??? Undefined function or method 'sign' for input arguments of type 'sym'.

Question

abhijit kulkarni 2013-8-25

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/85535-undefined-function-or-method-sign-for-input-arguments-of-type-sym

I am trying to generate a wave :

when time=0:5 then r(t)=1
when time=5:10 then r(t)=2
when time=10:15 then r(t)=1

and then I also want to take derivative of this wave (even double derivative also)

I tried it without giving t=0:15 and with syms

code:

t= 0:0.1:14.9;
aaa=sin(2*pi*0.1*t');
%  %r = [1,zeros(120,1)]+2-sign(aaa);
r =2.000-sign(aaa); 
size(r)
r1=diff(r,t);
size(r1)
rp=horzcat(0,r1);
size(rp)
size(t)
plot(t',rp)

------

error:when time=0:5 then r(t)=1

I also tried with if loop ,

if t>=0  && t<=5
r=1;
if t>=5  && t<=10
r=2;
if t>=10  && t<=15
r=1;

this aslo doesnot work; give some logical error message

please help

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Walter Roberson 2013-8-25

There is no such thing as an "if loop". There are "for loop" and "while loop" and "if statement".

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Answer 1

Walter Roberson 2013-8-25

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/85535-undefined-function-or-method-sign-for-input-arguments-of-type-sym#answer_95003

"when" is not a MATLAB or MuPAD command.

Your code does not use sym() or syms, so you could not be getting an error about sign of sym.

Your "t" is a vector, so "t>=0" is a vector result as is "t<=5". You cannot join vector results with "&&" as "&&" is reserved for scalars. You need to use "&" to join vector results. But you should probably read the definition of how "if" works carefully