Plot of ##\sin(x)/\log(x)##.

[MathematicalPhysicist]

Hi I want to plot the function $y=\sin(x)/\log(x)$ in MATLAB but I get the next error message:

Warning: Imaginary parts of complex X and/or Y arguments ignored

Here is the script that I used:

x = -2*pi:pi/100:2*pi;
y = sin(x)/log(x);
plot(x,y)

How to fix this that I'll get the plot on the real line?

Thanks!

 

[MathematicalPhysicist]

Never mind.

I want to delete this thread but I don't see how...

 

[kuruman]

What happens to the denominator when $x=1$?

 

[MathematicalPhysicist]

@kuruman Yeah, it's not defined there, then how plot this function?
when I type it in google I get a nice plot.

 

[jedishrfu]

What did you discover was your problem? It may help future posters.

One thing i see is that x may be negative and the log function will output complex values for that case.

https://www.rapidtables.com/math/algebra/ln/Ln_of_Negative_Number.html

 

[kuruman]

@kuruman Yeah, it's not defined there, then how plot this function?
when I type it in google I get a nice plot.

Can you do a piecewise plot? (I am not familiar with MATLAB).

 

[MathematicalPhysicist]

What did you discover was your problem? It may help future posters.

One thing i see is that x may be negative and the log function will output complex values for that case.

https://www.rapidtables.com/math/algebra/ln/Ln_of_Negative_Number.html

right, it's not defined for x<=0, but I also don't get a plot for x \in [0,2*pi].

 

[MathematicalPhysicist]

@jedishrfu when I try to plot for the domain [2,10] I still don't get a graph, I do get the plot window but without the graph line, it doesn't show up.

x = 2:1/100:10;
y = sin(x)/log(x);
plot(x,y)

 

[Wrichik Basu]

@jedishrfu when I try to plot for the domain [2,10] I still don't get a graph, I do get the plot window but without the graph line, it doesn't show up.

Use this:

y = sin(x)./log(x);

The thing I added is "./" so that $y$ becomes a matrix. Now you should get a good plot.

 

[jedishrfu]

the dot before the operator says to do an element by element evaluation.

as an example:

x=0:0.1:10
y=x.*x

generates the y=x^2 function and this plots it

plot(x,y)

In other programming languages, this simple convenience would be implemented as a for loop

for i in range(0, 100):
    x[i] = i / 10.0

for i in range(0, 100):
    y[i] = x[i] * x[i]

 

[MathematicalPhysicist]

Ah, yes forgot about the dot thing. Thanks folks!

 

[Orodruin]

as an example:

x=0:0.1:10
y=x.*x

generates the y=x^2 function

Note that the dot also works for exponential operators:

y = x.^2