plotting piecewise functions with logarithm

I am plotting a piecewise function as in below:

x_axis=[-40:0.01:40];
g=5;
if x_axis<0; 
 plot(x_axis, (1/40).*log(2.*(g-10)./x_axis))

else  x_axis>0;
 plot(x_axis, (1/40).*log(2.*(g+10)./x_axis))
 xlim([2.*(g-10) 2.*(g+10)])
ylim([0 0.2])
end

The thing is, I get an error message "Warning: Imaginary parts of complex X and/or Y arguments ignored > In pcwise at 8 "

And the graph does not show the correct plots. (How could there by complex values for a log function?) Please help. Thanks

Answer 1

[Youssef Khmou]

If the input vector X is negative, log produces complex result. Try the following version :

x=[-40:0.01:40];
g=5;
if x<0; 

    f1=(1/40).*log(2.*(g-10)./x);
    plot(x,abs(f1))
else 
    f2=(1/40).*log(2.*(g+10)./x);
    plot(x_axis,abs(f2))
    xlim([2.*(g-10) 2.*(g+10)])
    ylim([0 0.2])
end

Answer 2

[John D'Errico]

How could there be complex values for a log function? Um, what is the log(-2)? Or ANY negative number for the argument?

log(-2)
ans =
      0.693147180559945 +      3.14159265358979i

So what are you doing?

log(2.*(g-10)./x_axis)

g is 5, and x_axis varies from -40 to 40.

So to me, it looks like you are trying to evaluate the log of a negative number.

Part of your problem arises from the fact that an if statement does NOT apply separately to every member test you give it. For example, this does not do what you apparently think it does:

x = 1:10;
if x < 5
  stuff
else
  otherstuff
end

Perhaps you are confusing it with something that does do what you want:

for x = 1:10
  if x < 5
    stuff
  else
    otherstuff
  end
end

In the first example I showed, x is a vector. In the second, x is a scalar, that sequentially takes on values from 1 to 10.