Loops within loops (For Loops)

PHP
1

Hi,

I found a great resource for studying PHP - but I’m stuck on how to interpret “Loops within loops”.
The code I refer to is at the following page:

http://www.tuxradar.com/practicalphp/2/6/13

Can someone please explain the control flow? - I understand for (initialise, test, action) but not how this works with nested loops.

Any help appreciated.

2

for loops are simple, but in php they are set up kind of strange, here is a while loop that does the same thing:



$x = 0;
while($x < 10) {
    $x++;
    // Your other code
}

for($y = 0; $y < 10; $y++) {
    // Your other code
}

It’s more of a convenience thing…

So in nested loops the instructions continue as normal until they encounter the loop, then they continue in that loop, until they encounter the next, so:


for($y = 0; $y < 10; $y++) {
   echo 'got to a';

   for($y = 0; $y < 10; $y++) {
       echo 'got to c';

       for($y = 0; $y < 10; $y++) {
            got to 'e';
       }

       echo 'got to d';
   }

   echo 'got to b';
}

Your output will be:
got to a
got to c
got to e
got to d
got to b

3

Serj,
Thanks for that.
Below is my output from your code example. (I added an ‘echo’ for ‘go to e’ line - did you mean to leave it out?) and some <br / > tags

for($y = 0; $y < 10; $y++) {
   echo 'got to a <br />';

   for($y = 0; $y < 10; $y++) {
       echo 'got to c <br />';

       for($y = 0; $y < 10; $y++) {
		   echo 'got to e <br />';
       }

       echo 'got to d <br />';
   }

   echo 'got to b';
}

got to a
got to c
got to e
got to e
got to e
got to e
got to e
got to e
got to e
got to e
got to e
got to e
got to d
got to b

Can you tell me if I have this right?
for (initialise variable, test condition - if true execute statement, then increment this first instance or move on to the next nested loop).
Sorry I still don’t understand this and don’t know what questions to ask (isn’t that the definition of complete confusion?)

4

You would normally use a different variable for each for loop. In the above example, the 'got to e’s show 10 times because $y is reinitialised upon entering the loop, however the others are then not run due to $y already being above 10.

for($x = 0; $x < 10; $x++) {
   echo 'got to a <br />';
 
   for($y = 0; $y < 10; $y++) {
       echo 'got to c <br />';
 
       for($z = 0; $z < 10; $z++) {
		   echo 'got to e <br />';
       }
 
       echo 'got to d <br />';
   }
 
   echo 'got to b';
}

Is more practical, and is how it is done in the tutorial you mentioned. Individually however, your loops are perfect :slight_smile:

5

Thanks,

Could you please help me analyse the original code I posted. I would be interested to know if I have understood the flow correctly.

line 1: 	for ($i = 1; $i < 3; $i = $i + 1) {
Line 2:	****for ($j = 1; $j < 4; $j = $j + 1) {
Line 3:	********for ($k = 1; $k < 3; $k = $k + 1) {
line 4:************print "I: $i, J: $j, K: $k\
";
********}
****}
}

The output of that script is:

I: 1, J: 1, K: 1
I: 1, J: 1, K: 2
I: 1, J: 2, K: 1
I: 1, J: 2, K: 2
I: 2, J: 1, K: 1
I: 2, J: 1, K: 2
I: 2, J: 2, K: 1
I: 2, J: 2, K: 2

I understand how lines 1, 2 and 3 are output to read: I: 1, J: 1, K: 1 and how in this same iteration $k is incremented to the value of 2.
We then loop back to line 1 and follow the same method to output: I: 1, J: 1, K: 2

Don’t we then increment $k to 3 and in doing so render this loop inert as three is not less than three? - where do we go from here?

I don’t understand the next line of output: I: 1, J: 2, K: 1 (how does J become 2) and $k become 1 again.?

6

No, we don’t loop back to line 1. We stay inside line 3. The third loop isn’t exited while condition ($k < 3) is true.
So the first time you enter the ‘line 3’ loop, i = 1, j = 1 and k = 1.
Then k gets incremented to 2. 2 < 3 so the line 3 loop is executed again (i = 1, j = 1, k = 2).
Then k gets incremented to 3. 3 is not less than 3, so we exit the ‘line 3’ loop and go back to the ‘line 2’ loop.
j gets incremented to 2. 2 < 4 so the line 2 loop is executed again. $k is initialized at 1 (1 < 3) and the ‘line 3’ loop is executed (i = 1, j = 2, k = 1).

etc etc.

Once j gets incremented to 4 the line 2 loop is exited, and we go back to the line 1 loop. i is incremented to 2 (2 < 3) so the line 1 loop is executed again.
etc etc.


echo "start<br />";
for ($i = 1; $i < 3; $i = $i + 1) {
  echo "first loop, i = $i <br />";
  for ($j = 1; $j < 4; $j = $j + 1) {
    echo "second loop, j = $j <br />";
    for ($k = 1; $k < 3; $k = $k + 1) {
      echo "third loop, k = $k <br />";
      print "I: $i, J: $j, K: $k<br/>"; 
    }
    echo "third loop exited because k = $k<br/>";
  }
  echo "second loop exited because j = $j<br/>";
}
echo "first loop exited because i = $i<br/>";
echo "end<br />";
7

$i and $k wont equal 3 because your loop specifies less than… However with the code you posted it should display $j=3, I’m not sure why your output is different?

With the third output, when $j=1, $k loops through 1 and 2. Then $j is set to 2 and iterates through the k loop again. Seeing as the start parameter is $k=1, every time the loop is called it will reset at 1, no matter how high $j is set, $k will go through the exact same loop for every $j value

8

Thanks guys for all your input!
Guido,
I very much appreciate you taking the time to lay it all out for me, I understand now.
Travo,
Thank you for picking up the error:

However with the code you posted it should display $j=3, I’m not sure why your output is different?

I’ll have to inform the author http://www.tuxradar.com/practicalphp/2/6/13 of his error - wouldn’t you think he would test his code before publishing?
Thanks again guys!