algorithm - swap two variables. which way is faster? -

-

let's have 2 integers , b. way faster swapping values? 

c=a; a=b; b=c;//(edited typo)

or 

a=a+b; b=a-b; a=a-b;

or bitwise xor

a=a^b; b=a^b; a=a^b;

i'll test performance differences when i'll able i'd know now. bitwise?

firstly, cannot quantify speed of algorithm independent of program language, compiler , platform on run. algorithm mathematical abstraction.

having said that:

  • for typical programming language,
  • and typical compiler, and
  • a typical execution platform,

the first version typically faster because typically compile fewer native instructions take less clock cycles execute. first version requires load , save operations. other 2 versions have (at least) same number of loads , saves, , additional arithmetic or bit manipulation instructions.

however, not cut-and-dry.

the 2nd , 3rd examples performing swap without using temporary variable. might if using temporary variable expensive. might happen on machine didn't provide enough general purpose registers, , relative cost of loading / saving memory large. in circumstances, native code equivalents optimal.

however ... , real point ... best strategy leave kind of decision compiler. unless prepared put huge amount of effort micro-optimizing, compiler able better job can. indeed, writing code in "cunning ways" liable make harder compiler optimize. (in 3rd case example, compiler need figure out that sequence swapping 2 variables before can substitute optimal instruction sequence. chances optimizer won't able that.)


Comments

Post a Comment

Popular posts from this blog

qt - Using float or double for own QML classes -

-
i've created components , helper methods using c++ , qt used in qml gui. of methods calculate gui elements (size, transformations, scene graph items creation). target arm 32 bit using opengl / eglfs render output. opengl using float , qml using real (defined double ). what should use in c++ do not lose precision? if use qreal ( double ) shouldn't lose precision. if git grep float in qtbase.git , can find out bit more: dist/changes-5.2.0-**************************************************************************** dist/changes-5.2.0-* important behavior changes * dist/changes-5.2.0-**************************************************************************** dist/changes-5.2.0- dist/changes-5.2.0- - qt compiled qreal typedef'ed double on dist/changes-5.2.0: platforms. qreal float on arm chipsets before. guarantees more dist/changes-5.2.0- consistent behavior between platforms qt supports, binary dist/chan...
Read more

Create Outlook appointment via C# .Net -

-
i need create outlook appointment using microsoft.office.interop.outlook , while can work locally on own workstation , works if run web application via server's browser, not work when connect server externally. hence, think might permission issue. i changed application pool identity " localsystem " don't access denied error. unfortunately, doesn't work. the app behaves if appointment created, appointment doesn't appear in outlook. here include file on top of aspx.cs page using outlook = microsoft.office.interop.outlook; here code using pop appointment. outlook.application apptapp = new outlook.application(); outlook.appointmentitem appt = apptapp.createitem(outlook.olitemtype.olappointmentitem) outlook.appointmentitem; appt.subject = txtfirstname.text + " " + txtlastname.text; appt.body = txtcomment.text; appt.alldayevent = false; appt.start = datetime.parse(txtreminderdate.text + " 8:00 am"); appt.end = datetime.p...
Read more

ios - Swift Array Resetting Itself -

-
i have 2 arrays: pointhistory , datehistory . have nstimer , , when timer runs out, action called. in action appending int pointhistory , , current date string datehistory . here timer action: func timeraction() { let steppervalue = int(stepper.value) employee.numberofpoints = steppervalue networking.saveemployee(employee, completion: { (error) -> void in if error != nil { let alert = uialertcontroller(title: "error", message: "there error updating \(self.employee.name)'s points", preferredstyle: .alert) let alertaction = uialertaction(title: "ok", style: .cancel, handler: nil) alert.addaction(alertaction) self.presentviewcontroller(alert, animated: true, completion: nil) } }) var dateformatter = nsdateformatter() dateformatter.dateformat = "mm-dd-yyyy" dateformatter.timezone = nstimezone.localtimezone() var date = nsdate() var ...
Read more