If, else-if loops in matlab

Question

I'm having a bit of confusion with matlab. I'm trying to generate a random number between zero and one, and then map that number to the set $\{0,1,2,3,4\}$. Here's the code I have:

x=rand(1)

if( x < 3.2e-4)

    k = 0

elseif( 3.2001e-4 < x < 6.72e-3)

    k = 1
elseif(6.72001e-3 < x < 0.05792)

    k = 2

elseif(0.05792001 < x < 0.2627)

    k = 3

elseif(0.2627001 < x < 0.6723)

    k=4

else

    k = 5

end

Whenever I run the code, it only works part of the time. For example, when I got x=0.5469, it says that k=5 when in reality k should take on the value of 4.

Any help/links are appreciated. Thanks in advance.

Answer 1

The "if a < x < b" does not do what you think it does. Use "if a < x && x < b" instead. Extra credit: figure out how a < x < b actually evaluates.

Answer 2

To determine if a value falls within a range, you can use two separate comparison tests, joined with an AND condition, e.g.,

elseif ((3.2001e-4 < x) && (x < 6.72e-3))

There is another issue with the code above. if x is 3.20009e-4, you get k = 5, which is not what you want. What you should test for is:

elseif ((3.2e-4 <= x)  && (x < 6.72e-3))

However, you can simplify the code above further. If the first test (x < 3.2e-4) is false, then (3.2e-4 <= x) is true, and the execution would go to the next else statement anyway, so you can just check for:

elseif (x < 6.72e-3)

Answer 3

You can also simplify this as:

if( x < 3.2e-4)
  k = 0
elseif (x < 6.72e-3)
  k = 1
elseif (x < 0.05792)
  k = 2
elseif (x < 0.2627)
  k = 3
elseif (x < 0.6723)
  k = 4
else
  k = 5
end