for loops - newbie q's

I understand that for loops are used as
for(variable = value)(relational expression)(inc dec value) {loop}

my question is, do you have to increment or decrement the variable set at the first of the function? and do you even have to set a variable at all? Like could you just say..
for (i != 0) {loop that modifies i} ..or would that just be a for loop acting as a while loop.

I cant see a whole lot of uses for the for loop.. from what I see it's just a while loop with a preset number of loops.

could anyone give me a simple example of other uses for the for loop besides the plain for(i = 1)(i <=20) (i++)

The good thing about for loops is that they do three functions in one. They set the starting value, evaulate the expression, and make the incremental change.
With a while loop you have to set the value before the while loop and make sure you put in the incremental change in the loop. I can't count the number of times I've forgotten to put the incremental change in a while loop.
Every different typle of loop has its strong points and weak points. Just need to identify them and use the best loop for the job.

so the only valid expression on the third part of the function is (var)++ or (var)--?

Nope, you can do other ones.
ie

for(i=0;i<x;i+=15)

It seems like you can do any math function. I don't know all the limits but i++ or i-- are not the only ones that can be done.

>I cant see a whole lot of uses for the for loop.. from what I see it's just a while loop with a preset number of loops.

A for loop is a subset of the while loop - a for loop is only used for counter-controlled repetition, and has very little other use. A while loop can be used for counter controlled repetition, but not very often. Normally a while loop is used to evaluate a condition, like an if/else statement.

Code:

---

while (condition1 == true){
  do(someaction)
}
while (condition 1 !=  true){
  do(someotheraction)
}

---

Try here for some examples of the for loop (go onto the other sections as well).

With for loops, you can have a loop without the increment value, without the condition, without the intialization, or any combination of two, or none of the above.

Code:

---


for(  ;  ;  )  /*empty*/
{
  if(condition) break;
}

i = 0
for(; i < 5; ) /*conditional only*/
{
    printf("%d", i++);
 }

---

Just two of the permitations of the for loop.

Thats pretty cool bigtamscot, never knew that. Of course in both cases I would have used a while loop.

>Of course in both cases I would have used a while loop

Well a while loop generally saves you time, but bigtamscot is right in what he's saying.

Ofcourse we would use a while loop first of all, just demonstrating flexability of for loop.