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Program to evaluate Boolean expression from a string in Python?
Suppose we have a string s containing a boolean expression with operators "and" and "or", evaluate it and return the result. Here the expressions may have parentheses, which should be evaluated first.
So, if the input is like s = "T and (F or T)", then the output will be True
To solve this, we will follow these steps:
stack := a new list
t = list of elements of s split by blank spaces
for each v in t, do
if v[0] is same as "(", then
push true when v[from index of "(" to end] is same as "T", into stack
otherwise when ")" is found, then
ct := number of closing bracket ")" in v
push true when v[from index 0 to size of v - ct] is same as "T" into stack
for each value in range 0 to ct-1, do
right := pop from stack
:= pop from stack
left := pop from stack
perform operation (left o right) and push into stack
otherwise when v is either "T" or "F", then
push true when v is same as "T" into stack
otherwise,
push op[v] into stack
if element count in stack > 1, then
for i in range 0 to size of stack - 1, increase by 2, do
stack[i + 2] := stack[i + 1](stack[i], stack[i + 2])
return top element of stack
return bottom element of stack
Let us see the following implementation to get better understanding:
Example
class Solution:
def solve(self, s):
stack = []
op = {
"or": lambda x, y: x or y,
"and": lambda x, y: x and y,
}
for v in s.split():
if v[0] == "(":
stack.append(v[v.count("(") :] == "T")
elif v.count(")") > 0:
ct = v.count(")")
stack.append(v[:-ct] == "T")
for _ in range(ct):
right = stack.pop()
o = stack.pop()
left = stack.pop()
stack.append(o(left, right))
elif v in ["T", "F"]:
stack.append(v == "T")
else:
stack.append(op[v])
if len(stack) > 1:
for i in range(0, len(stack) - 1, 2):
stack[i + 2] = stack[i + 1](stack[i], stack[i + 2])
return stack[-1]
return stack[0]
ob = Solution()
s = "T and (F or T)"
print(ob.solve(s))Input
"T and (F or T)"
Output
True
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