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What's the best way to do a reverse 'for' loop with an unsigned index?

Asked 2009-03-20T11:26:08.860
70

My first attempt of reverse for loop that does something n times was something like:

for ( unsigned int i = n-1; i >= 0; i-- ) {
    ...     
}

This fails because in unsigned arithmetic i is guaranteed to be always greater or equal than zero, hence the loop condition will always be true. Fortunately, gcc compiler warned me about a 'pointless comparison' before I had to wonder why the loop was executing infinitely.

I'm looking for an elegant way of resolving this issue keeping in mind that:

  1. It should be a backwards for loop.
  2. The loop index should be unsigned.
  3. n is unsigned constant.
  4. It should not be based on the 'obscure' ring arithmetics of unsigned integers.
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2 Answers

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for ( unsigned int i = n; i > 0; i-- ) {
    ...  
    i-1 //wherever you've been using i   
}
answered 2009-03-20T11:30:10.210
1

Hm. Here are your options:

  1. Use i=0 as your break condition - Loop will not execute when i reaches 0, so execute 1 iteration of the loop contents for i=0 after the loop has exited. 
for ( unsigned int i = n-1; i > 0; i-- ) {
    doStuff(i);
}
doStuff(0);
  1. In the loop, test for i=0 and break out. Not recommended because now you're testing the value of i twice in the loop. Also using break within a loop is generally regarding as bad practice.
for ( unsigned int i = n-1; i >= 0; i-- ) {
    doStuff(i);
    if (i=0) break;
}
answered 2009-03-20T11:39:29.983

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