问题
I have trouble understanding why python returns different results for these 2 statements:
-1 // 3/4 and -1 // 0.75
The first one returns -0.25 and the second on returns -2.
The way i understand it, the / operator is executed before // , thus those 2 statements should have the same result.
edit: I was referring to a document provided by my university. I misinterpreted that. Official python documentation proves me wrong. Thanks for all the quick answers.
回答1:
The / and // operators have the same precedence according to the documentation so they are evaluated from left to right when used in the same expression. -1 // 3/4 is therefore equivalent to (-1 // 3)/4 rather than -1 // (3/4).
回答2:
The way i understand it, the
/operator is executed before//, thus those 2 statements should have the same result.
Your understanding is incorrect. / and // have the same precedence and have left associativity, meaning that Python performs the leftmost operation first - in your case, the /.
回答3:
The Expressions documentation has a section about Operator Precedence. Operators in the same box have the same precedence.
Thus, the table tells you that // and / have equal precedence, so
-1 // 3/4 parses as
>>> (-1//3)/4
>>> -0.25
回答4:
no, they have the same precedence, so they're evaluated from left to right.
-1//3 is rounded (to the least integer) integer division, so you get -1 divided by 4: 0.25
When you have doubts, it doesn't cost much to throw in a couple of parentheses.
回答5:
Think of these from an order of operations standpoint:
-1 // 3/4
This will perform -1 "floor" 3, which yields -1, which then divided by 4 yields -0.25.
Whereas:
-1 // 0.75
This will simply "floor" the operation straight away and yield -2.0.
回答6:
According to documentation, Multiplication *, matrix multiplication @, division /, floor division //, remainder % all have same precedence.
When two operators have the same precedence, associativity helps to determine the order of operations.
Now regarding your question; both / and // have same precedence, and if both of them are present in an expression, left one is evaluated first based on left-to-right associativity.
回答7:
// is essentially an operator for flooring division.
So 1 // 0.75 is essentially flooring 1.333 which is 1
-1 // 0.75 is essentially flooring -1.333 which is -2.
Hope this makes sense.
来源：https://stackoverflow.com/questions/53072731/operator-precedence-of-floor-division-and-division