mlz wrote: > I've been playing with the binomial function, and found that in the below > code, rs *= x does not behave the same way as rs = rs * x. When I set FAIL > to True, I get a different result. Both results are below. > > I had read that the two were equivalent. What am I missing? > > thanks, > -= miles =- > > > #!/usr/bin/python2 > > import sys > FAIL= True if len(sys.argv)>1 else False > > def bin(n,k): > rs=1 > k=min(k,n-k) > > for i in range(1,k+1): > if FAIL: rs *= (n-(i-1))/i # these should be the same,
This is evaluated as rs = rs * ((n - (i - 1)) / i) > else: rs = rs * (n-(i-1))/i # but apparently are not while this is evaluated as rs = (rs * (n - (i - 1))/i so the two expressions do not really result in the same calculation. A simpler example: >>> (2*2)/3 1 >>> 2*(2/3) 0 Now "hide" the the order of evaluation: >>> a, b = 2, 3 >>> a *= a/b >>> a 0 >>> a, b = 2, 3 >>> a = a*a/b >>> a 1 > return rs > > > for n in range(10): > for k in range(n+1): > print bin(n,k), > print'' > > ------------------- output ------------------------- > > > $ pascal2 > 1 > 1 1 > 1 2 1 > 1 3 3 1 > 1 4 6 4 1 > 1 5 10 10 5 1 > 1 6 15 20 15 6 1 > 1 7 21 35 35 21 7 1 > 1 8 28 56 70 56 28 8 1 > 1 9 36 84 126 126 84 36 9 1 > > $ pascal2 fail > 1 > 1 1 > 1 2 1 > 1 3 3 1 > 1 4 4 4 1 > 1 5 10 10 5 1 > 1 6 12 12 12 6 1 > 1 7 21 21 21 21 7 1 > 1 8 24 48 48 48 24 8 1 > 1 9 36 72 72 72 72 36 9 1 -- https://mail.python.org/mailman/listinfo/python-list
