How to insert values for a symbolic variable

I'm calculating some gradients with symbolic variables and after that I want to insert values for the variables.

So I have a function depandent of multiple variables x(1),..., x(20)

and I want to insert some values for the variables, something like

etc.

such that the function replaces all the x(1) values with 10 and x(20) values with 14

How is that done? Should be simple but I can't find anything helpful explaining how to solve this

Answer 1

[madhan ravi]

 subs(f, {x1, x20}, {10, 14})

[John Miller]

Thank you! But how can I handle this when I add the values to a vector x?

[madhan ravi]

 syms x [1 20]
 x_vars = num2cell(x);
 x_vals = num2cell(1:20); % example values for x vector
 subs(f, {x_vars{:}}, {x_vals{:}})
 
 % for older versions
 x =  sym('x', [1, 20]);
 syms(x)
 x_vars = num2cell(x);
 x_vals = num2cell(1:20); % example values for x vector
 subs(f, {x_vars{:}}, {x_vals{:}})

if it's not what you are looking for, you need to illustrate with a SHORT example and the desired output.

[John Miller]

syms x [1 20]
x_vars = num2cell(x);
x_vals(1) =   1.13;
....
subs(f1, {x_vars}, {x_vals})

Gives the following error

Error using sym/subs>normalize (line 231)
Entries in second argument must be scalar.

Error in sym/subs>mupadsubs (line 157)
[X2,Y2,symX,symY] = normalize(X,Y); %#ok

Error in sym/subs (line 145)
    G = mupadsubs(F,X,Y);

Error in new_file(2) (line 122)
subs(f1, {x_vars}, {x_vals})

[madhan ravi]

subs(f1, x_vars{1}, x_vals{1}) % the number of variables should be equal to the number of values that you want to substitute for.

[John Miller]

I have 20 Variables and the vector x_vals cotains 20 values (I wanted to indicate that with the "....")

[madhan ravi]

https://www.mathworks.com/matlabcentral/answers/552637-how-to-insert-values-for-a-symbolic-variable#comment_908746 Read this once again.

[John Miller]

I made a new answer to keep discussing. Thanks for the effort though

[madhan ravi]

P.S: By the way stop adding answers just to make a comment, it makes the thread inconsistent.

This is what happens when you refuse to read something.

 clear all
 x = sym('x_%d',[13 1],'real');
 y = sym('y_%d',[3 1],'real');
 f1 = x(1)*x(2)
 x_vars = num2cell(x(1:2));
 x_vals(1)=1.13
 x_vals(2)=1.23
 x_vals = num2cell(x_vals)
 Wanted = subs(f1, {x_vars{:}}, {x_vals{:}})

[John Miller]

Thank you. I am not refusing to read, but the statement

 x_vals = num2cell(1:20); % example values for x vector

was not well made in order to understand that each value has to be converted later on

[madhan ravi]

https://www.mathworks.com/matlabcentral/answers/552637-how-to-insert-values-for-a-symbolic-variable#comment_908773

Answer 2

[John Miller]

>> x = sym('x_%d',[13 1],'real');
>> y = sym('y_%d',[3 1],'real');
>> f1 = x(1)*x(2)
 
f1 =
 
x_1*x_2
 
>> x_vars = num2cell(x);
>> x_vals(1)=1.13

x_vals =

    1.1300

>> x_vals(2)=1.23

x_vals =

    1.1300    1.2300

So far so good

     
>> subs(f1, {x_vars{:}}, {x_vals{:}})
Brace indexing is not supported for variables of this type.

>> subs(f1, {x_vars}, {x_vals})
Error using sym/subs>normalize (line 231)
Entries in second argument must be scalar.

Error in sym/subs>mupadsubs (line 157)
[X2,Y2,symX,symY] = normalize(X,Y); %#ok

Error in sym/subs (line 145)
    G = mupadsubs(F,X,Y);

It is possible to pass in one variable at the time, but that is not what I'm trying to do