How to use gradient?

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what is the problem with the following code?
f=input('enter function: ','s')
[DX,DY] = -gradient(f);
f = str2func(['@(x,y)' vectorize(f)]);
[DX,DY] = -gradient(f);
hold on
quiver(X,Y,DX,DY)
hold off
I get the following errors:
Error using zeros CLASSNAME input must be a valid numeric or logical class name.
Error in gradient (line 56) g = zeros(size(f),class(f)); % case of singleton dimension
  2 Comments
geometry geometry
geometry geometry on 14 May 2017
I'm sorry because i'm so new to matlab.

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Accepted Answer

Star Strider
Star Strider on 13 May 2017
Edited: Star Strider on 13 May 2017
You have to evaluate ‘f’ to calculate the gradient.
The Code
f = 'log(x^2+y^2)';
f = str2func(['@(x,y)' vectorize(f)]);
x = -10:10;
y = x';
[DX,DY] = gradient(-f(x,y));
quiver(x,y,DX,DY,0); % The ‘0’ Argument Turns Off Arrow Scaling
The Plot
EDIT Added plot.
  7 Comments
Star Strider
Star Strider on 15 May 2017
Consider using the symvar function (in core MATLAB as well as the Symbolic Math Toolbox) to determine the variables in an expression. You can then use the size or length of that result with appropriate if or similar blocks to calculate the appropriate gradient of your function.
Example
f = 'log(x^2+y^2)';
vars1 = symvar(f)
f = str2func(['@(x,y)' vectorize(f)]);
syms f(x,y) x y
f(x,y) = x^4 + tan(x);
vars2 = symvar(f)
vars1 =
2×1 cell array
'x'
'y'
vars2 =
x
Note that symvar will not work on function handles, so you must put it before your str2func call for it to work.

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More Answers (1)

Stephen23
Stephen23 on 13 May 2017
Edited: Stephen23 on 13 May 2017
You need to start reading the MATLAB documentation, because guessing how to use MATLAB will not be an efficient use of your time. MATLAB has very accessible documentation: learn to use it:
When you actually open the gradient documentation and read the input description, it clearly describes the first argument as "F — Input array vector | matrix | multidimensional array"
So far you have tried two different inputs, neither of which are numeric arrays:
f=input('enter function: ','s')
[DX,DY] = -gradient(f); % <- here the input is a string
f = str2func(['@(x,y)' vectorize(f)]);
[DX,DY] = -gradient(f); % <- here the input is a function handle
The gradient documentation does not state that it accepts strings or function handles. It accepts numeric data only. In fact, the subtitle at the very top of the page states quite clearly "Numerical gradient", and it does not state symbolic or functional gradient.
Perhaps you were trying to do something like this (note that this is a numeric calculation):
>> [X,Y] = meshgrid(0:9);
>> Z = X.^2 + sqrt(Y); % function of X and Y
>> [DX,DY] = gradient(Z); % input numeric array to gradient
>> quiver(X,Y,DX,DY)
  2 Comments
Stephen23
Stephen23 on 14 May 2017
@geometry geometry: use str2func to generate a function handle from the input string. I am sure that you can find and read the str2func documentation yourself.

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