Python/data_structures/stacks/infix_to_prefix_conversion.py

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"""
Output:
Enter an Infix Equation = a + b ^c
Symbol | Stack | Postfix
----------------------------
c | | c
^ | ^ | c
b | ^ | cb
+ | + | cb^
a | + | cb^a
| | cb^a+
a+b^c (Infix) -> +a^bc (Prefix)
"""
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def infix_2_postfix(Infix):
Stack = []
Postfix = []
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priority = {
"^": 3,
"*": 2,
"/": 2,
"%": 2,
"+": 1,
"-": 1,
} # Priority of each operator
print_width = len(Infix) if (len(Infix) > 7) else 7
# Print table header for output
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print(
"Symbol".center(8),
"Stack".center(print_width),
"Postfix".center(print_width),
sep=" | ",
)
print("-" * (print_width * 3 + 7))
for x in Infix:
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if x.isalpha() or x.isdigit():
Postfix.append(x) # if x is Alphabet / Digit, add it to Postfix
elif x == "(":
Stack.append(x) # if x is "(" push to Stack
elif x == ")": # if x is ")" pop stack until "(" is encountered
while Stack[-1] != "(":
Postfix.append(Stack.pop()) # Pop stack & add the content to Postfix
Stack.pop()
else:
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if len(Stack) == 0:
Stack.append(x) # If stack is empty, push x to stack
else: # while priority of x is not > priority of element in the stack
while len(Stack) > 0 and priority[x] <= priority[Stack[-1]]:
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Postfix.append(Stack.pop()) # pop stack & add to Postfix
Stack.append(x) # push x to stack
print(
x.center(8),
("".join(Stack)).ljust(print_width),
("".join(Postfix)).ljust(print_width),
sep=" | ",
) # Output in tabular format
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while len(Stack) > 0: # while stack is not empty
Postfix.append(Stack.pop()) # pop stack & add to Postfix
print(
" ".center(8),
("".join(Stack)).ljust(print_width),
("".join(Postfix)).ljust(print_width),
sep=" | ",
) # Output in tabular format
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return "".join(Postfix) # return Postfix as str
def infix_2_prefix(Infix):
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Infix = list(Infix[::-1]) # reverse the infix equation
for i in range(len(Infix)):
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if Infix[i] == "(":
Infix[i] = ")" # change "(" to ")"
elif Infix[i] == ")":
Infix[i] = "(" # change ")" to "("
return (infix_2_postfix("".join(Infix)))[
::-1
] # call infix_2_postfix on Infix, return reverse of Postfix
if __name__ == "__main__":
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Infix = input("\nEnter an Infix Equation = ") # Input an Infix equation
Infix = "".join(Infix.split()) # Remove spaces from the input
print("\n\t", Infix, "(Infix) -> ", infix_2_prefix(Infix), "(Prefix)")