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pythonevaluationoperator-precedence
udpsunil
udpsunil

Reputation: 1375

Why does 1+++2 = 3?

How does Python evaluate the expression 1+++2?

How many ever + I put in between, it is printing 3 as the answer. Please can anyone explain this behavior

And for 1--2 it is printing 3 and for 1---2 it is printing -1

Upvotes: 32

Views: 7645

Answers (6)

Greg Hewgill
Greg Hewgill

Reputation: 992707

Your expression is the same as:

1+(+(+2))

Any numeric expression can be preceded by - to make it negative, or + to do nothing (the option is present for symmetry). With negative signs:

1-(-(2)) = 1-(-2)
         = 1+2
         = 3

and

1-(-(-2)) = 1-(2)
          = -1

I see you clarified your question to say that you come from a C background. In Python, there are no increment operators like ++ and -- in C, which was probably the source of your confusion. To increment or decrement a variable i or j in Python use this style:

i += 1
j -= 1

Upvotes: 63

Juha Syrjälä
Juha Syrjälä

Reputation: 34261

Think it as 1 + (+1*(+1*2))). The first + is operator and following plus signs are sign of second operand (= 2).

Just like 1---2 is same as 1 - -(-(2)) or 1- (-1*(-1*(2))

Upvotes: 1

Brian R. Bondy
Brian R. Bondy

Reputation: 347196

The extra +'s are not incrementors (like ++a or a++ in c++). They are just showing that the number is positive.

There is no such ++ operator. There is a unary + operator and a unary - operator though. The unary + operator has no effect on its argument. The unary - operator negates its operator or mulitplies it by -1. 

+1

-> 1

++1

-> 1

This is the same as +(+(1))

   1+++2

-> 3 Because it's the same as 1 + (+(+(2))

Likewise you can do --1 to mean - (-1) which is +1.

  --1

-> 1

For completeness there is no * unary opeartor. So *1 is an error. But there is a ** operator which is power of, it takes 2 arguments. 

 2**3

-> 8

Upvotes: 15

gimel
gimel

Reputation: 86344

Trying Unary Plus and Unary minus:

The unary - (minus) operator yields the negation of its numeric argument.

The unary + (plus) operator yields its numeric argument unchanged.

>>> +2
2
>>> ++2
2
>>> +++2
2
>>> -2
-2
>>> --2
2
>>> ---2
-2
>>> 1+(++2)
3

Upvotes: 4

James Curran
James Curran

Reputation: 103485

I believe it's being parsed as, the first + as a binary operation (add), and the rest as unary operations (make positive). 

 1 + (+(+2))

Upvotes: 1

alexwood
alexwood

Reputation: 612

1+(+(+2)) = 3

1 - (-2) = 3

1 - (-(-2)) = -1

Upvotes: 4

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