decremented loop counter
Author
What's the reason for this, the n-- in the while loop? Why is the -- added?
while(n-- > 0)
{
*p1 = *p2;
p1++;
p2++;
}
[Edited by - LessBread on June 2, 2007 12:23:20 PM]
n-- will decrease the variable n by 1.
However there is a twist, with it being after the n it will check that the number is above 0 first then decrease its value ready for the next loop.
However there is a twist, with it being after the n it will check that the number is above 0 first then decrease its value ready for the next loop.
This is just a compact form of the following:
It is however the same as:
The statement above is evaluated as:
Which can also be written just as:
But this aproach is unsafe, since it will fail for negative numbers, and works only if n is unsigned.
The exact value of n also depends on the loop. For some loops you might need to pre-decrement n before starting the loop, or you might need to compare it to something else than 0, such as -1 or 1.
while ( n > 0 ) { ... n--;}It is however the same as:
for ( ; n > 0; n--) { ...}The statement above is evaluated as:
while ( (n--) > 0 ) {}Which can also be written just as:
while (n--) {}But this aproach is unsafe, since it will fail for negative numbers, and works only if n is unsigned.
The exact value of n also depends on the loop. For some loops you might need to pre-decrement n before starting the loop, or you might need to compare it to something else than 0, such as -1 or 1.
Author
Quote:Original post by markg
What's the reason for this, the n-- in the while loop? Why is the -- added?
while(n-- > 0)
{
*p1 = *p2;
p1++;
p2++;
}
What would be a better loop to use besids theis one?
Author
Quote:Original post by markg
What's the reason for this, the n-- in the while loop? Why is the -- added?
while(n-- > 0)
{
*p1 = *p2;
p1++;
p2++;
}
What would be a better loop to use besids theis one?
Quote:Original post by markgQuote:Original post by markg
What's the reason for this, the n-- in the while loop? Why is the -- added?
while(n-- > 0)
{
*p1 = *p2;
p1++;
p2++;
}
What would be a better loop to use besids theis one?
Why loop?
In C, memcpy(p1,p2,n * sizeof(type of p1 and p2) );
In C++, std::copy(p2,p2 + n,p1);
If this is for strings use string copying routines, such as strcpy in C and std::string's copy constructor in C++.
Quote:Original post by Antheus
This is just a compact form of the following:while ( n > 0 ) { ... n--;}
Not quite. Your version only decrements n when the body of the loop is executed. The result is that n will have a different value after the loop is done. It doesn't make a difference in this case, but it can in others.
Quote:Original post by JohnBoltonQuote:Original post by Antheus
This is just a compact form of the following:while ( n > 0 ) { ... n--;}
Not quite. Your version only decrements n when the body of the loop is executed. The result is that n will have a different value after the loop is done. It doesn't make a difference in this case, but it can in others.
I mentioned at the end of the post that the value for n depends on the type of loop used.
I tend to avoid such shortcuts, since they can often result in weird boundary conditions which might not be immediately obvious.
C-syntax allows for lots of variations and bastardisations like this, many of which have their own particular gotchas. In general, it's a good idea to keep your code readable, so this syntax is best avoided. In this case, you're (for no good reason) iterating over a section of code with a decrementing variable. To most people, this suggests the use of a for loop:
Admiral
for (; n > 0; --n) { // Do your thing}int n = 50;while(n-- > 0){ //}for (int n = 50; n > 0; --n) { //}Admiral
Quote:Original post by JohnBoltonYou're correct about it only decrementing when the loop is run through, butQuote:Original post by AntheusNot quite. Your version only decrements n when the body of the loop is executed. The result is that n will have a different value after the loop is done. It doesn't make a difference in this case, but it can in others.
This is just a compact form of the following:while ( n > 0 ) { ... n--;}
while(n-- > 0)while(n > 0){ ... n--;}It might equal
while(n>0){ n--; ...}This topic is closed to new replies.
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