Programming homework and newbie help thread - Page 42 - Computer Technical Help

Two Plus Two Forums Other Topics Computer and Technical Help

Programming homework and newbie help thread

Post Reply Subscribe

...

Page 42 of 80

First

22 32 37 38 39 40 41 42 43 44 45 46 47 52 62

Last

Page 42 of 80

First

22 32 37 38 39 40 41 42 43 44 45 46 47 52 62

Last

03-30-2016 , 11:09 AM

#1026

RustyBrooks

Carpal \'Tunnel

Join Date: Feb 2006 Posts: 24,646

Quote:

Originally Posted by Mr.mmmKay

In that case it would be interesting if timeit also returned the standard deviation so you could get an idea if the difference in time between two functions is significant

You can do it yourself using the "repeat" method of timeit. However, they have this note:

Quote:

Note It’s tempting to calculate mean and standard deviation from the result vector and report these. However, this is not very useful. In a typical case, the lowest value gives a lower bound for how fast your machine can run the given code snippet; higher values in the result vector are typically not caused by variability in Python’s speed, but by other processes interfering with your timing accuracy. So the min() of the result is probably the only number you should be interested in. After that, you should look at the entire vector and apply common sense rather than statistics.

Quote

$25m Guaranteed WPM on CoinPoker

Join the action now

Daily Rewards • Splash Pots • CoinRaces

03-30-2016 , 12:14 PM

#1027

daveT

S.A.G.E. Master

Join Date: Jun 2005 Posts: 23,955

Quote:

Originally Posted by mackeleven

Hi guys. basic SQL question

Look a the same id with different colors. This is a composite key correct? It's ok to repeat the id if the corresponding colors are different?

I don't like this design.

I'd prefer to see an entry for each color item in the stock table. If you want to show that each shirt, pants, etc, are the same collection, then create a separate look up table and a view that adds up the total stock by item type.

Quote

03-30-2016 , 01:37 PM

#1028

mackeleven

old hand

Join Date: Aug 2013 Posts: 1,593

if you consider:

shirts come in a selection of black, white, yellow, red
and there are 50 of them in stock

10 black, 20 red etc

you mean like this? i don't understand what you mean by same collection,

item color qty price
shirt black 10 12.50
shirt red 20 12.50

and sum qty where item = shirt etc?

Last edited by mackeleven; 03-30-2016 at 01:44 PM.

Quote

03-30-2016 , 01:55 PM

#1029

daveT

S.A.G.E. Master

Join Date: Jun 2005 Posts: 23,955

yeah, that's what I mean. I also tend to thing of the world in skus and upcs. If you registered all of this with GS1, you'd end up having to create a new sku / upc for each product. So, more specifically, something like this:

Code:

create table products (
upc bigint primary key,
sku varchar unique,
description varchar
);

create table valid_product_type (
product_type varchar primary key  --- shirt, pants, etc
);

create table master_products (
master_id [could be serial, varchar] primary key,
master_name varchar unique,
product_type varchar references valid_product_type (product_type),
);

create table product_master_lookup (
master_id references master_products (master_id)
upc bigint references products (upc)
primary_key (master_id, upc)
);

The reason is that the master product isn't a real product. You can't, for example, sell a generic shirt, so it doesn't make sense to put it on sale. The above schema will give you the flexibility to represent each individual item and allow you to add up the inventory of the the main "type." Also, if you end up with two collections of shirts, you have no issues.

Quote

03-30-2016 , 02:01 PM

#1030

mackeleven

old hand

Join Date: Aug 2013 Posts: 1,593

Funny that's the way I was doing it. The spreadsheet there was my instructor's solution. He's throwing me off lol.

Quote

03-30-2016 , 02:25 PM

#1031

econophile

Carpal \'Tunnel

Join Date: Jul 2005 Posts: 11,306

Quote:

Originally Posted by JSLigon

There might be a bigger issue with h(), though (which I was not previously aware of):

Code:

>>> 5%5 == 0
True
>>> 5%5 == 0 is True
False
>>> (5%5 == 0) is True
True

Edit: And indeed at the end of the loop in h(), n remains empty.

It looks like the 'is' operator takes priority over the '==' operator.

Quote

03-30-2016 , 03:29 PM

#1032

daveT

S.A.G.E. Master

Join Date: Jun 2005 Posts: 23,955

Quote:

Originally Posted by mackeleven

Funny that's the way I was doing it. The spreadsheet there was my instructor's solution. He's throwing me off lol.

For purely instructional purposes, I don't mind his / her idea, but I don't think it is well-executed. The main issue is that there is a total stock and then an item stock column. This is going to cause massive headaches, regardless if you are using the processing language or not.

To answer the other question you had, a composite key means a primary, foreign, or unique key over two or more columns.

Quote

03-30-2016 , 04:56 PM

#1033

JSLigon

journeyman

Join Date: Nov 2014 Posts: 388

Quote:

Originally Posted by econophile

It looks like the 'is' operator takes priority over the '==' operator.

Code:

>>> 5%5 == 0 is True
False
>>> 5%5 == (0 is True)
True

Same issue without the modulus:

Code:

>>> 0 == 0 is True
False
>>> (0 == 0) is True
True
>>> 0 == (0 is True)
True

More fun:

Code:

>>> False is False
True
>>> False is False is False
True
>>> (False is False) is False
False
>>> False is (False is False)
False

What I think is happening: "is" tests if the two operands point to the same object, not just if they evaluate to the same thing. Putting an expression in parentheses forces the expression to be evaluated, somehow? So with the expression "5%5 == 0 is True", the left side of the "is" which is "5%5 == 0" as an unevaluated expression (like a tree with == at the root, 5%5 left branch, 0 right branch - or however Python represents expressions that haven't been evaluated yet) is not the same object as "True", even though it will evaluate to True.

Last edited by JSLigon; 03-30-2016 at 05:05 PM. Reason: added example

Quote

03-30-2016 , 05:14 PM

#1034

econophile

Carpal \'Tunnel

Join Date: Jul 2005 Posts: 11,306

Some instructive results

Code:

>>> 0 == False
True
>>> 0 is False
False

Quote

03-30-2016 , 05:15 PM

#1035

RustyBrooks

Carpal \'Tunnel

Join Date: Feb 2006 Posts: 24,646

Quote:

Originally Posted by econophile

Some instructive results

Code:

>>> 0 == False
True
>>> 0 is False
False

"is" is completely literal. Which becomes required when implicit conversion happens, such as with == etc.

Quote

03-30-2016 , 05:18 PM

#1036

JSLigon

journeyman

Join Date: Nov 2014 Posts: 388

I think I have a plausible explanation for all of the above, except:

Code:

>>> False is False is False
True

No idea what's happening there.

Quote

03-30-2016 , 05:24 PM

#1037

RustyBrooks

Carpal \'Tunnel

Join Date: Feb 2006 Posts: 24,646

Yeah that's weird. Also

Code:

>>> None is None is None
True

Quote

03-30-2016 , 05:24 PM

#1038

RustyBrooks

Carpal \'Tunnel

Join Date: Feb 2006 Posts: 24,646

OK, here's why
http://stackoverflow.com/questions/2...aluate-to-true

Quote

03-30-2016 , 05:31 PM

#1039

JSLigon

journeyman

Join Date: Nov 2014 Posts: 388

Thanks, that explains it, along with similar behavior with triple == that I plan to never use in my code:

Code:

>>> False == False == False
True
>>> (False == False) == False
False
>>> False == (False == False)
False

Actually I take back the part about never using multiple == like that. Could be useful as long as I'm clear on the semantics, less to type when testing whether several expressions are all equal.

Last edited by JSLigon; 03-30-2016 at 05:41 PM.

Quote

03-30-2016 , 07:31 PM

#1040

mackeleven

old hand

Join Date: Aug 2013 Posts: 1,593

Quote:

Originally Posted by daveT

That confusion is my fault. I believe he greyed out the column named stock in the stock table instead of deleting it. Cheers though.

Quote

03-31-2016 , 07:49 PM

#1041

mackeleven

old hand

Join Date: Aug 2013 Posts: 1,593

Code:

course_id   | courseName | courseCost | staff_id |
+-----------+------------+------------+----------+
|       401 | Boxing     |         30 |      100 |
|       402 | TKD        |         30 |      101 |
|       403 | BJJ        |         30 |      102 |
|       404 | Wrestling  |         30 |      103 |
+-----------+------------+------------+----------+

Hi. How do I sum two rows of a column. I need to sum the coursecost of boxing
and wrestling.........

Code:

select coursecost from course 
where coursename='boxing' or courseName = 'wrestling';

edit: i can create a new table:

Code:

create table cost as
(
select coursecost from course 
where coursename='boxing' or courseName = 'wrestling'
);

and then sum coursecost from that new table but that's necessary surely...?

Last edited by _dave_; 03-31-2016 at 10:36 PM. Reason: aligned your table :)

Quote

03-31-2016 , 08:50 PM

#1042

ChrisV

Carpal \'Tunnel

Join Date: Jul 2004 Posts: 40,336

Code:

select SUM(coursecost) from course 
where coursename='boxing' or courseName = 'wrestling';

Quote

03-31-2016 , 09:00 PM

#1043

mackeleven

old hand

Join Date: Aug 2013 Posts: 1,593

Jesus. I swear I tried that like 3 times and got an error each time.
Thanks

Quote

04-04-2016 , 02:02 AM

#1044

goofyballer

Carpal \'Tunnel

Join Date: Jun 2005 Posts: 70,719

JS question - say I have an object that inherits/uses (not sure of my terminology) a given prototype, which has functions, and I stash it in a mongo database. Upon later retrieval, it's just a plain old Object and doesn't have the prototype it did before, which makes me sad.

So, what I want to do is take that object and either give it the former glory it had as an instance of a class (am I allowed to say that in JS?) before being reduced to a pile of data fields. The only thing I could figure out how to do so far is make a new object and copy the data fields from the old object over:

Code:

var newUser = Object.assign(Object.create(user.prototype), userData);

but I'm not sure if that's ideal?

Quote

04-04-2016 , 08:19 AM

#1045

Mavoor

journeyman

Join Date: Jun 2014 Posts: 203

Question regarding databases (SQLite) -

I'm doing a school project that requires us to create a program that works with a database. I picked SQLite as the DBMS because it seemed simple to set up and work with. Disclaimer, I don't know a whole lot about databases, I'm familiar with SQL syntax but don't quite understand everything about databases, like what is the difference between say SQLite and MySQL. So bonus points for anyone who can explain that

Anyway, to create and initialize the database (with 1 table) I simply executed the following code in IntelliJ:

Code:

 Connection c = null;
    try {
      Class.forName("org.sqlite.JDBC");
      c = DriverManager.getConnection("jdbc:sqlite:MyDatabase");
    } catch ( Exception e ) {
      System.err.println( e.getClass().getName() + ": " + e.getMessage() );
      System.exit(0);
    }

This tries to connect with a database called MyDatabase, and if it doesn't exits then it "creates" a new database and connects with it.

Then I executed SQL statements on that database connection, and was able to create tables, insert into tables, retrieve from tables etc etc. Everything you could do with SQL.
And the database got stored in a file called MyDatabase, and had the file extension of .file (??)

But when I shipped the project out to another member of my team (it's a group project for school), he got an error which was something like "SQL_ERROR: No such table Trips exists". It was like the database had been corrupted/wiped out upon sending it over to him. I just .zipped the whole folder containing all the source code and the database. And he extracted it and ran it. So I'm wondering what might be the cause of this? Like, is there something fundamental thing about databases that I'm missing that could explain why you can't send a database to someone else like this? Also, is it expected that a database is a file with .file extension?
Perhaps this was just a problem with his computer/compiler/whatever, and we should try it on another computer?

Quote

04-04-2016 , 09:56 AM

#1046

candybar

Pooh-Bah

Join Date: Aug 2011 Posts: 4,375

Quote:

Originally Posted by goofyballer

Code:

var newUser = Object.assign(Object.create(user.prototype), userData);

There's this too but I wouldn't make an argument either way as to which is better:

https://developer.mozilla.org/en-US/...setPrototypeOf

Quote

04-05-2016 , 10:27 AM

#1047

gaming_mouse

Carpal \'Tunnel

Join Date: Oct 2004 Posts: 13,786

Quote:

Originally Posted by goofyballer

Code:

var newUser = Object.assign(Object.create(user.prototype), userData);

but I'm not sure if that's ideal?

That's a perfectly good solution, and in fact is philosophically in line with Crockford's latest object creation recommendations:

http://stackoverflow.com/questions/2...glas-crockford

I think calling it an instance of a class... isn't necessarily wrong per se, but there a better perspectives for thinking about JS. You really don't want to bring in Java/C++ baggage. Better to think of there being nothing but objects -- no classes -- but with the ability to compose and create new objects from the old ones. Crockford has said, and I agree, that the "class" keyword was ES6's biggest mistake.

Directly relevant to your question: http://raganwald.com/2015/05/11/javascript-classes.html

Quote

04-05-2016 , 03:04 PM

#1048

goofyballer

Carpal \'Tunnel

Join Date: Jun 2005 Posts: 70,719

Thanks, I'll look those over. re: terminology, I'm just not sure what the proper way to describe these things in JS parlance is - is there a term for an object with its own prototype methods, as opposed to an object that uses Object's prototype?

I've found the discussion of constructors somewhat confusing from the things I've read online so far, mostly because
- constructor functions look like regular functions in every way
- but magic things happen when they're called with 'new'!

Is there any meaningful difference between these two pieces of code?

Code:

var asdf = new constructorFunc();

var asdf = (function (obj) { constructorFunc.call(obj); return obj; })({});

Quote

04-05-2016 , 03:44 PM

#1049

candybar

Pooh-Bah

Join Date: Aug 2011 Posts: 4,375

Quote:

Originally Posted by goofyballer

Is there any meaningful difference between these two pieces of code?

Code:

var asdf = new constructorFunc();

var asdf = (function (obj) { constructorFunc.call(obj); return obj; })({});

Two differences as far as I can tell are that they don't have the same prototype; and if there's a return statement in the constructor, the first example will return that instead (only if it's an object though I think) whereas in the second example, it will still return {}.

Quote

04-05-2016 , 09:00 PM

#1050

gaming_mouse

Carpal \'Tunnel

Join Date: Oct 2004 Posts: 13,786

Quote:

Originally Posted by goofyballer

Code:

var asdf = new constructorFunc();

var asdf = (function (obj) { constructorFunc.call(obj); return obj; })({});

I don't think there's any special parlance. If you create an object with a "Dog" constructor, you'd just say it's an object of type Dog.

candy answered your other question. As far as actually writing code, you're best off not using "new" altogether. And definitely don't create a hierarchy of prototype chains.