How do I convert fractional display to decimal display ?

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Ibrahim A 2021-3-22

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评论： Steven Lord 2021-3-22

I am getting bunch of fractional values in my output matrix and I have other matrixes outputting the same way as this one. I cannot find a way to change it to decimal notation, something like 1.678.

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Answer 1

Steven Lord 2021-3-22

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Call double or vpa on your symbolic variable.

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Steven Lord 2021-3-22

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Looking more closely at the line of code where the error occurred I see that there are three possible causes of that error, only one of which I called out. You could also have defined a variable named int, so int(j1, t) is being interpreted as an attempt to index into the variable rather than an attempt to call the int function.

A third option is that t is your symbolic variable. When you assign something symbolic to an expression of the form "identifier(symbolicVariable)" on the left side of the equals sign, you create a symbolic function. Since vpa returns a symbolic expression, vpa will work here. But when you try to assign a number to that same expression, MATLAB tries to figure out to which element of the array whose name is specified the number should be assigned. But MATLAB can't figure out which element is element t and so throws an error.

syms t

f(t) = t*pi % creates a symbolic function, works

f(t) =

f(1:5) % evaluate f

ans =

g(t) = pi % assigns pi to the tth element of g, errors

Error using sym/subsindex (line 864)
Invalid indexing or function definition. Indexing must follow MATLAB indexing. Function arguments must be symbolic variables, and function body must be sym expression.

Ibrahim A 2021-3-22

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I use this function to find the integral of 1, I think this is what you mentioned above. Is this the problem?

j1(t) = (j_max * (t.^0))
qdd(t) = vpa(int(j1,t))

Steven Lord 2021-3-22

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two = sym(2);

sqrt2 = sqrt(two)

sqrt2 =

V = vpa(sqrt2)

V =

1.4142135623730950488016887242097

D = double(sqrt2)

D = 1.4142

whos two sqrt2 V D

Name Size Bytes Class Attributes D 1x1 8 double V 1x1 8 sym sqrt2 1x1 8 sym two 1x1 8 sym

V, sqrt2, and two are all sym. So in the code below the line assigning to f(t) creates a symbolic function:

syms t
f(t) = V
f(t) = 2.6651441426902251886502972498731

D is a double. In the code below assigning to g(t) attempts to store D in element t of the array g.

g(t) = D
Error using sym/subsindex (line 864)
Invalid indexing or function definition. Indexing must follow MATLAB indexing. Function arguments must be symbolic variables, and function body must be sym expression.

Compare:

qdd(t) = V % works
qdd(t) = D % errors

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Answer 2

Bjorn Gustavsson 2021-3-22

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You might have set format to rat somewhere, perhaps in a startup.m or the like. You could get back to more normal decimal output by something like this:

format short g

For additional options check the help and documentation to format.

HTH

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Ibrahim A 2021-3-22

Unfortunately, didn't work. I also tried to change the format on preferences -> matlab -> command window -> numeric format (Short g, compact), that also didn't work. I should also mention that I am using symbolic math toolbox, I don't know if it is related or not. But thank you for your answer.

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