Making a generalized script, very new MATLAB user!

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Andrew
Andrew 2013-10-31
评论： sixwwwwww 2013-10-31
Hi, I am a very new to MATLAB and I used python and C a lot for scientific computing. My colleagues encouraged me to get familiar with MATLAB and its powerful symbolic functions, so I have started using it. Anyway, I made a script for some arbitrary purpose and just thinking how to make a generalized one. Here is my code:
syms x1 x2 x3 a;
f=sym('x1^2+x2^2');
point=[1,1];
variable1 = [diff(f,x1),diff(f,x2)];
variable10 = double(subs(variable1,[x1,x2],point));
d=variable1;
variable3=subs(f,[x1,x2],point+a*d);
b=12.56
variable4 = double(subs(variable3,[x1,x2,a],[point,b]));
Here I have written f of only two variables and wish to make it generalized for n variable. Is there any method exists in MATLAB how can I do this? Any ideas or suggestions? Any help much appreciated. Thanks.

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回答（2 个）

Walter Roberson
Walter Roberson 2013-10-31
You can use matlabFunction() together with cell array expansion and num2cell
ThisCell = {'%08f', pi};
fprintf(ThisCell{:})
is equivalent to
fprintf('%08f', pi)
sixwwwwww
sixwwwwww 2013-10-31
Dear Andrew, one way to generalize the code is as follows:
syms a
n = 20; % Number of variables for function f
x = sym('x%d', [1 n]);
f = sum(x.^2);
point = ones(1, n);
variable1 = [];
for i = 1:n
variable1 = [variable1 diff(f, x(i))];
end
variable10 = double(subs(variable1,x,point));
d = variable1;
variable3 = subs(f, x ,point + a * d);
b=12.56;
variable4 = double(subs(variable3,[x, a],[point, b]));
I hope it helps. Good luck!
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Andrew
Andrew 2013-10-31
Thank you again for your help! I worked on your explanations, which were indeed helpful. And got rid of many errors. However I have still some problematic parts. Here is modified code:
f=sym('x1^2+x2^2+x3^2+x4^2');
point=[1,1,1,1];
num=4;
k=1;
syms a
tolxy=1e-2;
n=1;
valxy=10;
x = sym('x%d', [1 num]);
%f = sum(x.^2);
%point = ones(1, num);
while(valxy>tolxy)
variable1 = [];
for i = 1:num
variable1 = [variable1 diff(f, x(i))];
end
variable2 = double(subs(variable1,x,point));
n=n+1;
valxy=norm(variable2);
d = -1*variable1;
variable3 = subs(f, x ,point + a * d);
f2=matlabFunction(variable3);
[x,y]=functx(f2,0,5) %Calling another user defined function.
k=k+1;
b=12.56;
variable4 = double(subs(variable3,[x, a],[point, b]));
point = point+b*d;
end
I wish to substitute x's by (point+a*d). Both vectors have same dimensions. Can you tell me did I do right thing? or if I want to substitute all x's by (point+a*d), what is right way to do it? f2 should only have single variable a. But now it is variable of a,x1,x2,x3,x4. By the way, again thank you for our help!
sixwwwwww
sixwwwwww 2013-10-31
Basically what you are doing is as follow:
d = -1 * variable1;
Now let's look at it. What is variable1? It is as follows:
variable1 = [2*x1, 2*x2, 2*x3, 2*x4]
So d becomes:
d = -1 * variable1 = [-2*x1, -2*x2, -2*x3, -2*x4]
Now we compute point + a * d
point = [1, 1, 1, 1];
point + a * d = [1 - 2*a*x1, 1 - 2*a*x2, 1 - 2*a*x3, 1 - 2*a*x4]
Now when you try evaluate
variable3 = subs(f, x ,point + a * d)
then make following substitutions:
x1 --> 1 - 2*a*x1
...
...
x4 --> 1 - 2*a*x4
Due to this reason variable 3 is function of a, x1,...,x4. Now I hope you can understand the reason. Now if you have some values for x1...x4 and put them in variable3 then it will become function of just 'a'.
I hope it helps you figure out the problem. Good luck!

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