Python/data_structures/stacks/infix_to_prefix_conversion.py
hollowcrust 672fda9130
Fix bug and edit doctests for infix_to_prefix_conversion (#10259)
* Fix bug and edit doctests

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
2023-10-11 11:00:49 +02:00

194 lines
5.7 KiB
Python

"""
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)
"""
def infix_2_postfix(infix: str) -> str:
"""
>>> infix_2_postfix("a+b^c") # doctest: +NORMALIZE_WHITESPACE
Symbol | Stack | Postfix
----------------------------
a | | a
+ | + | a
b | + | ab
^ | +^ | ab
c | +^ | abc
| + | abc^
| | abc^+
'abc^+'
>>> infix_2_postfix("1*((-a)*2+b)") # doctest: +NORMALIZE_WHITESPACE
Symbol | Stack | Postfix
-------------------------------------------
1 | | 1
* | * | 1
( | *( | 1
( | *(( | 1
- | *((- | 1
a | *((- | 1a
) | *( | 1a-
* | *(* | 1a-
2 | *(* | 1a-2
+ | *(+ | 1a-2*
b | *(+ | 1a-2*b
) | * | 1a-2*b+
| | 1a-2*b+*
'1a-2*b+*'
>>> infix_2_postfix("")
Symbol | Stack | Postfix
----------------------------
''
>>> infix_2_postfix("(()")
Traceback (most recent call last):
...
ValueError: invalid expression
>>> infix_2_postfix("())")
Traceback (most recent call last):
...
IndexError: list index out of range
"""
stack = []
post_fix = []
priority = {
"^": 3,
"*": 2,
"/": 2,
"%": 2,
"+": 1,
"-": 1,
} # Priority of each operator
print_width = max(len(infix), 7)
# Print table header for output
print(
"Symbol".center(8),
"Stack".center(print_width),
"Postfix".center(print_width),
sep=" | ",
)
print("-" * (print_width * 3 + 7))
for x in infix:
if x.isalpha() or x.isdigit():
post_fix.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
if len(stack) == 0: # close bracket without open bracket
raise IndexError("list index out of range")
while stack[-1] != "(":
post_fix.append(stack.pop()) # Pop stack & add the content to Postfix
stack.pop()
else:
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 stack and stack[-1] != "(" and priority[x] <= priority[stack[-1]]:
post_fix.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(post_fix)).ljust(print_width),
sep=" | ",
) # Output in tabular format
while len(stack) > 0: # while stack is not empty
if stack[-1] == "(": # open bracket with no close bracket
raise ValueError("invalid expression")
post_fix.append(stack.pop()) # pop stack & add to Postfix
print(
" ".center(8),
("".join(stack)).ljust(print_width),
("".join(post_fix)).ljust(print_width),
sep=" | ",
) # Output in tabular format
return "".join(post_fix) # return Postfix as str
def infix_2_prefix(infix: str) -> str:
"""
>>> infix_2_prefix("a+b^c") # doctest: +NORMALIZE_WHITESPACE
Symbol | Stack | Postfix
----------------------------
c | | c
^ | ^ | c
b | ^ | cb
+ | + | cb^
a | + | cb^a
| | cb^a+
'+a^bc'
>>> infix_2_prefix("1*((-a)*2+b)") # doctest: +NORMALIZE_WHITESPACE
Symbol | Stack | Postfix
-------------------------------------------
( | ( |
b | ( | b
+ | (+ | b
2 | (+ | b2
* | (+* | b2
( | (+*( | b2
a | (+*( | b2a
- | (+*(- | b2a
) | (+* | b2a-
) | | b2a-*+
* | * | b2a-*+
1 | * | b2a-*+1
| | b2a-*+1*
'*1+*-a2b'
>>> infix_2_prefix('')
Symbol | Stack | Postfix
----------------------------
''
>>> infix_2_prefix('(()')
Traceback (most recent call last):
...
IndexError: list index out of range
>>> infix_2_prefix('())')
Traceback (most recent call last):
...
ValueError: invalid expression
"""
reversed_infix = list(infix[::-1]) # reverse the infix equation
for i in range(len(reversed_infix)):
if reversed_infix[i] == "(":
reversed_infix[i] = ")" # change "(" to ")"
elif reversed_infix[i] == ")":
reversed_infix[i] = "(" # change ")" to "("
# call infix_2_postfix on Infix, return reverse of Postfix
return (infix_2_postfix("".join(reversed_infix)))[::-1]
if __name__ == "__main__":
from doctest import testmod
testmod()
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)")