Python/data_structures/stacks/prefix_evaluation.py

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"""
Python3 program to evaluate a prefix expression.
"""
calc = {
"+": lambda x, y: x + y,
"-": lambda x, y: x - y,
"*": lambda x, y: x * y,
"/": lambda x, y: x / y,
}
def is_operand(c):
"""
Return True if the given char c is an operand, e.g. it is a number
>>> is_operand("1")
True
>>> is_operand("+")
False
"""
return c.isdigit()
def evaluate(expression):
"""
Evaluate a given expression in prefix notation.
Asserts that the given expression is valid.
>>> evaluate("+ 9 * 2 6")
21
>>> evaluate("/ * 10 2 + 4 1 ")
4.0
"""
stack = []
# iterate over the string in reverse order
for c in expression.split()[::-1]:
# push operand to stack
if is_operand(c):
stack.append(int(c))
else:
# pop values from stack can calculate the result
# push the result onto the stack again
o1 = stack.pop()
o2 = stack.pop()
stack.append(calc[c](o1, o2))
return stack.pop()
# Driver code
if __name__ == "__main__":
test_expression = "+ 9 * 2 6"
print(evaluate(test_expression))
test_expression = "/ * 10 2 + 4 1 "
print(evaluate(test_expression))