Python Shorthand If - 0 interpreted as false in calculation result-CodePudding

for i in range(0,len(A) - 1): 
   cand = (A[i]   A[i   1]) / 2
   cand2 = i   2 < len(A) and (A[i]   A[i   1]   A[i   2]) / 3 or cand
   A[i] = min(cand, cand2)

I've written the above code to compute the minimum average of the next two & three elements of an integer array. For example, if we have the array [2,3,5,6], on the first iteration cand will be 2.5 and cand2 will be 3.33 so A[0] will be updated to 2.5 and so on.

This code works, except when (A[i] A[i 1] A[i 2]) / 3 is 0. In this case, the or statement executes and cand2 gets set to cand.

I'm pretty sure this is because Python interprets the 0 as False and executes the or statement - how can I fix this though?

CodePudding user response：

It seems you wanted to do a ternary if here, that would work like this:

for i in range(0,len(A) - 1): 
   cand = (A[i]   A[i   1]) / 2
   cand2 = (A[i]   A[i   1]   A[i   2]) / 3 if i   2 < len(A) else cand
   A[i] = min(cand, cand2)

You chained them with boolean operators, which maybe can work, as in python they do some tricks (an and-chained expression will return the first Falsy value of the chain, or the last value if all are Truthy - - - an or-chained expression will return the first Truthy value or the value if all are Falsy), but that makes your program a lot harder to understand.

Anyway, you might want to consider using a regular if construct here for best readability.

Some unspoken rules are:

The if branch should be the most likely one.

Don’t use nested ternary operators (use plain multi-line if ... elif > ... then ... statements instead).

Don’t use long ternary operators with complicated expressions (again use multi-line if statements instead).

for i in range(0,len(A) - 1): 
    cand = (A[i]   A[i   1]) / 2
    if i   2 < len(A):
        cand2 = (A[i]   A[i   1]   A[i   2]) / 3
    else:
        cand2 = cand
    A[i] = min(cand, cand2)