# Plotting 2D contours syntax

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Tatte Berklee on 31 Aug 2022
Commented: Tatte Berklee on 31 Aug 2022
folks,
I have a few novice questions with plotting the following function:
My codes is:
zfun = @(x1,x2) x1./(x2.*(1-x1)-1);
zhandle = fcontour(zfun)
1. Is this the correct way to plot the contour? I am mainly concenred with having the "dot" after the variable.
2. Why do I get an error when I do this?
zfun = @(x1,x2) x1./(x2.*(1-x1.)-1);
Invalid expression. When calling a function or indexing a variable, use parentheses. Otherwise, check for mismatched delimiters.
zhandle = fcontour(zfun)
3. Is there a way to see how the contour changes, say fixing x2? For example, I would like to fix x2 to be say .5, and see how the contour plot changes with x1. Of course, this is just 1D plot, but is there a way to aggregate these graphs? is mesh the way to go?

Steven Lord on 31 Aug 2022
I am mainly concenred with having the "dot" after the variable.
The dot is not associated with the variable, it's associated with the operator. Don't think "x1.", think "./" See this documentation page for more information about the differences between the array operator ./ and the matrix operator / in MATLAB.
Why do I get an error when I do this?
Because x1. is not valid in MATLAB unless the next character is an array operator. There are situations where a value can have a trailing dot without an operator, like defining a literal numeric value: using 1. is equivalent to just plain 1 or 1.0. All three values in x below are equal to one.
x = [1 , 1. , 1.0]
x = 1×3
1 1 1
Is there a way to see how the contour changes, say fixing x2?
So you just want a line plot?
zfun = @(x1,x2) x1./(x2.*(1-x1)-1);
fplot(@(x) zfun(x, 5))
title('x2 equal to 5')
Tatte Berklee on 31 Aug 2022
Thanks a lot, Steve! It helped a lot!

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