I have the following array assignment:
advances = if advance == 0 then [advance] else [advance, (0 - advance)]
I wonder if there is a more coffeescript way of doing this?
What about:
advances = [advance]
advances.push (0 - advance) if advance != 0
I think this is a bit more readable. But I guess that a matter of taste.
It also uses coffee-script's nice if-at-the-end feature, which I makes some statements such as this "conditional array growing" more readable (for me at least).
PS: I also changed the == 0 to != 0 to avoid the unless statement. It just confuses me when reading code.
PPS: Don't put everything on one line just for the sake of it (even if is coffee-script is good at that). Always think of the most readable (and well performing) code.
I think your own proposal is perfectly fine. It's simple, clear, and pure-functional.
If you think minimizing punctuation is part of the coffeescript way then you could drop the parentheses
advances = if advance == 0 then [advance] else [advance, 0 - advance]
You're perhaps thinking about list comprehensions, but I can't imagine how to utilize them here. I believe you'd get better result utilizing underscore.js (or some other library providing collection utilities):
advances = _.uniq([advance, (0 - advance)])
http://underscorejs.org/#uniq
Related
Would the increment happen Left to right or right to left in the following case:
desc.src_addr = {rdata[i++],rdata[i++],rdata[i++],rdata[i++],rdata[i++],rdata[i++],rdata[i++],rdata[i++]};
Assuming i = 0 at the start of the above statement, would the end resolution be which of the two below:
desc.src_addr = {rdata[0],rdata[1],rdata[2],rdata[3],rdata[4],rdata[5],rdata[6],rdata[7]};
desc.src_addr = {rdata[7],rdata[6],rdata[5],rdata[4],rdata[3],rdata[2],rdata[1],rdata[0]};
Is there anything in LRM that talks about this?
Obviously, my aim is to avoid hardcoding the indices in the above statement to avoid typo and oversight issues.
Yes, it would be good to know what the SystemVerilog LRM says when writing SystemVerilog code. Do you have a copy?
See section 11.4.2 Increment and decrement operators and look for the word undefined.
A much better way of writing this code would be using the streaming operator, section 11.4.14 Streaming operators (pack/unpack)
desc.src_addr = {<<{rdata}}; //reverses bit order
Adam Ko has provided a magnificent solution to this question, thanks Adam Ko.
BTW if, like me, you love the c preprocessor (the thing that handles #defines), you may not be aware there is a handy thing in XCode: right click on the body of one of your open source files, go down near the bottom .. "Preprocess". It actually runs the preprocessor, showing you the overall "real deal" of what is going to be compiled. It's great!
This question is a matter of style and code clarity. Consider it similar to questions about subtle naming issues, or the best choice (more readable, more maintainable) among available idioms.
As a matter of course, one uses loops like this:
for(NSUInteger _i=0; _i<20; ++_i)
{
.. do this 20 times ..
}
To be clear, the effect is to to do something N times. (You are not using the index in the body.)
I want to signal clearly for the reader that this is a count-based loop -- ie, the index is irrelevant and algorithmically we are doing something N times.
Hence I want a clean way to do a body N times, with no imperial entanglements or romantic commitments. You could make a macro like this:
#define forCount(N) for(NSUinteger __neverused=0; __neverused<N; ++__neverused)
and that works. Hence,
forCount(20)
{
.. do this 20 times ..
}
However, conceivably the "hidden" variable used there could cause trouble if it collided with something in the future. (Perhaps if you nested the control structure in question, among other problems.)
To be clear efficiency, etc., is not the issue here. There are already a few different control structures (while, do, etc etc) that are actually of course exactly the same thing, but which exist only as a matter of style and to indicate clearly to the reader the intended algorithmic meaning of the code passage in question. "forCount" is another such needed control structure, because "index-irrelevant" count loops are completely basic in any algorithmic programming.
Does anyone know the really, really, REALLY cool solution to this? The #define mentioned is just not satisfying, and you've thrown in a variable name that inevitably someone will step on.
Thanks!
Later...
A couple of people have asked essentially "But why do it?"
Look at the following two code examples:
for ( spaceship = 3; spaceship < 8; ++spaceship )
{
beginWarpEffectForShip( spaceship )
}
forCount( 25 )
{
addARandomComet
}
Of course the effect is utterly and dramatically different for the reader.
After all, there are alresdy numerous (totally identical) control structures in c, where the only difference is style: that is to say, conveying content to the reader.
We all use "non-index-relative" loops ("do something 5 times") every time we touch a keyboard, it's as natural as pie.
So, the #define is an OKish solution, is there a better way to do it? Cheers
You could use blocks for that. For instance,
void forCount(NSUInteger count, void(^block)()) {
for (NSUInteger i = 0; i < count; i++) block();
}
and it could be used like:
forCount(5, ^{
// Do something in the outer loop
forCount(10, ^{
// Do something in the inner loop
});
});
Be warned that if you need to write to variables declared outside the blocks you need to specify the __block storage qualifier.
A better way is to do this to allow nested forCount structure -
#define $_TOKENPASTE(x,y) x##y
#define $$TOKENPASTE(x,y) $_TOKENPASTE(x, y)
#define $itr $$TOKENPASTE($_itr_,__LINE__)
#define forCount(N) for (NSUInteger $itr=0; $itr<N; ++$itr)
Then you can use it like this
forCount(5)
{
forCount(10)
{
printf("Hello, World!\n");
}
}
Edit:
The problem you suggested in your comment can be fixed easily. Simply change the above macro to become
#define $_TOKENPASTE(x,y) x##y
#define $$TOKENPASTE(x,y) $_TOKENPASTE(x, y)
#define UVAR(var) $$TOKENPASTE(var,__LINE__)
#define forCount(N) for (NSUInteger UVAR($itr)=0, UVAR($max)=(NSUInteger)(N); \
UVAR($itr)<UVAR($max); ++UVAR($itr))
What it does is that it reads the value of the expression you give in the parameter of forCount, and use the value to iterate, that way you avoid multiple evaluations.
On possibility would be to use dispatch_apply():
dispatch_apply(25, myQueue, ^(size_t iterationNumber) {
... do stuff ...
});
Note that this supports both concurrent and synchronous execution, depending on whether myQueue is one of the concurrent queues or a serial queue of your own creation.
To be honest, I think you're over addressing a non-issue.
If want to iterate over an entire collection use the Objective-C 2 style iterators, if you only want to iterate a finite number of times just use a standard for loop - the memory space you loose from an otherwise un-used integer is meaningless.
Wrapping such standard approaches up just feels un-necessary and counter-intuitive.
No, there is no cooler solution (not with Apple's GCC version anyways). The level at which C works requires you to explicitly have counters for every task that require counting, and the language defines no way to create new control structures.
Other compilers/versions of GCC have a __COUNTER__ macro that I suppose could somehow be used with preprocessor pasting to create unique identifiers, but I couldn't figure a way to use it to declare identifiers in a useful way.
What's so unclean about declaring a variable in the for and never using it in its body anyways?
FYI You could combine the below code with a define, or write something for the reader to the effect of:
//Assign an integer variable to 0.
int j = 0;
do{
//do something as many times as specified in the while part
}while(++j < 20);
Why not take the name of the variable in the macro? Something like this:
#define forCount(N, name) for(NSUInteger name; name < N; name++)
Then if you wanted to nest your control structures:
forCount(20, i) {
// Do some work.
forCount(100, j) {
// Do more work.
}
}
I was checking a friend's code, and this pattern showed up quite a bit whenever he wrote functions that returned a boolean value:
def multiple_of_three(n):
if (n % 3) is 0:
return True
else:
return False
I maintain that it's simpler (and perhaps a bit faster) to write:
def multiple_of_three(n):
return (n % 3) is 0
Am I correct that the second implementation is faster? Also, is it any less readable or somehow frowned upon?
I very much doubt there is a compiler or interpreter still out there where there is a significant speed difference - most will generate exactly the same code in both situations. But your "direct return" method is, in my opinion, clearer and more maintainable.
I can't speak of the Python interpreter's exact behavior, but doing one way over another like that (in any language) simply for reasons of "faster" is misguided, and qualifies as "premature optimization". As Paul Tomblin stated in another answer, the difference in speed, if any, is quite negligible. Common practice does dictate, however, that the second form in this case is more readable. If an expression is implicitly boolean, then the if statement wrapper is frivolous.
See also http://en.wikipedia.org/wiki/Program_optimization#When_to_optimize
The second form is the preferred form. In my experience the first form is usually the sign of an inexperienced programmer (and this does not apply solely to Python - this crops up in most languages).
I have been programming iPhone SDK for around 6 months but am a bit confused about a couple of things...one of which I am asking here:
Why are the following considered different?
if (varOrObject == nil)
{
}
vs
if (nil == varOrObject)
{
}
Coming from a perl background this is confusing to me...
Can someone please explain why one of the two (the second) would be true whereas the first would not if the two routines are placed one after the other within code. varOrObject would not have changed between the two if statements.
There is no specific code this is happening in, just that I have read in a lot of places that the two statements are different, but not why.
Thanks in advance.
They are the same. What you have read is probably talking about if you mistakenly write == as =, the former will assign the value to the variable and the if condition will be false, while the latter would be a compile time error:
if (variable = nil) // assigns nil to variable, not an error.
if (nil = variable) // compile time error
The latter gives you the chance to correct the mistake at compile time.
They might be different if you are using Objective-C++ and you were overriding the '==' operator for the object.
They are exactly the same, it is just a style difference. One would never be true if the other is false.
if it's a recommendation that you use the latter, it's because the second is easier to catch if you accidentally type =. otherwise, they are identical and your book is wrong.
They can only be different if the "==" - Operator is overloaded.
Let's say someone redefines "==" for a list, so that the content of two lists is checked rather than the memory adress (like "equals() in Java). Now the same person could theoretically define that "emptyList == NULL".
Now (emptyList==NULL) would be true but (NULL==emptyList) would still be false.
Sometimes I'm writing ugly if-else statements in C# 3.5; I'm aware of some different approaches to simplifying that with table-driven development, class hierarchy, anonimous methods and some more.
The problem is that alternatives are still less wide-spread than writing traditional ugly if-else statements because there is no convention for that.
What depth of nested if-else is normal for C# 3.5? What methods do you expect to see instead of nested if-else the first? the second?
if i have ten input parameters with 3 states in each, i should map functions to combination of each state of each parameter (really less, because not all the states are valid, but sometimes still a lot). I can express these states as a hashtable key and a handler (lambda) which will be called if key matches.
It is still mix of table-driven, data-driven dev. ideas and pattern matching.
what i'm looking for is extending for C# such approaches as this for scripting (C# 3.5 is rather like scripting)
http://blogs.msdn.com/ericlippert/archive/2004/02/24/79292.aspx
Good question. "Conditional Complexity" is a code smell. Polymorphism is your friend.
Conditional logic is innocent in its infancy, when it’s simple to understand and contained within a
few lines of code. Unfortunately, it rarely ages well. You implement several new features and
suddenly your conditional logic becomes complicated and expansive. [Joshua Kerevsky: Refactoring to Patterns]
One of the simplest things you can do to avoid nested if blocks is to learn to use Guard Clauses.
double getPayAmount() {
if (_isDead) return deadAmount();
if (_isSeparated) return separatedAmount();
if (_isRetired) return retiredAmount();
return normalPayAmount();
};
The other thing I have found simplifies things pretty well, and which makes your code self-documenting, is Consolidating conditionals.
double disabilityAmount() {
if (isNotEligableForDisability()) return 0;
// compute the disability amount
Other valuable refactoring techniques associated with conditional expressions include Decompose Conditional, Replace Conditional with Visitor, Specification Pattern, and Reverse Conditional.
There are very old "formalisms" for trying to encapsulate extremely complex expressions that evaluate many possibly independent variables, for example, "decision tables" :
http://en.wikipedia.org/wiki/Decision_table
But, I'll join in the choir here to second the ideas mentioned of judicious use of the ternary operator if possible, identifying the most unlikely conditions which if met allow you to terminate the rest of the evaluation by excluding them first, and add ... the reverse of that ... trying to factor out the most probable conditions and states that can allow you to proceed without testing of the "fringe" cases.
The suggestion by Miriam (above) is fascinating, even elegant, as "conceptual art;" and I am actually going to try it out, trying to "bracket" my suspicion that it will lead to code that is harder to maintain.
My pragmatic side says there is no "one size fits all" answer here in the absence of a pretty specific code example, and complete description of the conditions and their interactions.
I'm a fan of "flag setting" : meaning anytime my application goes into some less common "mode" or "state" I set a boolean flag (which might even be static for the class) : for me that simplifies writing complex if/then else evaluations later on.
best, Bill
Simple. Take the body of the if and make a method out of it.
This works because most if statements are of the form:
if (condition):
action()
In other cases, more specifically :
if (condition1):
if (condition2):
action()
simplify to:
if (condition1 && condition2):
action()
I'm a big fan of the ternary operator which get's overlooked by a lot of people. It's great for assigning values to variables based on conditions. like this
foobarString = (foo == bar) ? "foo equals bar" : "foo does not equal bar";
Try this article for more info.
It wont solve all your problems, but it is very economical.
I know that this is not the answer you are looking for, but without context your questions is very hard to answer. The problem is that the way to refactor such a thing really depends on your code, what it is doing, and what you are trying to accomplish. If you had said that you were checking the type of an object in these conditionals we could throw out an answer like 'use polymorphism', but sometimes you actually do just need some if statements, and sometimes those statements can be refactored into something more simple. Without a code sample it is hard to say which category you are in.
I was told years ago by an instructor that 3 is a magic number. And as he applied it it-else statements he suggested that if I needed more that 3 if's then I should probably use a case statement instead.
switch (testValue)
{
case = 1:
// do something
break;
case = 2:
// do something else
break;
case = 3:
// do something more
break;
case = 4
// do what?
break;
default:
throw new Exception("I didn't do anything");
}
If you're nesting if statements more than 3 deep then you should probably take that as a sign that there is a better way. Probably like Avirdlg suggested, separating the nested if statements into 1 or more methods. If you feel you are absolutely stuck with multiple if-else statements then I would wrap all the if-else statements into a single method so it didn't ugly up other code.
If the entire purpose is to assign a different value to some variable based upon the state of various conditionals, I use a ternery operator.
If the If Else clauses are performing separate chunks of functionality. and the conditions are complex, simplify by creating temporary boolean variables to hold the true/false value of the complex boolean expressions. These variables should be suitably named to represent the business sense of what the complex expression is calculating. Then use the boolean variables in the If else synatx instead of the complex boolean expressions.
One thing I find myself doing at times is inverting the condition followed by return; several such tests in a row can help reduce nesting of if and else.
Not a C# answer, but you probably would like pattern matching. With pattern matching, you can take several inputs, and do simultaneous matches on all of them. For example (F#):
let x=
match cond1, cond2, name with
| _, _, "Bob" -> 9000 // Bob gets 9000, regardless of cond1 or 2
| false, false, _ -> 0
| true, false, _ -> 1
| false, true, _ -> 2
| true, true, "" -> 0 // Both conds but no name gets 0
| true, true, _ -> 3 // Cond1&2 give 3
You can express any combination to create a match (this just scratches the surface). However, C# doesn't support this, and I doubt it will any time soon. Meanwhile, there are some attempts to try this in C#, such as here: http://codebetter.com/blogs/matthew.podwysocki/archive/2008/09/16/functional-c-pattern-matching.aspx. Google can turn up many more; perhaps one will suit you.
try to use patterns like strategy or command
In simple cases you should be able to get around with basic functional decomposition. For more complex scenarios I used Specification Pattern with great success.