Given an list A of objects and another list B which is identical to A except that one element is removed, find that removed element.

All these supposed answers are missing the point, and this question isn't even worded correctly. It should be lists of NUMBERS, not "objects". Anyway, the question is asking how you figure out the number that is missing from list B, which is identical to list A except one number is missing. Before getting into the coding, think about it logically - how would you find this? The answer of course is to sum all the numbers in A, sum all the numbers in B, subtract the sum of B from the sum of A, and that gives you the number.

select b.element from b left join a on b.element = a.element where a.element is null

In Python: (just numbers) def rem_elem_num(listA,listB): sumA = 0 sumB = 0 for i in listA: sumA += i for j in listB: sumB += j return sumA-sumB (general) def rem_elem(listA, listB): dictB = {} for j in listB: dictB[j] = None for i in listA: if i not in dictB: return i

How about this in python, will this work? x = set(listA)-set(listB) print(x)

missing_letters = [] for letter in A: if letter in B: pass else: missing_letters.append(letter) print (missing_letters)

In R: removed_element <- A[which(!A %in% B)] removed_element

Select * from A except Select * from B

I think it is a coding in algorithm rather than SQL query. So here is my take: def ret_miss(A, B): k = len(A) if k == 2: if A[1] == B[0]: return A[0] elif A[0] == B[0]: return A[1] n = k/2 print A[n], B[n] if A[n] == B[n]: A= A[n:] B=B[n:] else: A=A[:n+1] B=B[:n+1] print A,B return ret_miss(A,B) This works nicely actually.

# python code def missing_obj(original_lst, new_lst): for x in new_lst: original_lst.remove(x) return original_lst

In SQL: SELECT A.object FROM A LEFT JOIN B ON A.object = B.object WHERE B.object IS NULL;

Careful, there could be a repeated object that's being removed. i.e. A = [3, 4, 5, 6, 5] B = [3, 4, 6, 5] This is how I would do it on Python (works for numbers and strings) def missingval(lA, lB): a = sorted(lA) b = sorted(lB) c = None for i in range(len(b)): if a[i] != b[i]: c = a[i] break if c is None: c = a[-1] print(c, "was removed from list A")

SQL: select a.list as a, b.list as b from ListA as a full join ListB as b on a.list = b.list where a.list eq '' OR b.list eq "" ;

XOR all elements

A = [1,2,3,4,5,6,7,8] B = [1,2,3,4,5,6,8] [i for i in A if i not in B]

find the sum of the two list and subtract. ans = sum(a) - sum(b) where a and b are list of numbers.

Depends on the kind of elements in the lists. If they're numbers, sum(A) minus sum(B) will give the missing element. If they're characters/strings, just dump the elements of A into a dictionary and check each element in B for existence in A.

[i for i in A if i not in [j for j in B]]

Python: sum(A)-sum(B)