operators

问题 I study C language from "C Primer Plus" book by Stephen Prata and it came to the point : "A full expression is one that’s not a subexpression of a larger expression.Examples of full expressions include the expression in an expression statement and the expression serving as a test condition for a while loop" I can't understand clearly what is the exact definition of full expressions and why the book considers test conditions are full expressions. Could any one explain clearly what is meant by

问题 I am quite experienced with Python, but recently, when I was looking at the solutions for the codility sample tests I encountered the operators -= , += , ^= and I am unable to figure out what they do. Perhaps could anyone explain the context in which they are used? 回答1: As almost any modern language, python has Assignment Operators so they can use them every time you want to assign a value to a variable after doing some arithmetic or logical operation, both (assignment and operation)are

问题 I am quite experienced with Python, but recently, when I was looking at the solutions for the codility sample tests I encountered the operators -= , += , ^= and I am unable to figure out what they do. Perhaps could anyone explain the context in which they are used? 回答1: As almost any modern language, python has Assignment Operators so they can use them every time you want to assign a value to a variable after doing some arithmetic or logical operation, both (assignment and operation)are

问题 I am quite experienced with Python, but recently, when I was looking at the solutions for the codility sample tests I encountered the operators -= , += , ^= and I am unable to figure out what they do. Perhaps could anyone explain the context in which they are used? 回答1: As almost any modern language, python has Assignment Operators so they can use them every time you want to assign a value to a variable after doing some arithmetic or logical operation, both (assignment and operation)are

问题 For example: int age; cin >> age; cout << "You are " << age << " years old!" << endl; Why do we use the "<<" and ">>" operators here? What are they doing? I somewhat understand bit-shifting, but I don't get how that works here. 回答1: They are called the stream insertion operator ( << ) and the stream extraction operator ( >> ). These are the same operators as the left and right bit shift operators (even though they have different names). The bit shift operators are overloaded, so that when the

问题 I'm trying to understand the exact mechanism behind updating a python dictionary using d[key] += diff . I have some helper classes to trace magic method invocations: class sdict(dict): def __setitem__(self, *args, **kargs): print "sdict.__setitem__" return super(sdict, self).__setitem__(*args, **kargs) def __delitem__(self, *args, **kargs): print "sdict.__delitem__" return super(sdict, self).__delitem__(*args, **kargs) def __getitem__(self, *args, **kargs): print "sdict.__getitem__" return

问题 std::cout << (true ? "high pass" : false ? "fail" : "pass") is the same as std::cout << (true ? "high pass" : (false ? "fail" : "pass")) Since the ternary operator is right associative, why don't we perform the right-hand operation first? Shouldn't pass be printed instead of high pass ? 回答1: You misunderstood operator associativity. It's simply the way to group operators with the same precedence and doesn't affect order of evaluation in any way. So cond1 ? 1 : cond2 ? 2 : cond3 ? 3 : 4 will

问题 I recently answered a SO-question about using -lt or -gt with strings. My answer was based on something I've read earlier which said that -lt compares one char from each string at a time until a ASCII-value is not equal to the other. At that point the result (lower/equal/greater) decides. By that logic, "Less" -lt "less" should return True because L has a lower ASCII-byte-value than l , but it doesn't: [System.Text.Encoding]::ASCII.GetBytes("Less".ToCharArray()) 76 101 115 115 [System.Text

问题 How should I interpret this sentence in Python (in terms of operators precedence)? c = not a == 7 and b == 7 as c = not (a == 7 and b == 7) or c = (not a) == 7 and b == 7 ? thanks 回答1: Using dis module: >>> import dis >>> def func(): ... c = not a == 7 and b == 7 ... >>> dis.dis(func) 2 0 LOAD_GLOBAL 0 (a) 3 LOAD_CONST 1 (7) 6 COMPARE_OP 2 (==) 9 UNARY_NOT 10 JUMP_IF_FALSE_OR_POP 22 13 LOAD_GLOBAL 1 (b) 16 LOAD_CONST 1 (7) 19 COMPARE_OP 2 (==) >> 22 STORE_FAST 0 (c) 25 LOAD_CONST 0 (None) 28

问题 I am working on repeats classification project. I am calculating time lapse in between a repeat and fresh mail in days. I want to apply a function on this time lapse which states whether it is a fresh mail or repeat mail. function: days = df['days_difference'] if(days<30): return 'repeat' else: return 'fresh' I am getting error: not supported between instances of 'str' and 'int' 'days_difference' column contains integer values along with None values. looking for a solution ! 回答1: That