imitations

This is a discussion on imitations within the C++ Programming forums, part of the General Programming Boards category; Hi! I'm new to the boards, yet not new to forum posting and programming (have been doing both for almost ...

  1. #1
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    imitations

    Hi!

    I'm new to the boards, yet not new to forum posting and programming (have been doing both for almost half a decade), I hope I can have a nice stay here and see how it goes

    I'm currently learning C++ and would like to know something; I know that many of the loops in C/C++ can be represented using a regular while loop. In the following examples, please correct me if I'm wrong:

    Representing a while-statement using a do-statement
    Code:
    do
    {
         if (/* condition */) 
         {
              // ...
         }
         else
              break;
    } while (true);
    Representing a for-statement using a do-statement
    Code:
    int i = 0;
    do
    {
         if (i < MAX)
         {
              // ...
              i++;
         }
         else
              break;
    } while (true);
    Representing a while-statement using a for-statement
    Code:
    for (; /* condition */;)
    {
         // ...
    }
    Representing a do-statement using a for-statement
    Code:
    // the statements go here...
    for (; /* condition */;)
    {
         // same statements as above...
    }
    These are my guesses at the moment. Please feel free to correct me if I happened to make a mistake in any of those or if you see that any of them are inefficient (say, if I included something that was not necessary and there is a better way to represent the same thing, etc.)

    Thanks!

  2. #2
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    Your last one is somewhat incorrect, since do-while ALWAYS executes at least once, so the correct solution would be:
    Code:
    for(;;)
    {
       ...
       if(condition)
          break;
    }
    Of course, if condition is not true before the loop, you won't notice the difference.

    I tend to use "do - while" specifically when I want something done AT LEAST once - e.g. reading and validating data from user - you want to read in the value once at least, then if that's not an OK value, you repeat, until you get a valid value.

    I use while when I have an unknown number of iterations, and sometimes the loop is not needed at all.

    For-loops I choose when there is something that we can somehow "count" across. This is not strictly just incrementing a variable or some such - I'm happy using for-loops to iterate across a linked list for example.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  3. #3
    Algorithm Dissector iMalc's Avatar
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    Yeah a classic case of use for a do ... while comes from manually implementing a function for converting a number to string. You always want at least one digit.

    The first one could be simplified to:
    Code:
    do
    {
         if ( ! /* condition */) 
              break;
         // ...
    } while (true);
    But you could also do it like this:
    Code:
    if (/* condition */) 
         do
         {
              // ...
         } while (condition);
    The assembly that actually gets generated goes as far as doing it like this:
    Code:
    goto loopend;
    do
    {
         // ...
         loopend:
    } while (condition);
    The second one:
    Code:
    // initialisation part
    do
    {
         if (! /* condition */ )
              break;
         // ...
         // next iteration part
    } while (true);
    For loops don't have to count over values. They can for example, walk over a linked list.
    My homepage
    Advice: Take only as directed - If symptoms persist, please see your debugger

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  4. #4
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    Quote Originally Posted by iMalc View Post
    The assembly that actually gets generated goes as far as doing it like this:
    Code:
    goto loopend;
    do
    {
         // ...
         loopend:
    } while (condition);
    Yes, we can probably trust MOST compilers to do that - but not all.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

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