if and for condition statements

[Caprico]

Okay, so I was solving one of the Euler problems, and I came across a rather peculiar doubt... Till now, I had thought that:

for (i=1; i<=max; i++)
  if (some condition)
    do something;

And

for (i=1; i<=max && some condition; i++)
  do something;

would be equivalent, (and in a way, the latter be speed efficient in nested loops, since I'd check the condition before executing the loop for a certain value of i). However, I came to realize it is not so.
Can someone please explain why these two are not equivalent?

Thank you in advance... :)

[Moschops]

The difference is that in the first one some condition has nothing to do with when the loop ends, and in the second one, it does. They're not equivalent.


Here's a very simple example; let's say that some condition is "i is an even number", for example.

for (i=1; i<=max; i++) // For all values from i=1 to max...
  if (some condition) // if it's an even number....
    do something;  // do something 

This will cycle over all i from 1 to max, and do something will happen many many times.

for (i=1; i<=max && some condition; i++) // For all i from i to max, but stop when i is an even number...
  do something;

In this case, do something will not even happen once, as the very first check will find that some condition is false.

[Caprico]

Ooooh, now I get it.. So in the for loop, when the condition evaluates to false, it exits the for loop, and does NOT re-iterate, is that correct? Dunno why such a silly thing did not strike me... So dumb of me...

Thank you so much! :)