Friday Programming Quiz

[Josh Smith]

Shog9 wrote:

Yup.

Ok, then I have a programming question to ask you, but since this is the Lounge...oh screw it...this whole thread is a programming question. What the heck is this?

Transact.apply(this, [].slice.call(arguments, 1))

I assume it's the recursion point in the function, but I'll be damned if I can make heads or tails of it. I mean, Transact is a function, right? So what does .apply() mean when called on a function name? And what's the [].slice all about? :confused:

:josh: My WPF Blog[^] Without a strive for perfection I would be terribly bored.

[J Dunlap]

Step[] steps = new[]
{
new Step(
()=>Console.WriteLine("step 0"),
()=>Console.WriteLine("reset 0")
),
new Step(
()=>Console.WriteLine("step 1"),
()=>Console.WriteLine("reset 1")
),
new Step(
()=>Console.WriteLine("step 2"),
()=>Console.WriteLine("reset 2")
),
new Step(
()=>Console.WriteLine("step 3"),
()=>Console.WriteLine("reset 3")
),
new Step(
()=>Console.WriteLine("step 4"),
()=>Console.WriteLine("reset 4")
),
};

bool DoSteps(IEnumerable<step> steps)
{
Step step=steps.FirstOrDefault();
if(step==null || (step.Action() && DoSteps(steps.TakeAfter(0)))
return true;
step.Reset();
return false;
}

//EDIT: oops, forgot to define the step class! :o)
public class Step
{
public Step(Action action, Action reset)
{Action=action; Reset=reset;}

public Action Action{get;set;}
public Action Reset{get;set;}
}
public delegate bool Action();

To use it:

DoSteps(steps);

The neat thing about TakeAfter() is that it returns an enumerator method that iterates the existing array, rather than creating a new array. This saves on performance while still allowing elegance.

---

Last modified: 15mins after originally posted --

--Justin, Microsoft MVP, C#

C# / DHTML / VG.net / MyXaml expert available for consulting work[^] Get Quality Portraits Drawn From Your Photos[^]

[Nish Nishant]

A hopefully reusable solution in C#

class Program
{
static void Main(string[] args)
{
AtomicActions atomicActions = new AtomicActions();
atomicActions.AddActionPair(Actions.Step1, Actions.ResetStep1);
atomicActions.AddActionPair(Actions.Step2, Actions.ResetStep2);
atomicActions.AddActionPair(Actions.Step3, Actions.ResetStep3);
atomicActions.Run();
}
}

internal class AtomicActions
{
internal delegate bool Action();
internal delegate void ResetAction();

struct ActionPair
{
    internal Action action;
    internal ResetAction resetAction;

    internal ActionPair(Action action, ResetAction resetAction)
    {
        this.action = action;
        this.resetAction = resetAction;
    }            
}

Stack<ResetAction> undoStack = new Stack<ResetAction>();
List<ActionPair> actionList = new List<ActionPair>();

internal void AddActionPair(Action action, ResetAction resetAction)
{
    actionList.Add(new ActionPair(action, resetAction));
}

internal bool Run()
{
    bool runSuccess = true;

    foreach (ActionPair actionPair in actionList)
    {
        undoStack.Push(actionPair.resetAction);
        if (!(runSuccess = actionPair.action()))
            break;
    }

    if (!runSuccess)
        while (undoStack.Count > 0)
            undoStack.Pop()();

    return runSuccess;
}

}

internal static class Actions
{
public static bool Step1()
{
Console.WriteLine("Step1");
return true;
}

public static void ResetStep1()
{
    Console.WriteLine("ResetStep1");
}

public static bool Step2()
{
    Console.WriteLine("Step2");
    return true;
}

public static void ResetStep2()
{
    Console.WriteLine("ResetStep2");
}

public static bool Step3()
{
    Console.WriteLine("Step3");
    return true;
}

public static void ResetStep3()
{
    Console.WriteLine("ResetStep3");
}

}

Regards, Nish

---

Nish’s thoughts on MFC, C++/CLI and .NET (my blog)
My latest book :

[Nish Nishant]

Excellent solution :-)

Regards, Nish

---

Nish’s thoughts on MFC, C++/CLI and .NET (my blog)
My latest book : C++/CLI in Action / Amazon.com link

[Shog9 0]

[] is an empty array. In this case, i'm using it as shorthand for Array.prototype. Why? Because arguments isn't actually a proper array - while it does array-like things (indexed access to the current function's parameters), it's a separate type of object, one that also does other, function-specific things. So, it doesn't have a slice() method, which i wanted - slice() extracts a portion of an array (i want the second item and everything after it). call() and apply() are members of every function. call() takes at least one argument, which will become this when the function executes, while each subsequent argument becomes an argument to the function itself. In this way, i can call a function defined for Array objects on an arbitrary array-like object (arguments) and everything works just fine. apply() is very similar to call(), except that it takes two parameters, with the second being an array of arguments to the function. In this way, Transact() handles recursion with a variable (but ever-decreasing) number of arguments. A simpler definition for Transact might look like this:

function Transact(ops)
{
if ( ops.length < 1 )
return true;
var op = ops[0];
if ( op() && Transact(ops.slice(1)) )
return true;
eval("Reset"+op.name)();
return false;
}

Transact([Step1, Step2, Step3, Step4]);

...while a slightly more confusing implementation could take this form:

function Transact()
{
if ( arguments.length < 1 )
return true;
var op = arguments[0];
if ( op() && arguments.callee.apply(this, [].slice.call(arguments, 1)) )
return true;
eval("Reset"+op.name)();
return false;
}

;)

----

I don't care what you consider witty, but at least I do not blather on posting nonsense like Jim Crafton.

-- Stringcheese, humbled by Crafton's ability to string together multiple sentences

[Josh Smith]

Wow, thanks a lot for the in-depth explanations! I never realized that Javascript had so much going for it. Very cool! Too bad it's only used inside the browser, or else I'd give it a whirl.

:josh: My WPF Blog[^] Without a strive for perfection I would be terribly bored.

[ied]

I'm feeling nostolgic... bwa ha ha... -- Ian --------- bool Step1() {printf("Step1\n"); return true;} bool Step2() {printf("Step2\n"); return true;} bool Step3() {printf("Step3\n"); return true;} bool Step4() {printf("Step4\n"); return false;} bool Step5() {printf("Step5\n"); return true;} void ResetStep1() {printf("ResetStep1\n");} void ResetStep2() {printf("ResetStep2\n");} void ResetStep3() {printf("ResetStep3\n");} void ResetStep4() {printf("ResetStep4\n");} void ResetStep5() {printf("ResetStep5\n");} typedef bool (*steptype)(); steptype steplist[] = {Step1,Step2,Step3,Step4,Step5}; typedef void (*resetsteptype)(); resetsteptype resetsteplist[] = {ResetStep1,ResetStep2,ResetStep3,ResetStep4,ResetStep5}; int main(int argc, char* argv[]) { int i=0; while (i<5 && (*(steplist[i]))()) i++; if (i<5) while (i>0) (*(resetsteplist[--i]))(); return 0; } --------- Step1 Step2 Step3 Step4 ResetStep3 ResetStep2 ResetStep1

[David Stone]

Not so! Rhino JavaScript[^] :)

[Anton Afanasyev]

bool DoSteps(IEnumerable steps)
{
Step step=steps.FirstOrDefault();
if(step==null || (step.Action() && DoSteps(steps.TakeAfter(0))))
return true;
step.Reset();
return false;
}

could be changed to:

bool DoSteps(IEnumerable steps)
{
Step step=steps.FirstOrDefault();
return (step==null || (step.Action() && DoSteps(steps.TakeAfter(0)))|| (step.Reset() && false));
}

and btw, you forget a bracket there ;)

---

:badger:

[Gary R Wheeler]

In the K.I.S.S. tradition:

void (*step[])() = { Step1, Step2, Step3, Step4, Step5 };
void (*reset_step[])() = { ResetStep1, ResetStep2, ResetStep3, ResetStep4, ResetStep5 };

int index = 0;
int step_count = sizeof(step) / sizeof(step[0]);

while ((index < step_count) && step[index]()) index++;
if (index < step_count) {
while (index >= 0) reset_step[--index];
}

---

Software Zen: delete this;

Fold With Us![^]

[Gary R Wheeler]

Teach me not to read the whole thread... :-O Aha! Your solution doesn't work quite correctly. It will never call ResetStep1().

---

Software Zen: delete this;

Fold With Us![^]

[Eytukan]

omg! every other people here's going by the same way and much better than mine. I didn't like my code.:doh: deleted it. Feeling happy now :-O

---

The Advantage in work-from-home is that... we can blame the dog -Mark Salsbery Best wishes to Rexx[^]

[Josh Smith]

Shog9 wrote:

call() and apply() are members of every function.

I'm still shocked by this. In Javascript, functions have members. That is very strange to me, coming from a "classical" object-oriented background of C++ and C#. Do you commonly find a use for the members of functions, or is it an abstruse niche that you happen to be savvy about?

:josh: My WPF Blog[^] Without a strive for perfection I would be terribly bored.

[Shog9 0]

Nice! (missing parens in the call to reset though)

----

I don't care what you consider witty, but at least I do not blather on posting nonsense like Jim Crafton.

-- Stringcheese, humbled by Crafton's ability to string together multiple sentences

[Shog9 0]

It's just part of how Javascript works - there isn't the same distinction between functions, classes, and objects that you get with, say, C++. All functions are objects, derived from Function and inheriting whatever methods are defined for it. You can actually add methods to Function and immediately use them on any other function (this also works for other base types such as Array and Object; it's great for filling in the gaps of a sub-par (*cough*IE*cough*) JS engine). And you make your own types by - wait for it - defining a function that creates them! That said, you don't really get things like inheritance in the classical sense, although the language is flexible enough you can make it work with a bit of discipline. Honestly, it's a lot of fun. ;)

----

I don't care what you consider witty, but at least I do not blather on posting nonsense like Jim Crafton.

-- Stringcheese, humbled by Crafton's ability to string together multiple sentences

[Josh Smith]

Shog9 wrote:

And you make your own types by - wait for it - defining a function that creates them!

:wtf: Do Javascript developers have to drink the special Kool-aid before being "initiated"??? :)

Shog9 wrote:

Honestly, it's a lot of fun.

Sounds like it.

:josh: My WPF Blog[^] Without a strive for perfection I would be terribly bored.

[Shog9 0]

:) Btw - if you're interested, check out this link[^]. He demonstrates most of what i was talking about, and does a better job of explaining it.

----

I don't care what you consider witty, but at least I do not blather on posting nonsense like Jim Crafton.

-- Stringcheese, humbled by Crafton's ability to string together multiple sentences

[Josh Smith]

Shog9 wrote:

if you're interested, check out this link[^].

Thanks, I'll check it out. Going to see some apartments now (sigh).

:josh: My WPF Blog[^] Without a strive for perfection I would be terribly bored.

[J Dunlap]

Anton Afanasyev wrote:

could be changed to: [...]

Yeah... but it's less readable. ;)

Anton Afanasyev wrote:

and btw, you forget a bracket there ;)

Oops! :-O

--Justin, Microsoft MVP, C#

C# / DHTML / VG.net / MyXaml expert available for consulting work[^] Get Quality Portraits Drawn From Your Photos[^]

[ied]

Sure it does (I tested it). It predecrements the index before calling the resetstep function, so when i=1 it calls resetstep1. -- Ian