Do while / while

This is a discussion on Do while / while within the C++ Programming forums, part of the General Programming Boards category; Can sombody please explain to me who is correct in these statments: MY BOOK SAYS:: Using Do and while statments ...

  1. #1
    Its hard... But im here swgh's Avatar
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    Do while / while

    Can sombody please explain to me who is correct in these statments:

    MY BOOK SAYS::

    Using Do and while statments are a better way to achieve your goal as they are better than while loops alone.

    MY TUTOR AT UNI SAYS::

    Always use a While loop WITHOUT do, as a while loop can always be found near the top of a program, as you do not have to go searching through the program to find it.

    Who is correct?? Please help I am confused.... Do i use while or Do while??

  2. #2
    C++ Witch laserlight's Avatar
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    a better way to achieve your goal
    What's your goal?
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  3. #3
    Confused Magos's Avatar
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    Depends on what you want to achieve.
    I've never had any practical use of a do-while loop (doesn't mean they're useless though).
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  4. #4
    Its hard... But im here swgh's Avatar
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    The goal would be to make the program execute better, but they are just two reasons for a function "while" that would really achieve the same thing, I was wondering why a book would say use DO WHILE and my proffessor say always only use WHILE

  5. #5
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    The only difference is that the expression is checked at different points of the block. Use whatever you need to. Most of the time you will find that while() will be used more but maybe a do-while() will be needed. One is not better than the other.

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    C++ Witch laserlight's Avatar
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    The goal would be to make the program execute better
    Um, like what kind of problem are you trying to solve with the loop? Why not a for loop?
    Why use a loop at all?

    I was wondering why a book would say use DO WHILE and my proffessor say always only use WHILE
    From what you say, it looks like your book and your prof are talking in different contexts.
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  7. #7
    dra
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    I don't like do-while loops only because it executes the code no matter what at least once and then checks the condition.

    But then again, there's probably some good use for that.
    Last edited by dra; 07-23-2005 at 03:33 AM.

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    One example where a do{ ... }while(); loop was useful for me was when I needed to create a random string which wasn't already present in a database.

    So,

    do {
    //generate string
    //query database looking for that string
    } while(string_was_found_in_db);

    basically this loop will never run more than once, but I needed to make sure a unique string was generated.

    However, do-while and while loop translated in asm (correct me if I'm wrong):
    do-while
    Code:
    startloop:
     ;
     ; loop body
     ;
     CMP x,x
     JNZ startloop
    while
    Code:
    startloop:
     CMP x,x
     JGE endloop
     ;
     ; loop body
     ;
     JMP startloop
    endloop:

  9. #9
    Skunkmeister Stoned_Coder's Avatar
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    There are in general 3 types of loop. Theirs counted, top-tested and bottom-tested.
    for loops are counted.
    while loops are top-tested.
    do-while loops are bottom-tested.

    Just use whichever suits your needs best.
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  10. #10
    Registered User c0ldshadow's Avatar
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    if you want to get technical you could throw in a loop involving "goto", even tho they are highly frowned upon

  11. #11
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    I use do-while for my menu choice cause it makes sense to me.
    Code:
    int c;
    do
    {
         system("cls");
         cout<<"1- menu1\n";
         cout<<"2- menu2\n";
         cout<<"3- menu3\n";
         cout<<"4- quit\n";
         cin>>c;
         ... 
    
    }while(c!=4)
    to me it seems more sense.


    ofcourse you can always go

    Code:
    int c=-1;
    while(c!=4)
    {
        ...
        cin>>c;
    }

    whichever style u like or needed to get things done.

  12. #12
    Software Developer jverkoey's Avatar
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    I believe hoangvo gave the best example here. Do while's are most useful when you have a loop that you want to execute at least once, usually with user input.

    This way the user input will always execute at least once, and you then check the user input at the while part of the do..while.

  13. #13
    ... arjunajay's Avatar
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    My Teacher asks me to use forloop when ever possible...
    Hi...Hi...Hi...

  14. #14
    Skunkmeister Stoned_Coder's Avatar
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    you should probably favour while for general loops. for when you need a specific number of counted iterations and do/while when you need the loop body to execute before the looping test. Its that simple.
    Oh btw goto is not really a loop, its an unconditional jump. So even tho loops can be simulated using goto I would never include it in any discussion about loops.
    Free the weed!! Class B to class C is not good enough!!
    And the FAQ is here :- http://faq.cprogramming.com/cgi-bin/smartfaq.cgi

  15. #15
    ... arjunajay's Avatar
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    Isn't for loop better.
    Every thing like intialisation, condition, incrementation... etc is packed into a neat little capsule... easier to understand right?
    btw, I'm addicted to for loops

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