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I only started coding about a month ago and I am trying to add a code or program to my work in order to make it store user input and even after rerunning the program, the input will still be kept. Confused? For example, when you make an account on Facebook, they will keep your email and password so next time you just have to login without making the account again. Something like a database to store value I am guessing?
When a program terminates, it looses all the values stored in variables.
This is because we have a GC (Garbage Collection) mechanism which helps to clear up data which are not used.
In your case, to persist data post restart, we need to store to some external store (persistent storage) rather than keeping in the memory.
Your external store can be a database or can be a simple file.
Create a file and store your values in it. Once you restart the program, read the values again from it.
You would require a storage to store the data, as rerunning will erase all previous data. Any filesystem/db can be used to resolve your issue.
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I am making an online test with predefined questions and it's correct answer I let student enter their answer to each question and I took them and check them against correct form DB. is it better to call DB to for each answer to check or get all question right answer and make loop o(n)^2.I am using hibernate
A bulk operation is the best. It requires a single database roundtrip and you can do all the processing in the database so that you don't have to move one million records back and forth the DB.
For more details about Bulk Updates, check out this article.
In your example, it does not matter if the user had a test with 1000 questions. If they have a predefined right answer, you can match that in the DB automatically.
If you need to manually validate answers, do it with batch processing and process only N answers at a time and send all answers in a single batch to the DB.
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I wish to implement a variable (in Java) whose value is either stored somewhere or is not reset every time I run the program.
It's related to a "Booking reference Number" for a flight program. I know database connectivity but make a new data base for one variable is pretty pointless. Any ideas as to what I should/can do?
Also I don't want the numbers to be random I want them in order like if the first booking ID is 100 then the next one should be 101 and so on.
Organize your data in a structure and then serialize it.When you re-run your program, look for that serialized version in the file system, if there is any, read it. Viola.!
You should write the variable to a file and then read it from the file the next time you run the program.
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The app fetches data from a server. The data is returned in an Array. I need to check if one of the Array items starts with a value specified in the app. If it's possible without iterating because that slows down the app significantly.
If you have just an array data structure, there is no way to check what it contains without actually looking into it.
However if you use a different data structure such as a HashMap (which is built on to of an array) you can check/lookup a key like "101" in O(1) time typically. You can check map.isEmpty() in O(1) time.
In short, if it's taking too long to perform a simple operation, chances are you need to be using a different data structure (or possibly more than one)
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Let us say that I have a method that writes to a file or database. What if different parts of application calls this method too many times at the same time or in same interval of time. All those method calls are maintained in some stack/queue in memory and wait for previous requests to be served ?
Writing to the same file is platform dependent, like Unix allows concurrent writes to the same file.
You need to see the synchronization techniques - how you want to manage the read write operations.
If you see from DB perspective the db engine handles it properly - whichever comes first will be served. The next insert would depend on the first insert(in case you already inserted with the same key in the previous operation - then obviously it ll throw an exception)
Also I would say if different parts of your application are appending data to the same file at the same time - there could be design flaw and you need to reconsider the design
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So I have this app, a Java servlet. It uses a dictionary object that reads words from a file specified as a constructor parameter on instantiation and then serves queries.
I can do basically the same on PHP, but it's my understanding the class will be instantiated on each and every request, and the file will be read again every time. In fact, I did it and it works, but it collapses my humble amazon EC2 micro instance at the ridiculous amount of 11 requests per second or more.
My question is: Shouldn't some kind of compiler/file system optimization be kicking in and making the performance impact insignificant when the file does not change at all?
If the answer is no, I guess my design is quite poor and I should try to improve it. In that case, my second question is: What would be the best approach to improve it?
Building a servlet-like service so the code is properly reused?
Using memcached to keep the words file content in memory?
Using a RDBMS instead of a plain text file and have my dictionary querying it?
(despite the dictionary being only a few KB of static data and despite having to perform some complex queries such as selecting a
(cryptographically safe) random word from those having a length
higher than some per-request user setting and such?)
Something else?
Your best bet is to generate a PHP file which contains the final structure of the dictionary in PHP code. You could then include() that cache file into your code or write a new one when the file changes. You should store it on the filesystem, no databases. You could cache it in memory as well. But I don't think this is really needed at this point.