to do something slightly different. For discrete-valued sequences, I'd like to be able to exclude cases where the sequence reaches the target level but does not actually cross it. However, this feature of my code appears to not be functioning correctly.