CODEWITHSUNDEEP
pythonoperator-precedence
Terry Chern
Terry Chern

Reputation: 692

Evaluation of comparison operators in Python behaving unexpectedly

I'm TAing an introductory course on Python this semester (using 3.4) and recently came across an exercise about operator precedence and using parentheses to make a statement evaluate to true.

The exact question is: 

Add a single pair of parentheses to the expression so that it evaluates to true.
1 < -1 == 3 > 4

I assumed that the correct answer would be:

1 < -1 == (3 > 4)

Given that the comparison operators are all on the same level of precedence, they should evaluate from left to right, so it should evaluate as such:

1 < -1 == (3 > 4) 
1 < -1 == False
False == False
True

But when I run the code it still returns false. I saw this question comparison operators' priority in Python vs C/C++ and the result of that expression makes sense to me; but in this case I've forced the evaluation of the latter statement before evaluating the rest of the expression, so I don't get why I'm still getting the wrong answer.

I've been staring at it for the past hour and I feel like I might be overlooking something obvious; if anyone could provide some insight as to what the correct solution might be it'd be greatly appreciated.

Upvotes: 1

Views: 919

Answers (4)

w1cked
w1cked

Reputation: 1

(1 < -1) == (3 > 4)

False == False

True

Upvotes: 0

Imran
Imran

Reputation: 13458

You are on the right track, but due to comparison chaining you will have:

1 < -1 == (3 > 4) 
1 < -1 == False
1 < -1 and -1 == False
False and False
False

Notice that we don't evaluate the second line left to right, rather we chain the two comparisons together with an and.

There are only a few valid ways to add one pair of parentheses to this, so we can check all the others easily:

(1 < -1 == 3 > 4)
False #Trivially

(1 < -1) == 3 > 4
False == 3 > 4
False == 3 and 3 > 4
False and False
False

1 < (-1 == 3) > 4
1 < False > 4
1 < False and False > 4
False and False
False

It looks like there is no answer to this problem!

Edit:

Whoops! Rob points out we forgot:

1 < -(1 == 3) > 4
1 < -False > 4
1 < 0 > 4
1 < 0 and 0 > 4
False and False
False

Upvotes: 1

Robᵩ
Robᵩ

Reputation: 168596

The task is provably impossible. Consider these three cases:

  1. There is an open paren immediately before -1.

  2. There is an open paren between - and 1.

  3. There is an open paren anywhere else.

These three cases represent every possible location for the parentheses.

In the first case, we have 1 < ( ... ), where the ellipsis is a boolean expression. Since 1 is not less than either True or False, the entire expression is False.

In the second case, we have 1 < -( ...), where the ellipsis is a boolean expression. Since 1 is not less than either -True nor -False, the entire expression is False.

In the third case, we have 1 < -1 == .... Since all legs of a expression of chained operators must be True, and and since 1 < -1 is False, the entire expression is False.

So, in every possible case, the result is False.

Upvotes: 2

jithinodattu
jithinodattu

Reputation: 90

3>4 gives False. -1 == False gives False. 1 < False gives False. Hence, 1 < -1 == 3 > 4 becomes False.

Give proper brackets to make the statement semantically correct.

(1 < -1) == (3 > 4)

Upvotes: 1

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