php causing me 'HEADACHES!!!!! plz help!

Paul_help_

I have been reading a php book titled: 'beginning php 5.3,, by matt doyle' and I am stumped by a piece of code he uses as part of a script - to justify lines of text.

The code is below. Basically, what this code does is take some text and justifies the text
aligning the text within a column so that the text is flush with both the left and right margins.

The line of code which I cannot understand is:

$originalLineLength = strpos( $myText, “\n”, $startOfLine ) - $startOfLine;

...... to be more specific -------> - $startOfLine; <-------- i do not understand why this piece of code is needed or what it actually does!!

Can someone please HELP!!

PS,

I don't have any visuals to show you but I have a link to the author's book (which contains visuals ( which you can view online for free),,,, page 82. http://www.cs.karelia.ru/~musen/php/2013/Books/Beginning%20PHP%205.3%20by%20Matt%20Doyle.pdf

Paul.

    $numLines = substr_count( $myText, “\n” );
    $startOfLine = 0;
    // Move through each line in turn
    for ( $i=0; $i < $numLines; $i++ ) {
    $originalLineLength = strpos( $myText, “\n”, $startOfLine ) - $startOfLine;
    $justifiedLine = substr( $myText, $startOfLine, $originalLineLength );
    $justifiedLineLength = $originalLineLength;
    // Keep adding spaces between words until the desired
    // line length is reached
    while ( $i < $numLines - 1 & & $justifiedLineLength < $lineLength ) {
    for ( $j=0; $j < $justifiedLineLength; $j++ ) {
    if ( $justifiedLineLength < $lineLength & & $justifiedLine[$j] == “ “ ) {
    $justifiedLine = substr_replace( $justifiedLine, “ “, $j, 0 );
    $justifiedLineLength++;
    $j++;
    }
    }
    }
    // Add the justified line to the string and move to the
    // start of the next line
    $myTextJustified .= “$justifiedLine\n”;
    $startOfLine += $originalLineLength + 1;
    }
    ?>
    < h2 > Original text: < /h2 >
    < pre > <?php echo $myText ?> < /pre >
    < h2 > Justified text: < /h2 >
    < pre > <?php echo $myTextJustified ?>< /pre >
    $numLines = substr_count( $myText, “\n” );
    $startOfLine = 0;
    // Move through each line in turn
    for ( $i=0; $i < $numLines; $i++ ) {
    $originalLineLength = strpos( $myText, “\n”, $startOfLine ) - $startOfLine;
    $justifiedLine = substr( $myText, $startOfLine, $originalLineLength );
    $justifiedLineLength = $originalLineLength;
    // Keep adding spaces between words until the desired
    // line length is reached
    while ( $i < $numLines - 1 & & $justifiedLineLength < $lineLength ) {
    for ( $j=0; $j < $justifiedLineLength; $j++ ) {
    if ( $justifiedLineLength < $lineLength & & $justifiedLine[$j] == “ “ ) {
    $justifiedLine = substr_replace( $justifiedLine, “ “, $j, 0 );
    $justifiedLineLength++;
    $j++;
    }
    }
    }
    // Add the justified line to the string and move to the
    // start of the next line
    $myTextJustified .= “$justifiedLine\n”;
    $startOfLine += $originalLineLength + 1;
    }
    ?>
    < h2 > Original text: < /h2 >
    < pre > <?php echo $myText ?> < /pre >
    < h2 > Justified text: < /h2 >
    < pre > <?php echo $myTextJustified ?>< /pre >

NogDog

See this:

$startOfLine = 0;

Then see the manual page of [man]strpos/man, notice what the 3rd parameter is, and from the description of that parameter ("offset"), see if it makes sense now.

(Hint: it would not change anything in this particular case if you did not call strpos() with that 3rd parameter at all, and instead let it use its default value.)

Paul_help_

but what does - $startOfLine; at the end of $originalLineLength = strpos( $myText, “\n”, $startOfLine ) .... do exactly?????

because, to me it looks like its subtracting '0' (zero) away from the value inside the stpos() function.

NogDog

/ EDITED /

The value gets incremented, so it's not always zero:

$startOfLine += $originalLineLength + 1;

Paul_help_

You see, this is where im getting extremely confused.
The third argument in the 'strpos()' function is used to tell the function where to start the search.
So, surely when the 'for loop' gets to the second line in the paragraph (that needs to be justified), its going to be using the result of this:

$startOfLine += $originalLineLength + 1;

......when its subtracts in the following line (at the beginning of the loop):

$originalLineLength = strpos( $myText, “\n”, $startOfLine ) - $startOfLine;

the thing i don't understand is that its going to be subtracting the value of the previous line from the next line in the paragraph.

so, how on earth does it store the length of the next line (in the paragraph) if its just going to delete the value it gets from:

$originalLineLength = strpos( $myText, “\n”, $startOfLine )

...... with this:

$startOfLine; ?????

Weedpacket

Because [man]strpos[/man] measures from the start of the string. The length of the next line is the length of the string from the start up to the next "\n" found (as returned by strpos), less the position (measured from the start of the string) of the previous "\n" found (as stored in $startOfLine).

Paul_help_

sorry if im being a pain. lol

but surely if the third argument in ----> $originalLineLength = strpos( $myText, “\n”, $startOfLine ) ..... is this ---> $startOfLine ............
...... wouldn't this mean that the search will start from the integer value which is stored in ----> $startOfLine...... from the previous line.

what if this integer is greater then the integer in the next line? surely the search for "\n" will not be found becoz the integer is just to high???

Weedpacket

Sure, the search will start there, but the value returned is measured from the start of the string. To quote from the manual (which exists for a reason):

[man]strpos[/man]
offset
If specified, search will start this number of characters counted from the beginning of the string. [/quote]
Return Values

Returns the position of where the needle exists relative to the beginning of the haystack string (independent of offset).

what if this integer is greater then the integer in the next line? surely the search for "\n" will not be found becoz the integer is just to high???

You could try it and see what happens....

Paul_help_

I think I understand most of what ur saying.

the only thing im still unsure of is why the following is used at the end of the strpos() function:

$startOfLine; ????

whats with the subtraction??

i don't get it.

laserlight

Paul help! wrote:
the only thing im still unsure of is why the following is used at the end of the strpos() function:

$startOfLine; ????

whats with the subtraction??

So that the substr call on the next line would take a substring with the correct number of characters. In other words, you subtract the position of the start of the line in order to get the length of the substring.

Consider:

$myText = "abc\ndefg\nhi\n"

On the first iteration, we have:

$startOfLine = 0
$originalLineLength = strpos($myText, "\n", $startOfLine) - $startOfLine
                    = strpos($myText, "\n", 0) - 0
                    = 3 - 0
                    = 3

This makes sense, since the first substring is "abc", which indeed has a length of 3.

On the second iteration, we have:

$startOfLine += $originalLineLength + 1
              = 0 + 3 + 1
              = 4
$originalLineLength = strpos($myText, "\n", $startOfLine) - $startOfLine
                    = strpos($myText, "\n", 4) - 4
                    = 8 - 4
                    = 4

The second substring is "defg", which has a length of 4. You have to recognise here that the 8 is the position (0-indexed) of the second "\n".

Paul_help_

thanks.

I think i understand most of what you said.

but where does the '8' come from in:

= 8 - 4 ????

Paul_help_

am i correct in thinking that if:

-$startOfLine

..... didn't exist at the end:

$originalLineLength = strpos($myText, "\n", $startOfLine)

..... the string position would just keep doubling up at evey iteration of the loop??

laserlight

Paul help! wrote:
but where does the '8' come from in:

= 8 - 4 ????

I explained this in my previous post:
You have to recognise here that the 8 is the position (0-indexed) of the second "\n".

Paul help! wrote:
am i correct in thinking that if:

-$startOfLine

..... didn't exist at the end:

$originalLineLength = strpos($myText, "\n", $startOfLine)

..... the string position would just keep doubling up at evey iteration of the loop??

I gave you a concrete example. In my example, the second substring is "defg". This substring has a length of 4: 'd', 'e', 'f' and 'g'. If you don't have that subtraction, then:

$originalLineLength = strpos($myText, "\n", $startOfLine)
                    = strpos($myText, "\n", 4)
                    = 8

Therefore, the length of the line is 8. 4 is not equal to 8, therefore the calculation is wrong.

This is something you need to do: take a concrete example, and carefully work through the code. It will help you to understand what is going on. If you don't do this, and yet you are unable to think at the abstract level required to understand what is happening without a concrete example, then you will fail to understand.

Paul_help_

thanks. this makes more sense now.

Hey. Im new to this site. Ive only been on here a few days and i don't know how to add friends. Do you know how to add friend requests and upload a profile pics? as ive tried and this site doesn't seem very user friendly when editing your profile.

Paul.

laserlight

You should be able to edit your profile by going to the settings link. If the options are not available to you yet, it means you need to stay around a bit longer and be active 🙂

Paul_help_

thanks.

also thanku to 'weedpacket' for your help.

Paul_help_

Ok guys.

I know its been a while.
but i still need some help with this.

This whole $startOfLine variable issue is still causing me confusion.

Everything in the script makes sense apart from the variable $startOfline.

Ok. I'll explain how i understand things and you can tell me where im going wrong or what im missing.

When i read the book (which there is a reference to at the top of this thread), it explains how $startOfLine has an initial value of '0' zero.

From here the script does its thing in order to justify the current line its working on. then at the bottom of the script we have:

$startOfLine += $originalLineLength + 1;

In the book it explains about the above line of code by telling us the following:

"the $startOfLine pointer is moved to the
start of the next line (adding 1 to the index to skip over the newline character)"

I don't understand how this can be at the start of the next line when $startOfLine will no longer be equal to '0' zero????????

I don''t understand what this will mean for the 2nd and 3rd (etc) iterations as the code progresses through the lines.

so we've now reached the 2nd iteration,, and this is where things confuse the hell out of me.

at the start of the 2nd iteration we come to these 2 lines of code which are causing me most of the confusion:

$originalLineLength = strpos( $myText, “\n”, $startOfLine ) - $startOfLine;
$justifiedLine = substr( $myText, $startOfLine, $originalLineLength );

..... so at the 2nd iteration, how is the $startOfLine still considered to be 'at the start of the line'?

because it is no longer going to be equal to '0' is it???

let me guess what the below code does. and just tell me if im correct.

in the below piece of code it uses the 'string position function'. i have learned about this function and its parameters. Maybe the book missed something important out,, but to me it looks like its taking the '$myText' variable as the text its going to work on and its searching for \n starting from the numerical value position of $startOfLine.

now i need to know,,, is the deduction of - $startOfLine; at the end of the below line, the first thing to happen, before strpos( $myText, “\n”, $startOfLine ) happens ???? or is it the other way around???

$originalLineLength = strpos( $myText, “\n”, $startOfLine ) - $startOfLine;

...... and the piece of code below. I need to ask about this. Is the $startOfLine variable now equal to the deducted sum above? or is it still set to the number it was set to after the first iteration???

$justifiedLine = substr( $myText, $startOfLine, $originalLineLength );

Weedpacket

Paul help! wrote:
I don't understand how this can be at the start of the next line when $startOfLine will no longer be equal to '0' zero????????

Because the offset [man]strpos[/man] uses ([font=monospace]$startOfLine[/font]) is measured from the start of the entire string. Not the "start of the line" (which [man]strpos[/man] has no concept of anyway). [font=monospace]$startOfLine[/font] would obviously have to be nonzero, because the next line obviously doesn't start at the same position in the string as the first line because that's where the first line starts, and the next has to start somewhere after that (at a different offset, i.e., a different value of [font=monospace]$startOfLine/[font]).

Paul_help_

ok. so the beginning string position of the next line is NOT going to be equal to zero?

is it going to be a number that keeps adding up from the line before??

so if the final string position of the first line is 36,, the beginning sting position of the second line would be 37, etc??

dalecosp

Paul help!;11032931 wrote:
ok. so the beginning string position of the next line is NOT going to be equal to zero?

is it going to be a number that keeps adding up from the line before??

so if the final string position of the first line is 36,, the beginning sting position of the second line would be 37, etc??

Yes, IF the lines are all in the same string:

$long_string = "This is a long string of characters and it has more than one line.
This is the second line of the long string.";

$short1="This is a short string.";
$short2="This is a second short string.";


echo "the length of the long string is ".strlen($long_string)." characters.\n";
echo "the length of the 1st short string is ".strlen($short1)." characters.\n";
echo "the length of the 2nd short string is ".strlen($short2)." characters.\n";

If you happen to do "echo strpos($long_string,'second');" you'll get 79, not 13.