Lecture#14
Chapter 15 - Strings and Sorting

index/rindex

$a = "Hi there there";
$pos = index($a,"the");     #pos =3
$pos2 = index($a, "the", $pos+1);    #pos2=9
$pos3 = index($a, "z");    #pos3=-1 (not found)

rindex is EXACTLY the same, except it starts searching from the right

substr

$mystring = substr("Joe Blow",4,4); #gets 'Blow'
$mystring = substr("Joe Blow",4,20); #gets 'Blow'
$mystring = substr("Joe Blow",-2,4); #counts from right hand side first

substr can be used as an lvalue
$string = "Hello";
substr($string,1,4) = "i"; #$string now equals "Hi"

Another shortcut...
$prev_string=substr($string,$start, $len, $replacement);

$prev_string has the result
$string is the string we're manipulating
$start is to where we should start
$len is how many chars to take
$replacement is what we should replace this substring with

sprintf

sprintf is printf to a string (instead of an output stream)

$amt=sprintf "%.2f", 6.9994;

Advanced Sorting

You can give your own comparison to be used, so if you want to say, sort a hash by the values, you can do it like this:

%scores = ("Bowler1" => 100, "Bowler2" => 200, "Bowler3" => 300);
@leaders = sort by_score keys %scores;

That 2nd line is saying to sort the keys of %scores, but use the comparision defined in the by_score subroutine.....that subroutine looks like this.

sub by_score {
$scores{$b} <=> $scores{$a}
}

$a and $b are special variables...the subroutine you make has to use them. The normal compare just looks to see if $a if less than $b, ours does something a little different...

1. The first time it's executed, $a=Bowler1 and $b=Bowler2
2. The values that are being compared are 200 on the left and 100 on the right
3. <=> compares numeric values...if they are out of order, it rearranges them to sort...in this case 200 is not less than 100, so $a and $b are rearranged
4. This continues so that the keys end up being sorted in reverse order by value.

Another example...sorting on multiple keys....

@winners = sort by_score_and_name keys %scores;

sub by_score_and_name {
$scores{$b} <=> $scores{$a}

$a cmp $b
}

if <=> sees 2 identical values, it returns 0, so the 2nd part would be executed. That part then compares the names (cmp works on strings). So Alan bowling 200 would come before Bob bowling 200.

Quick Slice Intro (From Ch. 17)

List slice
($one, $five) = (0,1,2,3,4,5) [1,5]; #$one=1, $five=5

Array Slice
@array=(0,1,2,3,4,5);
print @array[1,5]; #prints 15