Why the result does change when ı add other rows in Matrix?

Question

Merve Ala 2023-1-13

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1893660-why-the-result-does-change-when-i-add-other-rows-in-matrix

评论： Steven Lord 2023-1-13

I am trying basic multiplication on Matlab. For the first row, it calculates in true way. But when ı add other rows, it calculates first row wrong.

l_1=600  
A= 48.6
K =  [12, 6*(l_1), -12 ,  6*(l_1) ;
     6*(l_1),  4*(l_1)^2 , -6*(l_1),  2*(l_1)^2 ;
     -12,  -6*(l_1), (12),  -6*(l_1)  ; 
     6*(l_1),  2*(l_1)^2,  -6*(l_1), 4*(l_1)^2 ]
K_result=K.*A
and the result is 
K_result =
   1.0e+07 *
    0.0001    0.0175   -0.0001    0.0175
    0.0175    7.0000   -0.0175    3.5000
   -0.0001   -0.0175    0.0001   -0.0175
    0.0175    3.5000   -0.0175    7.0000
    
    it doesnt calculate A*12 the result must be 583.3 but it shows 0.0001. When i just calculate first row it calculates in true. 
    K =  [12, 6*(l_1), -(12),  6*(l_1)]
    K_result=K.*A
   
    K_result =
   1.0e+05 *
    0.0058    1.7500   -0.0058    1.7500

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Answer 1

Kevin Holly 2023-1-13

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1893660-why-the-result-does-change-when-i-add-other-rows-in-matrix#answer_1148175

在 MATLAB Online 中打开

It is giving the correct result. It is just rounded. I changed format to long below.

l_1=600; 
A= 48.6;
K =  [12, 6*(l_1), -12 ,  6*(l_1) ;
     6*(l_1),  4*(l_1)^2 , -6*(l_1),  2*(l_1)^2 ;
     -12,  -6*(l_1), (12),  -6*(l_1)  ; 
     6*(l_1),  2*(l_1)^2,  -6*(l_1), 4*(l_1)^2 ];
K_result=K.*A
K_result = 4×4
1.0e+07 *

    0.0001    0.0175   -0.0001    0.0175
    0.0175    6.9984   -0.0175    3.4992
   -0.0001   -0.0175    0.0001   -0.0175
    0.0175    3.4992   -0.0175    6.9984
format long
K_result=K.*A
K_result = 4×4
1.0e+07 *

   0.000058320000000   0.017496000000000  -0.000058320000000   0.017496000000000
   0.017496000000000   6.998400000000000  -0.017496000000000   3.499200000000000
  -0.000058320000000  -0.017496000000000   0.000058320000000  -0.017496000000000
   0.017496000000000   3.499200000000000  -0.017496000000000   6.998400000000000

2 个评论
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Merve Ala 2023-1-13

Oh thank you so much

Steven Lord 2023-1-13

在 MATLAB Online 中打开

In this case IMO format longg is even better.

l_1=600; 
A= 48.6;
K =  [12, 6*(l_1), -12 ,  6*(l_1) ;
     6*(l_1),  4*(l_1)^2 , -6*(l_1),  2*(l_1)^2 ;
     -12,  -6*(l_1), (12),  -6*(l_1)  ; 
     6*(l_1),  2*(l_1)^2,  -6*(l_1), 4*(l_1)^2 ];
format longg
K_result=K.*A
K_result = 4×4
1.0e+00 *

                     583.2                    174960                    -583.2                    174960
                    174960                  69984000                   -174960                  34992000
                    -583.2                   -174960                     583.2                   -174960
                    174960                  34992000                   -174960                  69984000