Python/data_structures/stacks/postfix_evaluation.py

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"""
Output:
Enter a Postfix Equation (space separated) = 5 6 9 * +
Symbol | Action | Stack
-----------------------------------
5 | push(5) | 5
6 | push(6) | 5,6
9 | push(9) | 5,6,9
| pop(9) | 5,6
| pop(6) | 5
* | push(6*9) | 5,54
| pop(54) | 5
| pop(5) |
+ | push(5+54) | 59
Result = 59
"""
import operator as op
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def Solve(Postfix):
Stack = []
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Div = lambda x, y: int(x / y) # integer division operation
Opr = {
"^": op.pow,
"*": op.mul,
"/": Div,
"+": op.add,
"-": op.sub,
} # operators & their respective operation
# print table header
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print("Symbol".center(8), "Action".center(12), "Stack", sep=" | ")
print("-" * (30 + len(Postfix)))
for x in Postfix:
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if x.isdigit(): # if x in digit
Stack.append(x) # append x to stack
print(
x.rjust(8), ("push(" + x + ")").ljust(12), ",".join(Stack), sep=" | "
) # output in tabular format
else:
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B = Stack.pop() # pop stack
print(
"".rjust(8), ("pop(" + B + ")").ljust(12), ",".join(Stack), sep=" | "
) # output in tabular format
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A = Stack.pop() # pop stack
print(
"".rjust(8), ("pop(" + A + ")").ljust(12), ",".join(Stack), sep=" | "
) # output in tabular format
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Stack.append(
str(Opr[x](int(A), int(B)))
) # evaluate the 2 values poped from stack & push result to stack
print(
x.rjust(8),
("push(" + A + x + B + ")").ljust(12),
",".join(Stack),
sep=" | ",
) # output in tabular format
return int(Stack[0])
if __name__ == "__main__":
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Postfix = input("\n\nEnter a Postfix Equation (space separated) = ").split(" ")
print("\n\tResult = ", Solve(Postfix))