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!

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

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.

Quote:

---

Originally Posted by iMalc

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