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Queries on the use of "double"

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Doc Ed
Doc Ed on 17 Sep 2020
Commented: Ameer Hamza on 30 Sep 2020
Hi MATLAB, during our laboratory simulation, when we evaluate sin(x) when x=2, and use the "subs" command, some students reported the answer as sin(2) instead of 0.9093. How come? But when they use the "double" command, they arrived 0.9093. But other computers don;t need double command.
Case 1:
Script:
y=input('Input f(x): ');
x=input('Input x for f(x): ');
val=subs(y,x)
Result:
Input f(x): sin(x)
Input x for f(x): 2
val=
sin(2)
Case 2:
Script:
y=input('Input f(x): ');
x=input('Input x for f(x): ');
val=subs(y,x)
Result:
Input f(x): sin(x)
Input x for f(x): 2
val=
0.9093
Why did some computers reported sin(2) instead of 0.9093 with the same script? Why do some programs need double command but other computeres don't? Thank you

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Accepted Answer

Ameer Hamza
Ameer Hamza on 17 Sep 2020
This is probably caused by symbolic preferences on your machines. I guess you have defined x as a symbolic variable before this code.
syms x
By default, in the case of symbolic variables, MATLAB does not convert to a floating-point value; therefore, sin(2) is the default answer. If you are getting a value of 0.9093, then that means that somehow the preference has been changed by running
sympref("FloatingPointOutput", 1)
If you want always to get floating-point output, run the above line on all machines. To get the default preferences, run
sympref("default")

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Ameer Hamza
Ameer Hamza on 30 Sep 2020
I am glad to be of help!

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More Answers (1)

BOB MATHEW SYJI
BOB MATHEW SYJI on 17 Sep 2020
I think the first one uses syms command anywhere in the script while the second one does not. While you use syms command the entire expression is given as output. So you have to manually give double(output) to get the value.

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