# Index issue while using gradient of a function

조회 수: 6(최근 30일)
댓글: Braden Kerr 2020년 11월 4일
Hello,
I am attempting to take the gradient of a function and use that as a varaible in matlab function in order to calculate a specific matrix of values.
I currently am taking the gradient by the following code, where fun is the function I need to take the gradient of:
syms x [1 2]
fun = -9*x(1) -10*x(2) + theta*(-log(100-x(1)-x(2))-log(x(1))-log(x(2)) - log(50-x(1)+x(2)));
x0 = [8 90]'; % change this for different sub-problems in the homework
choice = 1; % 1 = bisection; 2 = Armijo
I then have another script that I am writing a rather long function within that needs to use the grad_fun at various points, I thus have the following:
function [out1, out2, out3] = steepestdescent(fun,grad_fun,eps,x0,choice,stp_eps)
if choice == 1
X=[]; D=[]; A=[]; F=[];
X(:,1)= x0; %initialize state
However, in my debugging process, I keep getting an error when I reach D(:,1) that states
Index exceeds the number of array elements (2).
Error in sym/subsref (line 902)
R_tilde = builtin('subsref',L_tilde,Idx);
Error in steepestdescent (line 7)
Error in HW4_3 (line 45)
The last element of the error, where it states Error HW4_3 (line 45) I know is associated with my use of grad_fun in my use of the function at the bottom of the first block of code, but I cant figure out why grad_fun is not returning the proper elements.
The gradient should hopefully take in the function fun, take its gradient, save that symbolic gradeint in a 2x1 matrix, then when the grad_fun function is called, be able to plug in the values for x0 and return a 2x1 matrix of two doubles that can then be stored and used elsewhere in the code.
Let me know if I need to clarify anything, thank you.

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### 채택된 답변

Walter Roberson 2020년 11월 4일
grad_fun is a vector not a function. You cannot use () notation to evaluate it, you have to use subs. Or use matlabFunction to turn it into a function handle (and be sure to use the option 'vars',{x} for that)
##### 댓글 수: 5표시숨기기 이전 댓글 수: 4
Actually, just figued it out. If anyone reads this thread, the issue is that x0 is a 2x1 matric while the function grad_fun needed a 1x2 matrix. By adjusting the argument of grad_fun(x0) to grad_fun(x0') it worked

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