Histogram on two theards

Hi guys i want generate histogram on two theards i want split the table
“a” and generate half table on one theard and second table one theard
but i have
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#Thread:0x00000001d4d2e8
2 wątek
#Thread:0x00000001d4d090
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a = [0,1,2,2,2,2,2,3,3,3,3,3,3,4,4,4,4,5,5,6,6,6,7,7,7,7,7,8,9,9,10]
a.count
half = (a.count/2)-1
half+1
b=a[0…half]
c=a[half+1…20]
tab1 = Hash[*b.group_by{ |v| v }.flat_map{ |k, v| [k, v.size] }]
c=a[half+1…20]
tab2 = Hash[*c.group_by{ |v| v }.flat_map{ |k, v| [k, v.size] }]
puts t1=Thread.new{c}
puts “2 theard”
puts t2=Thread.new{b}
t1.join
t2.join