logical-operators

问题 I am doing review questions which ask me "What is the output of the following," and I am having some trouble understanding something about this function: int a = 1, b = 1, c = -1; c = --a && b++; printf("%d %d %d", a, b, c); The output is 010. My question is about line 2, c = --a && b++ . How is this line processed, and how does it work/change the values? And if it were c = --a || b++ ? From my understanding I thought the output would be 020. 回答1: The key concept to understanding the result

问题 I would like to make a subset of a data frame in R that is based on one OR another value in a column of factors but it seems I cannot use | with factor values. Example: # fake data x <- sample(1:100, 9) nm <- c("a", "a", "a", "b", "b", "b", "c", "c", "c") fake <- cbind(as.data.frame(nm), as.data.frame(x)) # subset fake to only rows with name equal to a or b fake.trunk <- fake[fake$nm == "a" | "b", ] produces the error: Error in fake$nm == "a" | "b" : operations are possible only for numeric,

问题 I'm learning AngularJS, trying to make a simple calculator, and I'm trying to condense this if / else if statement to 1-2 lines, using the Javascript logical Operators ( && , || , ! ) Given this example, how could I reduce it? (if you don't understand $scope , ignore it. Its basically a view. So when someone clicks 9, the calculator will display 9 on the answer screen) $scope.setOperand = function (operandEntered) { if ($scope.leftOperand === null) { $scope.leftOperand = operandEntered; }

问题 I was under the impression it was possible to overload and in Python, but reading through the docs just now, I realized that __and__ refers to the bitwise & operator, not logical and . Am I overlooking something, or is it not possible to overload logical and in Python? 回答1: No this is not possible. There is a proposal that adds this functionality but for now, it is rejected. 回答2: No, it is not possible. See here. 回答3: There is no straight way to do this. You could override __nonzero__ for

问题 I have a quick question about using logical operators in an if statement. Currently I have an if statement that checks if x equals to 5 or 4 or 78: if ((x == 5) || (x == 4) || (x == 78)) { blah } And I was wondering if I could just condense all that to: if (x == 5 || 4 || 78) { blah } Sorry for such a basic question, I've just started learning C. 回答1: There is no shortcut, but you need to fix your equality operator. if ((x == 5) || (x == 4) || (x == 78)) { 回答2: First, you're using assignments

问题 I get a weird result and I try to apply the and or the or operator to 2 Boolean lists in python. I actually get the exact opposite of what I was expecting. [True, False, False] and [True, True, False] > [True, True, False] [True, False, False] or [True, True, False] > [True, False, False] Is that normal, and if yes, why? 回答1: Your lists aren't comparing each individual value, they're comparing the existence of values in the list. For any truthy variables a and b : a and b > b #The program

问题 I'm learning the book named Data Structures & Algorithms in Python . On Page 12 that introduce the Logical Operators , it writes: The and and or operators short-circuit , (I thinks it should add is called ), in that they do not evaluate the second operand if the result can be determined based on the value of the first operand. This feature is useful when constructing Boolean expressions in which we first test that a certain condition holds (such as a reference not being None), and then test a

问题 Strange question, but someone showed me this, I was wondering can you use the not ! operator for int in C++? (its strange to me). #include <iostream> using namespace std; int main() { int a=5, b=4, c=4, d; d = !( a > b && b <= c) || a > c && !b; cout << d; system ("pause"); return 0; } 回答1: Yes. For integral types, ! returns true if the operand is zero, and false otherwise. So !b here just means b == 0 . This is a particular case where a value is converted to a bool . The !b can be viewed as

问题 I am trying to use a Boolean mask to get a match from 2 different dataframes. U Using the logical OR operator: x = df[(df['A'].isin(df2['B'])) or df['A'].isin(df2['C'])] Output: ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all(). However using the bitwise OR operator, the results are returned successfully. x = df[(df['A'].isin(df2['B'])) | df['A'].isin(df2['C'])] Output: x Is there a difference in both and would bitwise OR be the best

问题 Here is some example data: data = data.frame(series = c("1a", "1b", "1e"), reading = c(0.1, 0.4, 0.6)) > data series reading 1 1a 0.1 2 1b 0.4 3 1e 0.6 Which I can pull out selective single rows using subset: > subset (data, series == "1a") series reading 1 1a 0.1 And pull out multiple rows using a logical OR > subset (data, series == "1a" | series == "1e") series reading 1 1a 0.1 3 1e 0.6 But if I have a long list of series expressions, this gets really annoying to input, so I'd prefer to