Count[] with conditionals [closed]

Question

I run the following code and I got what I want

a = Table[
   EuclideanDistance[RandomVariate[UniformDistribution[]], 
    RandomVariate[UniformDistribution[]]], 1000];
Length[Select[a, # < 0.5 &]]

I don't know why when I run

Count[a,# < 0.5 &]

I got 0, shouldn't Count[a,# < 0.5 &]=Length[Select[a, # < 0.5 &]]?

Answer 1

The second arg of Count expects a pattern, not a function. You could use either or the following:

In[29]:= Count[a, _?(# < 0.5 &)]

Out[29]= 763

In[30]:= Count[a, x_ /; x < 0.5]

Out[30]= 763

Answer 2

Help says Count argument should be a pattern.

So do this

Count[a, x_ /; (x < 0.5)]

# < 0.5 & is not a pattern. It is a function.

Answer 3

You can also use the CountsBy function, where you can use a test directly:

{f, t} = Values@CountsBy[a, # < 0.5 &]

{246, 754}