What does this for-loop mean? - php

Question

I've stumbled upon a syntax in a code example I have never seen in for-loops before in PHP.
What does this do? WHY should I use this?
for(;$i<$max;){
$i++;
//code..
}
I could figure out that it was not the same as
for($i=0;$i<$max;$i++) {
//code...
}
I don't understand the difference between the two examples above.
If for being more specific about my thoughts.
If we have this code (taken from a solution from adventofcode):https://www.reddit.com/r/adventofcode/comments/kdf85p/2020_day_15_solutions/
<?php
$cap = 2020;
$bits = [5,1,9,18,13,8,0];
$i=0;
$time = [];
foreach($bits as $bit) {
$i++;
$time[$bit] = $i;
$say = 0;
}
for(;$i<$cap-1;){
$i++;
if(isset($time[$say])) {
$last = $time[$say];
}
else {
$last = $i;
}
$time[$say] = $i;
$say = $i - $last;
}
echo $say;
?>
and compare that to:
<?php
$cap = 2020;
$bits = [5,1,9,18,13,8,0];
$i=0;
$time = [];
foreach($bits as $bit) {
$i++;
$time[$bit] = $i;
$say = 0;
}
for($i=0;$i<$cap-1;$i++){
if(isset($time[$say])) {
$last = $time[$say];
}
else {
$last = $i;
}
$time[$say] = $i;
$say = $i - $last;
}
echo $say;
?>
I get different results in $say.(376 in first example and 38 in last example).
Why do I get different values?

Answer 1

Well based on the PHP docs:
https://www.php.net/manual/en/control-structures.for.php
for loops are the most complex loops in PHP. They behave like their C
counterparts. The syntax of a for loop is:
for (expr1; expr2; expr3)
statement
While:
Each of the expressions can be empty or contain multiple expressions
separated by commas.
So it's just a way to "save code" when your first expression of the for-loop is "obvious", So instead of mentioning the default obvious expression - you just "skip" it.
According to your example:
for(;$i<$cap-1;)
we skipped the first expression (expr1) as $i has been already defined as 0 ($i = 0;) earlier in the code block so it's "skippable". Doesn't affect the code.
But, The third expression:
At the end of each iteration, expr3 is evaluated (executed).
Since we don't mention it in the loop, we are responsible to handle the "increasement" (in this case) of $i in the loop itself.
However - the main difference in the code is that if you mention the third expression - it's evaluated at the end of each iteration but in the code block (your example) - we increase the $i variable at the beginning of the code.

Answer 2

The main difference between the two blocks (assuming $i is initialised to 0 before the for(;$i<$max;){ loop) is that the first loop increments $i before running the loop code, where the second loop increments $i after running the loop code. As a trivial example:
function code($i) {
echo "$i\n";
}
$max = 3;
$i = 0;
for(;$i<$max;){
$i++;
code($i);
}
for($i=0;$i<$max;$i++) {
code($i);
}
The output of the first loop is:
1
2
3
while the output of the second loop is:
0
1
2
Note (as pointed out by #IMSoP), the condition clause is executed at the beginning of the loop, and can have side-effects, so you could also emulate the first loop with this code:
for($i=0;$i++<$max;) {
code($i);
}

Answer 3

C-style for loops are a rather peculiar piece of syntax. They actually consist of three expressions, any of which can do anything you like, or even be empty:
An expression to execute for its side effects once before the loop begins
An expression to evaluate at the beginning of each iteration, to see if the loop should terminate
An expression to execute for its side effects at the end of each iteration
The most common way to use them is:
Initialise a counter of some sort
Check if the counter has reached some value
Increment or decrement the counter
But those aren't built into the language at all, and leaving one of the expressions out doesn't apply any default behaviour, it just does nothing - except that leaving the second expression empty always evaluates to true.
So, for instance:
for(;true;) is an infinite loop: it does nothing, checks true, and does nothing again
for(;;) is the same infinite loop, because the empty expression in the middle is considered true
for($i=0;;) is just the same infinite loop, but with $i initialised to 0 before running it
for(;true;foo()) is the same loop, but with the function foo() run at the end of every iteration
In your case, the loop is for(;$i<$max;) which breaks down to:
Before starting the loop, do nothing
At the beginning of each iteration, terminate if the expression $i<$max happens to be false
At the end of each iteration, do nothing
It doesn't do anything to control what values $i and $max have before or during the loop.
In fact, it's just the same as while($i<$max) and would probably be much clearer if written that way.
The example shown seems to have ended up that way because the author wanted to move the $i++ to the beginning of each iteration, rather than the end (although why they also left out the $i=0 I'm not sure). But the for syntax supports that too! You can actually include multiple expressions with a comma between in any of the three positions; if they're in the second position, the last one is what determines if the loop exits.
So you can do this:
for($i=0; $i++,$i<=$max; )
Now, $i will be incremented at the beginning of the loop; since it will be incremented before the test, you have to also adjust the condition from < to <= to make sure it runs for one more iteration.
For that particular case, there's another way as well: incrementing a variable returns a value. If you write ++$i it returns the value after incrementing, and if you write $i++ it returns the value before incrementing. So both of these would also work:
for($i=0; ++$i <= 10; ) echo $i, PHP_EOL;
for($i=0; $i++ < 10; ) echo $i, PHP_EOL;

Answer 4

In your first example, $i is registered somewhere before. so $i can be 0 or other zero, because if you use code like
for(;$i<$max;){
$i++;
//code..
}
you got error Undefined variable $i in and later $i is registered.
/* example 1 */
for ($i = 1; $i <= 10; $i++) {
echo $i;
}
/* example 2 */
for ($i = 1; ; $i++) {
if ($i > 10) {
break;
}
echo $i;
}
/* example 3 */
$i = 1;
for (; ; ) {
if ($i > 10) {
break;
}
echo $i;
$i++;
}
https://www.php.net/manual/en/control-structures.for.php
what is your point?