How do I manually insert variables I have declared symbolically into a zero matrix?

I have been trying to insert symbolic variable into a zero matrix.

    clear all
    clc
    syms k1 k2 k3 k4
    global_k1 = zeros(4,4);
    global_k2 = zeros(4,4);
    global_k3 = zeros(4,4);
    global_k4 = zeros(4,4);

    global_k1(1,1) =  k1;
    global_k1(1,2) = -k1;
    global_k1(2,1) = -k1;
    global_k1(2,2) =  k1;

    global_k2(2,2) =  k2;
    global_k2(2,3) = -k2;
    global_k2(3,2) = -k2;
    global_k2(3,3) =  k2;

    global_k3(2,2) =  k3;
    global_k3(2,3) = -k3;
    global_k3(3,2) = -k3;
    global_k3(3,3) =  k3;

    global_k4(2,2) =  k4;
    global_k4(2,4) = -k4;
    global_k4(4,2) = -k4;
    global_k4(4,4) =  k4;

    k_stiff = global_k1+global_k2+global_k3+global_k4;
    disp('The stiffness matrix is: ');
    disp(k_stiff);

It displays the following error when I run the program:

The following error occurred converting from sym to double:
Unable to convert expression into double array.

Error in HW1_1 (line 9)
global_k1(1,1) =  k1;

How should I resolve this error?

Answer 1

[madhan ravi]

>> syms k1 k2 k3 k4
        global_k1 = sym(zeros(4,4));
        global_k2 = sym(zeros(4,4));
        global_k3 = sym(zeros(4,4));
        global_k4 = sym(zeros(4,4));
        global_k1(1,1) =  k1;
        global_k1(1,2) = -k1;
        global_k1(2,1) = -k1;
        global_k1(2,2) =  k1;
        global_k2(2,2) =  k2;
        global_k2(2,3) = -k2;
        global_k2(3,2) = -k2;
        global_k2(3,3) =  k2;
        global_k3(2,2) =  k3;
        global_k3(2,3) = -k3;
        global_k3(3,2) = -k3;
        global_k3(3,3) =  k3;
        global_k4(2,2) =  k4;
        global_k4(2,4) = -k4;
        global_k4(4,2) = -k4;
        global_k4(4,4) =  k4;
        global_k1
        global_k2
        global_k3
        global_k4
        k_stiff = global_k1+global_k2+global_k3+global_k4;
        disp('The stiffness matrix is: ')
        disp(k_stiff);

global_k1 =

[  k1, -k1, 0, 0]
[ -k1,  k1, 0, 0]
[   0,   0, 0, 0]
[   0,   0, 0, 0]

global_k2 =

[ 0,   0,   0, 0]
[ 0,  k2, -k2, 0]
[ 0, -k2,  k2, 0]
[ 0,   0,   0, 0]

global_k3 =

[ 0,   0,   0, 0]
[ 0,  k3, -k3, 0]
[ 0, -k3,  k3, 0]
[ 0,   0,   0, 0]

global_k4 =

[ 0,   0, 0,   0]
[ 0,  k4, 0, -k4]
[ 0,   0, 0,   0]
[ 0, -k4, 0,  k4]

The stiffness matrix is: 
[  k1,               -k1,         0,   0]
[ -k1, k1 + k2 + k3 + k4, - k2 - k3, -k4]
[   0,         - k2 - k3,   k2 + k3,   0]
[   0,               -k4,         0,  k4]

[Salil Chawla]

I wanted to insert the indexed values in the zero matrix, not just add them. Thanks anyway

[madhan ravi]

Check the code now , it works.

[Salil Chawla]

Not the result I intended. The addition is based on the indexed position. The way you suggested would just add the matrices

[madhan ravi]

Can you show the result you intend? Just to understand what you actually want.

[madhan ravi]

Plus your actual question was How to resolve the error?

[Salil Chawla]

This is basically to find the elemental and global stiffness matrix for finite element analysis.

[madhan ravi]

Can you show an example of the indexed addition ? If you define the process what has to be found ? It would be narrowed instead of generalisation because it’s totally vague for now.

[Salil Chawla]

Something of the effect of this:

clear all
clc
syms k1 k2 k3 k4 
global_k1 = [k1 -k1 0 0; -k1 k1 0 0; 0 0 0 0; 0 0 0 0];
global_k2 = [0 0 0 0; 0 k2 -k2 0; 0 -k2 k2 0; 0 0 0 0];
global_k3 = [0 0 0 0; 0 k3 -k3 0; 0 -k3 k3 0; 0 0 0 0];
global_k4 = [0 0 0 0; 0 k4 0 -k4 ; 0 0 0 0; 0 -k4 0 k4];

k_stiff = global_k1 + global_k2 + global_k3 + global_k4;
disp('The stiffness matrix is: ');
disp(k_stiff);

The difference is that here I had to manually input the symbolic values into a zero matrix and then add them. What I intended to do was to declare zero matrices for variable global_k1, k2 etc and then put variables into the matrices by calling variables into different positions by calling the function

   global_k1(1,1) =  k1;
   global_k1(1,2) = -k1;
   global_k1(2,1) = -k1;
   global_k1(2,2) =  k1;

And so on. Hope that clears the picture.

[madhan ravi]

Code edited - first zero matrices were made and then symbolic variables inserted according to the index.If still you didn’t get the required result, just type the desired result in the next comment. I’ll give you the apt code.

[Salil Chawla]

sym(zeros(4,4)); is an incorrect command

[Salil Chawla]

The stiffness matrix is: 
[  k1,               -k1,         0,   0]
[ -k1, k1 + k2 + k3 + k4, - k2 - k3, -k4]
[   0,         - k2 - k3,   k2 + k3,   0]
[   0,               -k4,         0,  k4]

This is the result I am looking for

[madhan ravi]

see the code I just posted the results you wanted and it works fine for me for your information , I am using Matlab 2017a release .

[Salil Chawla]

Got it, thanks