How to use recursion on a nested list in order to add/subtract/multiply/divide?-CodePudding

If I have:

['add', 12, 12]

I would just do:

if i[0] == 'add':
   a = i[1]   i[2]
   return a

but what if it's something like

['add', 12, ['add', 12, 12]]

I know that I need to solve the ['add', 12, 12] on the inside first using a recursion but I'm not sure how to do it. And if it's for something like this:

['add', 12, ['mul', 6, ['sub', 6, 4]]]

How would I solve this using recursion?

CodePudding user response：

You can use recursion:

import operator as op
def run_op(l):
   if not isinstance(l, list):
      return l
   return getattr(op, l[0])(*map(run_op, l[1:]))

r = ['add', 12, ['mul', 6, ['sub', 6, 4]]]
print(run_op(r)) #24

CodePudding user response：

Just check if the elements are lists via isinstance. If so, recursively call your solve function, otherwise just leave it as is.

def solve(i):
    if i[0] == "add":
        if isinstance(i[1], list):
            x = solve(i[1])
        else:
            x = i[1]
        if isinstance(i[2], list):
            y = solve(i[2])
        else:
            y = i[2]
        return x   y

To implement multiplication, subtraction, etc, just add more cases to the outer if, following similar logic.

CodePudding user response：

Check if the parameter is a list. If it is, assume that the two operands need to go through recursion to get their respective values, then perform the specified operation. If the parameter is not a list simply return it as is.

def calc(A):
    if not isinstance(A,list): return A  # not a list, return value
    op,left,right = map(calc,A)          # recurse to get values
    if op == "add": return left right    # perform operation...
    if op == "sub": return left-right
    if op == "mul": return left*right
    if op == "div": return left/right

print(calc(['add', 12, ['mul', 6, ['sub', 6, 4]]])) # 24