conditional-statements | 易学教程

Conditional mean statement

阅读更多关于 Conditional mean statement

问题 I have a dataset named bwght which contains the variable cigs (cigarattes smoked per day) When I calculate the mean of cigs in the dataset bwght using: mean(bwght$cigs) , I get a number 2.08. Only 212 of the 1388 women in the sample smoke (and 1176 does not smoke): summary(bwght$cigs>0) gives the result: Mode FALSE TRUE NA's logical 1176 212 0 I'm asked to find the average of cigs among the women who smoke (the 212). I'm having a hard time finding the right syntax for excluding the non

filter HABTM-associated model with conditions

阅读更多关于 filter HABTM-associated model with conditions

问题 preamble: a few days ago I asked a question to solve a HABTM-filter, I'm not able to do it even with tutorials, so "Obi Kwan Kenobi youre my only hope". What I want to achieve: Filtering Staff by GroupID which is used in StaffStaffgroup I'm having the following tablelayout staffs (a person can belong to many groups) staff_staffgroups (HABTM-linking table) staffgroups (has a groupname) The variable $tmp gets me a working array, but the problem is that Staff is a child object of

How to use Fortran statement labels well?

阅读更多关于 How to use Fortran statement labels well?

问题 I'm working on a model written in Fortran 95, which I am completely new to. The concept of statement labels seems strange, and I've so far only found the explanation that the labels can be arbitrarily decided by the author, usually incrementing by 10's. Are there any practical uses of these labels, other than picking out more easily where a statement is ending? AND a generally accepted standard on how to label. 回答1: The only way I can think of statement labels being useful in modern Fortran

Ant conditions - which comes first 'if' or 'unless'

阅读更多关于 Ant conditions - which comes first 'if' or 'unless'

问题 Question If an ant target uses both if and unless , which is evaluated first? Example What comes first, the chicken or the egg? . . . <target name="prepare" if="chicken" unless="egg" > <echo>Dinner time. Chicken is served!</echo> </target> Would ant evaluate the chicken property first? Or the egg property? 回答1: It isn't really a question of evaluation, since the properties either are or are not set before the target gets called. EDIT: I looked at the 1.8.1 source and the logic is as follows:

SQL: conditioning multiple rows on an inner joined table

阅读更多关于 SQL: conditioning multiple rows on an inner joined table

I am needing some help with SQL syntax. Say I have a members table, a questions table and an answers table, basics of the tables are as follows: Members table: memberId: primary key Questions table: questionId: primary key, questionText:varchar Answers table: answerId: primary key, questionId: int (relating to the row of the question in the questions table) memberId: int (relating to the row of the member in the members table) answerValue:varchar The tables will have more columns but for the purpose of this problem these should suffice. Now In some instances I will want to run a query that

Conditional statements in a class, but outside of scope of the function

阅读更多关于 Conditional statements in a class, but outside of scope of the function

问题 We know that with notation: class Foo(object): a = 1 def __init__(self): self.b = 2 def c(self): print('c') we can create static variable Foo.a , 'normal' variable b , which will be available after creating and instance of Foo , and method c Today I was really surprised, that I can use conditional statements in a class, but outside of scope of the function class C(): if True: a = 1 b = 2 Languages like C++/Java, taught me that legal notation is similar to: class Name(): variable = <expression

Is it safe to check if a pointer is null, then dereference it in the same if statement?

阅读更多关于 Is it safe to check if a pointer is null, then dereference it in the same if statement?

问题 Is the following code safe if a null pointer is passed in? if(ptr && *ptr == value) { //do something } Does the order of the checks matter? Does it work if I change it to this? if(*ptr == value && ptr) { //do something } 回答1: The former is correct and safe, the latter is not. The built-in && operator has short-circuit semantics , meaning that the second argument is evaluated if and only if the first one is true. (This is not the case for overloaded operators.) 回答2: Yes (1) is safe because of

Pointer dereferencing overhead vs branching / conditional statements

阅读更多关于 Pointer dereferencing overhead vs branching / conditional statements

问题 In heavy loops, such as ones found in game applications, there could be many factors that decide what part of the loop body is executed (for example, a character object will be updated differently depending on its current state) and so instead of doing: void my_loop_function(int dt) { if (conditionX && conditionY) doFoo(); else doBar(); ... } I am used to using a function pointer that points to a certain logic function corresponding to the character's current state , as in: void (*updater)

Is there a possibility of multiple statements inside a conditional statement's body?

阅读更多关于 Is there a possibility of multiple statements inside a conditional statement's body?

问题 I'm primarily a C++ (thus an OO/imperative) programmer and I find it quite bizarre that you can only have one statement per evaluation in a conditional statement such as an if-statement in Scheme, a functional language. For example: (let ((arg1 0) (arg2 1)) (if (> arg1 arg2) arg1 arg2))) Erroneous example: (let ((arg1 0) (arg2 1)) (if (> arg1 arg2) (arg1 (display "cool")) (arg2 (display "not cool")))) gives me an error of a type "procedure application: expected procedure, given: 2; arguments

Dynamically changing conditions of an if statement in C#

阅读更多关于 Dynamically changing conditions of an if statement in C#

问题 I'm working on an algorithm whose purpose is to create a collection of dates on which a certain event occurs (so that I can bold them on a MonthCalendar control, or display them in some other way). Now, for simple time intervals that can be described with regular functions (for instance if the event occurs every Monday or every 10 days) this is not a problem. The real problem occurs when I have an event with irregular occurrences (for instance if event occurs every Friday if the first day of