++ operator (no wonder, having the language name on mind).
i++; ++i; i--; --i;Inline
-- and ++ constitute sort of a standard in quite a variety of languages, so let's get a grip on Python (2.7) as well. Move on to the snippet below:
A = range(0,10) i = 0 print A[--i]What is the output?
Having experience with C++/C#, etc. one might say it's an attempt to read at an out-of-bounds index. i would be decremented before accessing the list. However, in Python a negative index corresponds to a position counted from the back of the list. A[-1] is the last element in the collection, which in this example equals 9. The above script prints 0, though. Why is that?
The reason is related to the fact that ++ and -- are neither operators in Python, nor is there any kind of syntactic sugar in it. Therefore --i should be split into two separate occurrences of - unary operator that returns negated number. It has no effect on the value of the variable i, though. The critical statement could be also written as:
print A[-(-i)]What it really does is take the value of
i and negate it twice without modifying the variable itself.
For instance, a typical C++ while-loop could become an infinite loop in Python:
i = len(some_collection) while (--i > 0) ...Finally notice that
i++ or i-- are not legal in Python.
Read on CodeProject
