How to get unknown number of variables to be all same length through interpolation

I have N variables, all of different lengths. I have no prior knowledge of which variables are what lengths BUT I need to get all variables interploated to the same length.

I load in a file that contain a time of length M and a data field of length M, then another with time length M+1 and data field length M+1. The first and last time of every file are the same, but the number of data points is not (different sampling rates and/or invalid samples removed). So I cant just add NaNs to the beginning or end to make them all the same.

If it is like 2 variables I can easily just check the lengths, have an if statement for which one is longer and interpolate the longer one to the length of the shorter one (or the shorter to the longer but I prefer to downsample).

I was trying to do some sort of loop over the number of variables or something but I could not figure out an easy solution that didnt involve manually inputting the lengths or something similiar.

Answer 1

[Star Strider]

Try this approach:

V1 = sort(rand(1,5));                                               % Original Vector
V2 = sort(rand(1,10));                                              % Original Vector

N = 20;                                                             % Required Vector Length
V1i = interp1((1:numel(V1)), V1, linspace(1, numel(V1), N));        % Interpolated Vector
V2i = interp1((1:numel(V2)), V2, linspace(1, numel(V2), N));        % Interpolated Vector

EDIT — (14 Jan 2021 at 14:22)

Plotting them (in one random example):

V1xi = linspace(1, numel(V1), N);
V2xi = linspace(1, numel(V2), N);

figure
hold on
plot((1:numel(V1)), V1, 'o-')
plot((1:numel(V2)), V2, 'o-')
plot(V1xi, V1i, 'x-')
plot(V2xi, V2i, 'x-')
hold off
grid

produces:

.

[Matthew Schmidt]

My problem is finding N and then figuring out which vectors are not length N for 4+ different time vectors (and their associated data)

[Star Strider]

Make ‘N’ at least the length of the longest vector. I would create ‘N’ as a convenient length for the code using it.

My code shows how to interpolate the vectors, and then to use them approopriately.

[Matthew Schmidt]

The problem is that N needs to be found through the code knowing the minimum (or max) length of some unknown amount of variables. I think my code below is how I can fix my problem

I added a bit more explainiation here: foo# and tfoo# are the data and its corresponding time. Each foo# and tfoo# are the same length. I am using 3 but my code I need to do 5+ AND between data sets (which I have many of) the foo# that is longest or shortest is not the same.

% my variables have been loaded from .dat files already...

[foo1, tfoo1, foo2, tfoo2, foo3, tfoo3 ] = ...
    get_my_vars_all_same_length(foo1, tfoo1, foo2, tfoo2, foo3, tfoo3 )

function varargout = get_my_vars_all_same_length(varargin)

for i = 1:2:length(varargin)
    lengths = length(varargin(i))
end

% find the min
min = min(lengths)
% find the mins position in the varargin
min_idx = find(.....)

% loop over the varagin and interpolate them to the right length
for i = 1:2:length(varargin)
    if i == min_index
        % dont do the interpolation
        
    else
        % do the interpolation to the smallest length variable using the min_idx
    end
    
end

[Star Strider]

It doesn’t need to be that complicated! You can do the same thing in a cell array:

V1 = sort(rand(1,5));                                               % Original Vector
V2 = sort(rand(1,10));                                              % Original Vector

vector_cell = {V1,V2};
max_len = max(cellfun(@numel, vector_cell))

producing:

max_len =
    10

So just put all of them in a cell array and use my ‘max_len’ statement.

You couild put it in an anonymous function if you want:

max_len_fcn = @(vector_cell) max(cellfun(@numel, vector_cell));
max_len = max_len_fcn(vector_cell)

with the same result, however that would llikely not be necessary, especially if you only wanted to do it once in each run of your script.

[Matthew Schmidt]

I will give this a go, thank you

[Star Strider]

My pleasure!