While loop to detect 0 slope

Hello everyone,

so I'm currently trying to make a code that will separate some data. To do so, I came up with a code that I thought would work, and I've tried looking at the what the error message mean, but I cant understand. My code is the follwoing

*g=0; j=0; A=[5 5]; h=0; slope=5; while (g<1785) & (A(1,1)>0) & (h<0.15) h = AverageStrain(1+g:20+g,:); i = AverageStress(1+g:20+g,:); A = polyfit(h,i,1); j = g+10; g = g+20; end

Cycle1UpStrain=AverageStrain(0:j,1);*

The error message i receive is ;

*Subscript indices must either be real positive integers or logicals.

Error in MTSAnalyser (line 43) Cycle1UpStrain=AverageStrain(0:j,1);*

here, the files AverageStress and AverageStraain are 1785x1 variables. I tried doing a linear fit of 20 data points at the time so that when the linear slope calculated is 0 are negative, it stops and extracts the number j.

j would be used to create a variable made of a fraction of AverageStress and AverageStrain, data points 1 to j. (afterwards I would make it something like j to a new variable limit)

Also, my value of A always ends in something like [574, -64], which is weird since I wanted it to stop at a slope equal 0 or smaller, which is the first row, first column of A.

I'm quite new to Matlab, so try not to assume I know a whole bunch.

Thank you for your help! N

[Star Strider]

Formatted code:

g=0; 
j=0; 
A=[5 5]; 
h=0; 
slope=5; 
while (g<1785) & (A(1,1)>0) & (h<0.15) 
  h = AverageStrain(1+g:20+g,:); 
  i = AverageStress(1+g:20+g,:); 
  A = polyfit(h,i,1); 
  j = g+10; 
  g = g+20; 
end

Answer 1

[Star Strider]

Here’s the problem:

Cycle1UpStrain=AverageStrain(0:j,1);

Zero is not a positive integer. Your (variable or index) ‘j’ is also defined as zero, as best I can see from your code (difficult to read because it’s not formatted).

If you want to find the indices where AverageStrain is equal to zero and one respectively, try this:

AvStn0Idx = find(AverageStrain == 0);
AvStn1Idx = find(AverageStrain == 1);

[Neil]

Thanks, the mistake was in fact the 0 there. it needed to start at 1. Meanwhile, my A still ends at a value of [571, -644]. Ideas?

[Star Strider]

I’m not entirely understand what you’re doing, but I see some problems. One is that h is a vector, not a scalar (at least as I read your code), so if any value of h is less than 0.15, that test will be true.

Consider:

h = [0.1 randi(10,1,5)];
htest = h < 0.15

yields:

htest =

       1     0     0     0     0     0

You may want to consider how you’re testing h. What information do you really want from it? First value? Last value? Mean? Norm? Minimum? Maximum?

I would solve the h problem (if it is a problem) first, then see if your while loop runs as it should.