++ is not an operator. It is two + operators. The + operator is the identity operator, which does nothing. (Clarification: the + and - unary operators only work on numbers, but I presume that you wouldn't expect a hypothetical ++ operator to work on strings.)
++count
Parses as
+(+count)
Which translates to
count
You have to use the slightly longer += operator to do what you want to do:
count += 1
I suspect the ++ and -- operators were left out for consistency and simplicity. I don't know the exact argument Guido van Rossum gave for the decision, but I can imagine a few arguments:
- Simpler parsing. Technically, parsing
++countis ambiguous, as it could be+,+,count(two unary+operators) just as easily as it could be++,count(one unary++operator). It's not a significant syntactic ambiguity, but it does exist. - Simpler language.
++is nothing more than a synonym for+= 1. It was a shorthand invented because C compilers were stupid and didn't know how to optimizea += 1into theincinstruction most computers have. In this day of optimizing compilers and bytecode interpreted languages, adding operators to a language to allow programmers to optimize their code is usually frowned upon, especially in a language like Python that is designed to be consistent and readable. - Confusing side-effects. One common newbie error in languages with
++operators is mixing up the differences (both in precedence and in return value) between the pre- and post-increment/decrement operators, and Python likes to eliminate language "gotcha"-s. The precedence issues of pre-/post-increment in C are pretty hairy, and incredibly easy to mess up.
++ is not an operator. It is two + operators. The + operator is the identity operator, which does nothing. (Clarification: the + and - unary operators only work on numbers, but I presume that you wouldn't expect a hypothetical ++ operator to work on strings.)
++count
Parses as
+(+count)
Which translates to
count
You have to use the slightly longer += operator to do what you want to do:
count += 1
I suspect the ++ and -- operators were left out for consistency and simplicity. I don't know the exact argument Guido van Rossum gave for the decision, but I can imagine a few arguments:
- Simpler parsing. Technically, parsing
++countis ambiguous, as it could be+,+,count(two unary+operators) just as easily as it could be++,count(one unary++operator). It's not a significant syntactic ambiguity, but it does exist. - Simpler language.
++is nothing more than a synonym for+= 1. It was a shorthand invented because C compilers were stupid and didn't know how to optimizea += 1into theincinstruction most computers have. In this day of optimizing compilers and bytecode interpreted languages, adding operators to a language to allow programmers to optimize their code is usually frowned upon, especially in a language like Python that is designed to be consistent and readable. - Confusing side-effects. One common newbie error in languages with
++operators is mixing up the differences (both in precedence and in return value) between the pre- and post-increment/decrement operators, and Python likes to eliminate language "gotcha"-s. The precedence issues of pre-/post-increment in C are pretty hairy, and incredibly easy to mess up.
Python does not have pre and post increment operators.
In Python, integers are immutable. That is you can't change them. This is because the integer objects can be used under several names. Try this:
>>> b = 5
>>> a = 5
>>> id(a)
162334512
>>> id(b)
162334512
>>> a is b
True
a and b above are actually the same object. If you incremented a, you would also increment b. That's not what you want. So you have to reassign. Like this:
b = b + 1
Many C programmers who used python wanted an increment operator, but that operator would look like it incremented the object, while it actually reassigns it. Therefore the -= and += operators where added, to be shorter than the b = b + 1, while being clearer and more flexible than b++, so most people will increment with:
b += 1
Which will reassign b to b+1. That is not an increment operator, because it does not increment b, it reassigns it.
In short: Python behaves differently here, because it is not C, and is not a low level wrapper around machine code, but a high-level dynamic language, where increments don't make sense, and also are not as necessary as in C, where you use them every time you have a loop, for example.
[Python] How do you increment/decrement values in a list?
x -= 1 conceptually means "create a new number object that has the value one less than x and make x refer to that object." This does not affect the list at all, because the list is still referring to the old object.
Use a list comprehension or iterate by index. See this thread from just the other day.
More on reddit.comPython lacks pre-increment/decrement; `++count` is syntactically valid but inert - TestMu AI Community
How do increment and decrement operators work in Python? - Python - Data Science Dojo Discussions
How to decrement using a for or while loop?
Videos
When I put a += in a for loop, the += doesn't actually get run. My goal is to lower each value in the list by one each loop, so long as it's greater than 0.
http://ideone.com/8lHUit
x -= 1 conceptually means "create a new number object that has the value one less than x and make x refer to that object." This does not affect the list at all, because the list is still referring to the old object.
Use a list comprehension or iterate by index. See this thread from just the other day.
the nice way would be to use a list comprehension.
list = [item-1 if item > 0 else item for item in list]Okay, some of you might argue that it’s because Python should be clear in terms of understanding, but I am not convinced that a simple ++ sign would mess around with the programmer’s interpretation. Any real arguments in favor of keeping it out of the language?