Understanding For()

[Sharif]

I'm still trying to get how for() works. I've always used while() for my loops (I've never had to loop anything except database results) and now in my recent experiments to create a calender, I find that I need to use the for statement.

So go with me here,

for(initial var value; number of times to loop; amount to add to loop)

Is that how it works? I tried to understand what the manual says but I don't get it. Please help.

[ahundiak]

Almost. The middle clause is the terminating condition i.e. when this condition is false then exit the loop;

for(
  $i =  20; // Initialization
  $i < 100; // Terminate when false
  $i += 10) // Do this on every pass
{
  echo $i . ' ';
}

[Gary_King]

You can always just use a while statement to replace a for() one, especially if it's hard to remember what goes in the for() 🙂

[Sharif]

ok so the middle clause is sort of like an IF() statement...

[cruz610]

well using the FOR statement will make something continue to do something else. ie:

for( 
  $i =  0;  $i <= 100;) {
  echo $i . ' ';
 $i++;
}

will output
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100

[phprock]

Here is another example.

Let say your age is 24 and you want to display all the years
from year 1 to 24.

So your intial value is 1 and last value is 24

for($myage=1;$myage<=24;$myage++){

echo"$myage<br>";

}

The above code print age from 1 to 24..Its increment by 1 year
$myag++ could write as $mysage=$myage+1

if i crement by 2 , How many output you will get ?

write the following code and see

for($myage=1;$myage<=24;$myage=$myage+2){

echo"$myage<br>";

}

Here is example to printout 7 days from now on ..

<?php

$starttime=time();
$endtime=$starttime+6*86400;

// 86400 seconds a  day, so for (6 days +today )= 7days 

for($time=$starttime;$time<=$endtime;$time=$time+86400){

echo"".date("d-m-Y","$time")."<br>";

}

?>

[Gary_King]

All those examples can be done using a while() statement 🙂

[ahundiak]

Gary W,
It's true that a while loop can do the same thing as a for loop. And vice versa. They both have their uses and a good programer knows which one to pick for a given situation.

BTW, would you care to post an example of how to initialize a variable, test for an end condition and increment the counter in a "while statement"? No fair doing this:
$i = 20;
while($i < 100) {
$i += 10;
}
Since the initialization and incrementing require their own statements and you seem to feel that a single while statement can do the same.

[Gary_King]

Those lines of code are fine..?

[pjleonhardt]

his point his with while loops you need to use 3 statements, while with for loops you can do it in only one...

they each have there purposes, but both can be used for the same most of the time

[Gary_King]

Okay.

Anyways, it was all just a suggestion to use while() if for() was a bit too confusing, and while() you sort of have more control over.

It's not like I use while() when a for() would be better 🙂

[Weedpacket]

Originally posted by Gary W
Anyways, it was all just a suggestion to use while() if for() was a bit too confusing, and while() you sort of have more control over.

Well yeah, any loop could be coded with a while() - it's the most primitive looping construct there.

[zapa]

The for Statement you cannot achieve anything with a for statement that you cannot do with a while statement. On the other hand, the for statement is often a neater and safer way of
achieving the same effect. Earlier, Listing 5.6 initialized a variable outside the while statement. The while statement then tested the variable in its expression. The variable was incremented within the code block. The for statement allows you to
achieve this on a single line. This allows for more compact code and makes it less likely that you forget to increment a counter variable, thereby creating an infinite loop.
for ( variable assignment; test expression; variable increment )
{
// code to be executed
}

Each of the expressions within the parentheses of the for statement is separated by semicolons. Usually, the first expression initializes a counter variable, the second expression is the test condition for the loop, and the third expression increments the counter. Listing 5.8 shows a for statement that re-creates the example in Listing 5.6,
which multiplies 12 numbers by 2.
Listing 5.8: Using the for Statement
1: <html>
2: <head>
3: <title>Listing 5.8</title>
4: </head>
5: <body>
6: <?php
7: for ( $counter=1; $counter<=12; $counter++ )
8: {
9: print "$counter times 2 is ".($counter*2)."<br>";
10: }
11: ?>
12: </body>
13: </html>
The results of Listings 5.6 and 5.8 are exactly the same. The for statement, though, makes the code more compact. Because $counter is initialized and incremented at the top of the statement, the logic of the loop is clear at a glance. Within the for
statement's parentheses, the first expression initializes the $counter variable and sets it to 1. The test expression checks that $counter contains a value that is less than or equal to 12. The final expression increments the $counter variable.
When program flow reaches the for loop, the $counter variable is initialized, and the test expression is evaluated. If the expression evaluates to true, the code block is executed. The $counter variable is then incremented and the test expression
evaluated again. This process continues until the test expression evaluates to false.