Problem with Polyval, potential bug

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Robert on 17 Feb 2021
Answered: Steven Lord on 17 Feb 2021
So i am having a bug with poly val.
I am issuing the function two vectors. The first being a vector of coeficients.
The second is a vector that is 1023X1 long. and those are my ex coordinates. It is of type double.
each number in the vector is a very small number 2.0e-9 for example
When polyval runs, it is running the x coordinates as 1,2,3,4,.....1023. so somehow, someway it is using the index of the vector instead of the data in the vector location.
I cant for the life of me figure out why. The vector is 1023X1 so there is no other data in the vector. Clicking on the vector and bringing it up in the table format of the workspace varieified whats in it. I think this is a bug.
Can anyone help?
Robert on 17 Feb 2021
so in short, its evaluating the polynomial at the index not the data in the index
I am on 2018rb

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Accepted Answer

Steven Lord
Steven Lord on 17 Feb 2021
This is not a bug and this behavior is not caused by polyval but by plot. What happens if you call plot with one input argument? According to the documentation:
"plot(Y) creates a 2-D line plot of the data in Y versus the index of each value.
  • If Y is a vector, then the x-axis scale ranges from 1 to length(Y)."
So if you want to plot the values of the polynomial at the specified X coordinates, you need to pass those X coordinates into both polyval (to determine the Y coordinates of the points in the polynomial) and plot (to plot those points at the appropriate X coordinates.)
p = [1 2 1];
x = (1:12)./4;
subplot(2, 1, 1)
plot(polyval(p, x), 'o-')
title('One input to plot uses indices')
subplot(2, 1, 2)
plot(x, polyval(p, x), 'o-')
title('Two inputs to plot uses data')

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