Math.random()
when a>3, 40% probability output 4
> 3
< 0.4
4
when a>3, 60% probability output 5
> 3
>= 0.4
5
whern a<= 3, 20% probability output 4
<= 3
< 0.2
4
when a<= 3, 80% probability output 5
<= 3
>= 0.2
5
Math.random()
when b=4, 30% probability output "label1"
4
< 0.3
"label1"
when b=4, 70% probability output "label2"
4
>= 0.3
"label2"
when b=5, 50% probability output "label3"
5
< 0.5
"label3"
when b=5, 50% probability output "label4"
5
>= 0.5
"label4"