MATLAB: Rand function taking a long time

Question

So I have this code below for demonstration a Poisson Distribution. Using an if loop

if rand < (1/AmountRnd)

is much faster than generating a random array

y=rand(10000);

Does anyone know why this is? I have poster both copies of my full code below. Faster Loop Method:

%Set Amount of random numbers to generate

  AmountRnd=10000;
  %Set length of distribution
  Lpos=30;
  %Create X array
  X=1:1:Lpos;
  %Initialize Poisson array
  Poisson = zeros(1,Lpos);  
  %Loop 

    for ii = 1:100
        %Initialise Count

        Count = 1;
        %Create loop for as many random number are set
        for jj = 1:AmountRnd
            %Count amount of times rand is less than given amount
            if rand < (1/AmountRnd)
                Count = Count + 1;
            end
        end
        %Create Poisson array using values from each count
        Poisson(Count) = Poisson(Count) + 1;
    end
    %Graph Histogram of distribution
    stairs(Poisson);

Slower Method:

%Set Amount of random numbers to generate
AmountRnd=10000;
Lpos=30;
X=1:1:Lpos;
Poisson = zeros(1,Lpos);
%Loop 
for j=1:100
    %Initialise Count
    Count = 1;
    y=rand(10000);
    Poisson(Count) = sum(y(1:10000)<(1/10000)) + Poisson(Count) + 1;
end

Answer 1

rand(10000) is the same as rand(10000,10000), not the same as rand(1,10000)