conditional-statements | 易学教程

Put one thread to sleep until a condition is resolved in another thread

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问题 Here are two chunks of code that accomplish (what I think is) the same thing. I basically am trying to learn how to use Java 1.5's concurrency to get away from Thread.sleep(long). The first example uses ReentrantLock, and the second example uses CountDownLatch. The jist of what I am trying to do is put one thread to sleep until a condition is resolved in another thread. The ReentrantLock provides a lock on the boolean I'm using to decide whether to wake the other thread or not, and then I use

How much does the order of case labels affect the efficiency of switch statements?

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Consider: if (condition1) { // Code block 1 } else { // Code block 2 } If I know that condition1 will be true the majority of the time, then I should code the logic as written, instead of: if (!condition1) { // Code block 2 } else { // Code block 1 } since I will avoid the penalty of the jump to the second code block (note: I have limited knowledge of assembly language). Does this idea carry forward to switch statements and case labels? switch (myCaseValue) { case Case1: // Code block 1 break; case Case2: // Code block 2 break; // etc. } If I know that one of the cases will happen more often,

In Rails, what is the best way to update a record or create a new one if it doesn't exist?

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问题 I have a create statement for some models, but it’s creating a record within a join table regardless of whether the record already exists. Here is what my code looks like: @user = User.find(current_user) @event = Event.find(params[:id]) for interest in @event.interests @user.choices.create(:interest => interest, :score => 4) end The problem is that it creates records no matter what. I would like it to create a record only if no record already exists; if a record does exist, I would like it to

How to know which condition was true in an if statement?

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问题 How can I know which condition in an if statement in JavaScript was true? if(a === b || c === d){ console.log(correctValue) } How can I know if it was either a === b or c === d ? Edit: I wanted to know if there was any way of doing this besides checking each condition on it's own if statement. 回答1: You can't. If it matters, it needs to be two different conditions. if (a == b) { // it was a == b return true; } if (c == d) { // it was c == d return true; } Note that even so, you won't know if

MySQL “Or” Condition

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Check out this MySQL Query and then I'll show you what I really want it to do... mysql_query(" SELECT * FROM Drinks WHERE email='$Email' AND date='$Date_Today' OR date='$Date_Yesterday' OR date='$Date_TwoDaysAgo' OR date='$Date_ThreeDaysAgo' OR date='$Date_FourDaysAgo' OR date='$Date_FiveDaysAgo' OR date='$Date_SixDaysAgo' OR date='$Date_SevenDaysAgo'"); The problem with it is that I want it to always match the email. In this case (for example) if the date equals $Date_SixDaysAgo then it will be selected from the query even if $Email doesn't equal the email column. So, in short, I want the

What is condition synchronization?

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Could someone explain condition synchronization to me? An example (preferably in C#) would be greatly appreciated also. It sounds like your professor is talking about threading. Threading enables computer programs to do more than one thing at a time. The act of starting a new thread while one is already running is called "spinning up a thread" by computer programmers. Threads can share the same memory space. Condition Synchronization (or merely synchronization) is any mechanism that protects areas of memory from being modified by two different threads at the same time. Let's say you are out

Rails ActiveRecord conditions

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Is there a way to create a condition like this? @products = Product.find(:all, :limit => 5, :conditions => { :products => { :locale => 'en', :id NOT '1' }, :tags => { :name => ['a','b']}) I would like to list all products not including product 1. Thx. Rails 3 Use squeel gem. Product.where( :products => { :locale => 'en', :id.not_in => '1' }, :tags => { :name => ['a','b']} ).limit(5) Rails 2 Use AR Extensions for this. It supports the following condition modifiers: * _lt => less than * _gt => greater than * _lte => less than or equal to * _gte => greater than or equal to * _ne => not equal to *

Multiple 'or' condition in Python [duplicate]

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This question already has an answer here: How to test multiple variables against a value? 22 answers I have a little code issue and it works with IDLE and not with Eclipse, can I write this : if fields[9] != ('A' or 'D' or 'E' or 'N' or 'R'): instead of this : if fields[9] != 'A' and fields[9] != 'D' and fields[9] != 'E' and fields[9] != 'N' and fields[9] != 'R': Thank you. Use not in and a sequence: if fields[9] not in ('A', 'D', 'E', 'N', 'R'): which tests against a tuple, which Python will conveniently and efficiently store as one constant. You could also use a set literal: if fields[9] not

Conditional INSERT INTO statement in postgres

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问题 I'm writing a booking procedure for a mock airline booking database and what I really want to do is something like this: IF EXISTS (SELECT * FROM LeadCustomer WHERE FirstName = 'John' AND Surname = 'Smith') THEN INSERT INTO LeadCustomer (Firstname, Surname, BillingAddress, email) VALUES ('John', 'Smith', '6 Brewery close, Buxton, Norfolk', cmp.testing@example.com'); But Postgres doesn't support IF statements without loading the PL/pgSQL extension. I was wondering if there was a way to do some

Two conditions in one if statement does the second matter if the first is false?

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问题 Okay, so I have this piece of code tested and I found there isn't any exception thrown out. public static void main(String[] args) { int[] list = {1,2}; if (list.length>2 && list[3] == 2){ System.out.println(list[1]); } } Does the statement here if (list.length>2 && list[3] == 2) mean that if the first condition is false we don't even have to check the second condition? Or it equals to if (list.length>2){ if (list[3] == 2){ ... } } ? And, what if it is written in C or python or some other