english.sbv

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Hi, and welcome back to the channel of Perl Tutorial

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This time I'm going to talk about

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a number of subroutines

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functions, that can handle strings in perl

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There are couple of basic ones like...

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Let's switch to the other window

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So, there are couple of basic functions, for example

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lc, that will return a lowercase version of a string

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uc, that will return uppercase version

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and length that will return a number of characters in that string

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So, I wrote a couple example files

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What you can see here is $str, which is a string

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HeLlo with a capital L here and lowercase l

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And if you run the script

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Lets run it, so you can see, how it acually works

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You see, that the first will return "hello" in lowercase

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The second one in all uppercase

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and the last one just returns the number of characters

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Neither of these acually change the string

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if you want to change the string, than you type in:

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$str = lc $str;

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and than you print out the $str again

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If I run this now than you will see that

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now $str is already lowercase

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These are really the simple ones

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There is one slightly more complex function

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which is called index

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Index will get normally two parameters

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one of them is string like this one

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Let's enlarge the font

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the first parameter is a string

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and that's what you can see here

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and the second one is also a string

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that is usually shorter

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because you're looking for it's location within the first parameter

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so, if you run index on this longer string, giving it "cat"

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it will return you 10

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because "cat" starts from the 10. character

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Index counts characters starting with 0

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so this capital T is actually 0

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that's how it comes to 10 at "c"

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If I run this same index function, now giving it "dog"

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than it will return -1

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because in this string there is only a cat

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there is no "dog"

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The way it returns error or faliture is giving us -1

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If you call this function giving a string

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that doesn't appear in the original string you'll get -1

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It might be not the most convenient way

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but that's what we have to use

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If I run index on the string "The", capital T

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it will return 0

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That's just showing, that the index is 0 based

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So it will find this "The" at the beginning

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and it will return 0, because that's how it starts

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Next call, this one, returns 26

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because index is looking for the exact, same string

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and it's case sensitive as well

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so first "The" doesn't fit, because that's a capital T

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so it will look for another "the" and there it will find it

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Now, you might think index is looking for the word

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but it's not the case, it just looks

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for a substring, which is just characters one after another

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In the next example you can see

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that we are looking for the string

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letter "e" and the space afterwards

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and it will return us 2

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because here you can see

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highlighted the place where it appears

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but as you can see there is a "e " here

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the way you can reach that is index can get third parameter

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that's here the number 3, but can be any number

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and it will start searching from the third position

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which is acually 4th character

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beacuse you start counting form 0

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so the third place, place number three

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and therefore it will find the "the"  here

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at this spot, which is appearentely the 28. place

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Similarly, if I call index and give it character "e"

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just for looking

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but I give it the number 3 to start from

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than it will find the letter "e"

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lets see...

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so this in the word "jumped"

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there is character "e"

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so that's what its going to be found

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Obviously this 3 can be any number here

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so if I can start from 5

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and it will return the same value

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Lets run this script

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So, you can see that the values are:

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10, -1, 0, 26, 2, 28, 18

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this is the same value

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It doesn't matter where you start exactly

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as long as you start before

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the appearence of the substring

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And that's what index does

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but there is also function called rindex

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that does the same just looking

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from the right hand side

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It's interesting

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if there're multiple apperances of the same string

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and it will find the rightmost one of it

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Running rindex on the same string

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and giving it the substring "e"

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than it will return 39

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because it will find this letter "e"

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And rindex can also have a third parameter

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that will tell it where to start from

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The position to start from looking for

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So if I give it 38

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than it will find this letter "e" here

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returning 38

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And if I start from 37 here

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than it will, probably, find this "e" returning 33

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So that's about index

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The last one, which is probably the most interesting one

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out of the functions that I'm showing now

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is substring

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Substring is actually the opposite of index

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In index you would know what you're looking for

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what string you're looking for

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just don't know where it is

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So you're looking for a location

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In substring you know the location

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you just don't know what is there

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and you want to see what is in that location

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Substring will get normally a string,

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again this "The black cat climbed the green tree";

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a number, which is an offset, so it is the location

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to beginning of the substring we are looking for

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in this original string,

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again it's 0 based, so 0 would mean the first character

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and 4 means this "b";

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and the third parameter is the length

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of the substring we are fetching

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So because it has 5 characters

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it will return the string, starting from this letter "b"

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...5 characters, so that's the word "black"

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Substring returns those values

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and obviously I can assign it to...

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what acually I showed you in lc and index example

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... you can assign that value to some kind of variable

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Here too, I'm just lazy so I'm just printing it out immediately

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This call will return the word "black"

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Now in the next example you can see

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that the length is a negative number

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So how would a negative number come there?

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Instead of using that as a real length

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what Perl does, is saying that this "4" means

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how many characters from the left to leave off

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and this number now says

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how many characters from the right not to include

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So, basically, it says count 4 from the left,

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count 11 from the right

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and whatever is in the middle

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- that's what I want to get back

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So that's how the "black cat climbed the" is returned

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You can even leave off the whole third parameter at all

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and that would mean in this case, as you can see:

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just give me string from the position 14 to the end

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because I didn't see how long it should take

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So you just give it: from the position to the end

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And here you can see,

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that even the first parameter, the location

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can be a negative number

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and that, as you propably already guessed,

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is counting from the end

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So instead of saying it's a the length less 4

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you just say less 4

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and that means: start from this position

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And because in this example here

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I didn't give the number of characters

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so, it will give me all the string till the end

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that's just the 4 letters "tree"

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Obviously, as you can see in this example,

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you can give the third parameter as well

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and than it will return you

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the two characters from the last four

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and that's "tr"

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Let's just run it, so we can see

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that's the "tr" here, right

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And, if I change it a little bit,

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for example put "1" here,

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than run it and you can get only the "t"

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And, if I would give "3" and run,

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than I get "tre", so that you can understand it

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The last example is a bit funky

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So up untill now this call returned substring,

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but the original $str, the original string didn't change

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In the last example, the return value is still the same

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it's defined by the first three parameters

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but in the last example, this example

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we also have fourth parameter,

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which is, in this case, a string containing "jumped from"

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And what happends is that, in this case,

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Perl substring function will replace the string

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that was defined by this three parameters

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will replace that substring by the new string

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So, because 14, 7 here defines the word "climbed" here,

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the return value, in $z, will be "climbed",

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but the $str will have a different content

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and the word "climbed" will be replaced by the word "jumped from",

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by the two words, substring actually

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In this case $str will contain:

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"The black cat jumped from the green tree"

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And as you can see, Perl doesn't care that

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the new string, new substring is actually longer

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than the original one

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it will just make enough space in the string

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or if you leave it out, so for example

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Let's just run it again

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so you can see that

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"The black cat jumped from the green tree"

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that works

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What happens, if I just put in the letter "j"

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and run it? Than you will see that

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"The black cat j the green tree"

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whatever that would mean

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It doesn't really matter, what the lengths are relative to each other

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You can input an empty string,

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but you have to have it here

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and than Perl substring will cut out that part of the string

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and leave, in this case, two spaces,

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because the space before word "climbed"

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and the space after

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So, that's about substring

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I hope you've learned something

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and we will return to the next part

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and if you are watching it on YouTube

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please subscribe to the channel

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Thank you, bye bye