beginner:referencing a cell containing a zero matlab - matlab

Question

So far I have got this code:
clear all; % clears all variables from your workspace
close all; % closes all figure windows
clc; % clears command window
%%=============================================
%%define number of trials and subjects
%%=============================================
njump=81;
nsbj=6;
%%
%%==============================================
%%defining size of cell that will be created
%%==============================================
data=cell(njump,1);
%%
%%==============================================
%%defining gravity and frame rate per second (fps)
%%==============================================
fps=0.001; %frames per second
g=-9.81; %acceleration
%%
%%==============================================
%%read in excel data in CSV format
%%===============================================
for i=1:njump;
x=sprintf('Trial%02d.csv',i),sprintf('Trial%02d',i),'A1:E7000';;% jump data
data{i}=xlsread(x,'A1:E7000');
%%===============================================
%%defining total no. of frames and time of flight
%tnf=total number of frames equal to zero
%n = nnz(X) returns the number of nonzero elements in matrix X.
%%===============================================
% myMax(i) = nanmax(data{i}(:,5));
% vals = find(data{i}(:,5) > (myMax(i) - 10));
% pointsInAir = numel(vals,i);
tnf(i,1) = size(data{i,1},1) - nnz(data{i,1}(:,5)); %number of zeros
tof(i)=tnf(i)*fps; %Time of flight is equal to this
jh(i,1)=(-g*(tof(i).^2)/8); %defining jump height
%%=================================================
%%to find average power first use "find" function to find the first zero in
%%Fz, have the cell referenced
%%then use nanmean for average power(av_pwr)
%%use nanmin for peak power (peak_pwr)
%%=================================================
n = 1; % the following call to find retrieves only the n first element(s) found.
ref= find(data{i,1}(:,5)==0, n, 'first');
% ref=find(data(:,5)==0);
peak_pwr(i,1) = nanmin (data {i,1}(1:ref,5)); %preak power in coloumn E1 in all data with reference to cell found
av_pwr(i,1)=nanmean(data {i,1}(1:ref,5));%average power in coloumn E1 in all data with reference to cell found
%%==================================================
%%Plot the results onto a time vs jump height, time vs average power and
%%time vs peak power
However the part that is hard is trying to find the first zero in column E which is the 5th column to use as a reference cell. I want to use this reference cell so that I can do my average and peak power calcs. that use the numbers before this zero.

Answer 1

In this case ref is empty so you cannot access the first element.
If you think that ref should not be empty you need to go back further to see where things go wrong. Otherwise, you can use something like:
if any(ref)
%Do something
else
%Return the default value/do alternative action
end

Answer 2

It could help to have an example of what's in data. I have created one which might be similar to yours :
data{1,1}=magic(6)-10
Now in this matrix, column 5 actually does have a zero element so ref= find(data{1,1}(:,5)==0); ref(1) actually works and retrieves the first index of a zero element. However, if it didn't, you would be trying to access the first element of an empty matrix.
Try instead using the second (and perhaps third) arguments of find to achieve this :
n = 1;
% the following call to find retrieves only the n first element(s) found.
ref= find(data{1,1}(:,5)==0, n, 'first');
The rest of your code seems like it should work, although from the looks of it i have a feeling your loop (i take it you are using a loop for i) could maybe be vectorized.
Hope this helps :)
Tepp