Update evaluate_postfix_notations.py

[rohan472000]

Describe your change:

Fixes #8754

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: {#8754}.

[amirsoroush]

Hi, Please add Fixes #8754 to the PR's description so that GitHub can attach this PR to the issue.

[amirsoroush]

I couldn't pass the doctests. Here are two failed test cases:

**********************************************************************
File ".../evaluate_postfix_notations.py", line 18, in __main__.evaluate_postfix
Failed example:
    evaluate_postfix(["4", "13", "5", "/", "+"])
Expected:
    6
Got:
    6.6
**********************************************************************
File ".../evaluate_postfix_notations.py", line 20, in __main__.evaluate_postfix
Failed example:
    evaluate_postfix(["2", "+"])
Exception raised:
    Traceback (most recent call last):
      File "/usr/lib/python3.10/doctest.py", line 1350, in __run
        exec(compile(example.source, filename, "single",
      File "<doctest __main__.evaluate_postfix[2]>", line 1, in <module>
        evaluate_postfix(["2", "+"])
      File ".../evaluate_postfix_notations.py", line 46, in evaluate_postfix
        raise ValueError(
    ValueError: Invalid expression: insufficient operands for binary operator
**********************************************************************
1 items had failures:
   2 of   5 in __main__.evaluate_postfix
***Test Failed*** 2 failures.

[amirsoroush]

unary_operations is not currently being used.

[rohan472000]

now I have made several changes, kindly look into it.

[tianyizheng02]

This doesn't fix the problem with unary negation operators. We want to support negation, not raise an error if it's present.

[rohan472000]

made several changes, look into that.

[tianyizheng02]

What is the purpose of this elif block?

[rohan472000]

This block will handle +,*,/ operators as above block(if block) is handling "-" operator.

[amirsoroush]

I guess that way is more readable.

[amirsoroush]

elif (True) and len(stack) < 2 can be simplified to just len(stack) < 2.

[rohan472000]

@cclauss, look into it again in order to merge.

[cclauss]

Doing this work without using https://docs.python.org/3/library/operator.html seems suboptimal.

We should be able to deal with both floats and ints.

Please add tests for negative numbers, zeros, floats and

• evaluate_postfix(["5"])
• evaluate_postfix(["-"])
• evaluate_postfix(["5", "A"])
• evaluate_postfix(["5", "4", "A"])
• evaluate_postfix(["A"])

[rohan472000]

for >>> evaluate_postfix(["-"]) what output should I give, 0 or -1 or anything else??

[tianyizheng02]

for >>> evaluate_postfix(["-"]) what output should I give, 0 or -1 or anything else??

@rohan472000 See my comment above. It should raise an error because the negation operator has nothing to negate.

[tianyizheng02]

Suggested change

	if len(stack) < 2:
	operand = stack.pop()
	if token == "-":
	operand = -operand
	else:
	raise ValueError(f"Unrecognized {token = }")
	stack.append(operand)
	if token == "-":
	if len(stack) == 1:
	operand = stack.pop()
	stack.append(-operand)
	elif len(stack) == 0:
	raise ValueError(f"Negation operator with no operand")

Currently, if there's only 1 operand on the stack but the token is not -, then it just appends the operand back onto the stack. This effectively leaves the stack exactly as it already was, so you can skip that altogether.

Furthermore, checking len(stack) < 2 includes the case in which the stack is empty. This should raise an error since there's no operand to negate.

[rohan472000]

I'm not able to commit above change requested...!!

[rohan472000]

@tianyizheng02 ,as your suggestion is making the build fail(doctest not passing), so I have again pushed my previous code.

[tianyizheng02]

The build was failing because this test was failing:

>>> evaluate_postfix(["2", "+"])

UNEXPECTED EXCEPTION: IndexError('pop from empty list')
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.11.3/x64/lib/python3.11/doctest.py", line 1351, in __run
    exec(compile(example.source, filename, "single",
  File "<doctest data_structures.stacks.evaluate_postfix_notations.evaluate_postfix[2]>", line 1, in <module>
  File "/home/runner/work/Python/Python/data_structures/stacks/evaluate_postfix_notations.py", line 55, in evaluate_postfix
    b, a = stack.pop(), stack.pop()
                        ^^^^^^^^^^^
IndexError: pop from empty list
/home/runner/work/Python/Python/data_structures/stacks/evaluate_postfix_notations.py:20: UnexpectedException

I believe my suggestion was still correct, but you need to add an if-statement to handle this edge case as well. This test should raise an exception because + is a binary operator and there is only 1 operand.

[rohan472000]

ok...but main concern is for >>> evaluate_postfix(["-"]) case....which is still not resolved..

[tianyizheng02]

ok...but main concern is for >>> evaluate_postfix(["-"]) case....which is still not resolved..

I believe that test should've already been passing. Again, my suggestion (specifically the elif len(stack) == 0: line) should've handled it.

[tianyizheng02]

Issue has already been fixed by another PR