Download Python Strings: A Sequence of Characters and more Lecture notes Programming Languages in PDF only on Docsity!
Programming, Data Structure and Algorithms in Python Prof. Madhavan Mukund Department of Computer Science and Engineering Indian Institute of Technology, Madras Week - 02 Lecture - 02 Strings We have seen now that python uses names to remember values. Values are the actual quantities that we manipulate in our program, these are stored in names. Values have types, and essentially the type of a value determines what operations are allowed. (Refer Slide Time: 00:02) The types we have seen are the basic numeric types - int and float, and the logical type bool which takes values true or false. So, for the numeric types, we have arithmetic operations, we also have other operations which are more complicated. For the Boolean types we have and, or, not, which allows us to manipulate true and false values. And then we have these comparison operators equal to, greater than and so on, which allows us to check the relative values of two different quantities, and decide whether they are in some order with each other. The important thing that we said was that in python the names themselves do not have a
fixed type. So, we cannot say that i is of type int or x is of type float, rather it depends on what values assigned and in particular, if a name is used for the first time without assigning a value then python will complain. We do not have to announce names in advance like other programming languages, but whenever we first use a new name; its first use must be in an assignment statement on the left hand side. So, before we use a name in an expression on the right hand side it must be assigned a valid value. (Refer Slide Time: 01:36) Numeric types by no means the only things that are of interest these days in computation. A lot of the computation we do is actually dealing with text. So, whenever we prepare a document, for example, using a word processor or some other things for presentation, then we are actually manipulating text; so we are moving text around, searching for something to replace and so on. Also when we are manipulating data itself, very often data comes from multiple sources. We might have tables of values which are typed in by somebody or generated by a device and we have to import them in a spreadsheet. And then if we want to manipulate them by using another program, we might want to export them from a spreadsheet this is typically done using text files in which the columns of the spreadsheet are stored in a systematic way separated by say commas. So, this also involves text processing.
The values of this type are written as we would normally do in English using quotes. We use quotation marks to demarcate the beginning and at the end of a string when we want to write down an explicit value. So, we can use any type of quote, so a single quote would denote in this case the name city is assigned the string ‘Chennai’. Note that when we write symbols like this capital C is different from small c and so on. So, we have seen exactly seen the symbols within these two quotes as the value assigned to the string to the name city. Now we can also use double quotes; and one reason to use double quotes is if you actually need to use a single quote as part of the string. This is one way to do it; and the other way to do it is actually to write a back slash. If you write a back slash and s quote in the middle of the string, it means that this quote is to be taken as a symbol and not at the end of the string, but a much simpler way to include special things like quotes inside other quotes is to change the quotation. So, a single quote can include double quotes, and the double quote can include single quote without any confusion. So, this says that the name title is assigned a value “Hitchhiker’s Guide to the Galaxy”. Now, what if you wanted to combine both double quotes and single quotes in a string? So, python has a very convenient thing called a triple quote. So, you can open three single quotes, and then you can write whatever you want with multiple double quotes and single quotes. So, if you want to say ‘‘‘He said his favorite book is within quotes “Hitchhiker’s Guide to the Galaxy” ’”. Then this value string has both double quotes inside it and it also has a single quote inside it. So, we cannot enclose it in double quotes and we cannot enclose it in single quotes, because either of them will be ambiguous unless we use this back slash as I said before. So, if we do not want to use back slash, you can use a triple quote.
(Refer Slide Time: 06:06) Let see how this works in python interpreter. So, we can say s equal to ‘Chennai’ and now been asked the value of this and we see that it is reported with single quote. If we ask for the type of s, it says that s is class of str. So, this tells us that internally python realizes that s is a string. If we say t is equal to say just the letter x, then the type of t is also a string. So, there is no distinction between single character and multiple characters. Now if we say let us just shorten it say title is equal to “Hitchhiker’s” then if you ask for the value of title, it shows it to you with double quotes outside and a single quote outside. So, this indicates that this is a single string and again the type of title is str. And finally, if I say myquote is equal to and I use three quotes and I use ‘‘‘ “Hitchhiker’s” ’’’. So, I have “Hitchhiker’s” in double quotes and Hitchhiker’s itself contains a single quote. And I use triple quotes around it then my quote is correctly shown. Now notice that when it displays my quote, it does not show triple quotes. It includes puts another single quote outside and it shows this internal single quote has been highlighted with the back slash. So, back slash single quote is python's way and many programming languages’ way of saying that the next character should not be treated as what it stands for, but as it is. So, just take the next single quote as a single quote, do not treat as the end of the quotation.
end, it is convenient to say take the last character. So, take the minus 1th character. So, we actually saw this and we did the gcd, we talked about the last element of the list say the list of common characters, and we said the minus 1th element n the list is the last element. This numbering scheme that we use for list informally in the gcd example without formally explaining, it is actually the same numbering scheme that is used for positions in the string. We have minus 1, minus 2, minus 3, minus 4, minus 5, so the important thing to remember is that going forward, you start at 0, and coming backward you start at minus 1, because obviously, minus 0 is same as 0. So, if we use minus 0 for the right most thing there would be terrible confusion as to whether we are talking about the first value or the last value. So, the forward position start from 0 from the beginning and the reverse position start from minus 1 from the last element. (Refer Slide Time: 10:37) Once we have this then we can see that we use this square bracket notation to extract individual positions. So, s 1, so that is the character at position 1 is an e and if I walk backwards then s minus 2 is an l.
(Refer Slide Time: 10:59) One of the most basic things one can do with strings is to put them together; to combine two values into a larger string and this is called concatenation; putting them one after the other. And the operator that is used for this is plus. So, plus, we saw for numeric values add them; for strings the same symbol plus does not add strings; obviously, it does not make sense to add strings, but it juxtaposes them, puts them one after the other. So, if we have a string hello as we did before, and we take this, and we take a new string and we add it to s. Then we get a string t, whose value is the part that was in hello plus the part that was added. So, plus is just the simple operator which takes two strings and sticks them side by side.
(Refer Slide Time: 12:47) We can get length of the string using the function len. So, len(s) returns the length of s. So, this is the number of characters. So, remember that if the number of characters is n then the positions are 0 to n minus 1. So, the length of the string s here would be 5, the length of the string t here would be 5 plus 7 – 12. There are many other interesting functions that one can use to manipulate strings, you can search and replace things, you can find the first occurrence of something and so on, and we will see some of these later on, when we get into strings and text processing and reading data from files in more details.
(Refer Slide Time: 13:26) A very common thing that we want to do with strings is to extract the part of a string. We might want to extract the beginning, the first word and things like that. The most simple way to do this in python is to take what is called a slice. Slice is a segment, a segment means I take a long string which I can think of as a list of character and I want the portion from some starting point to some ending point. (Refer Slide Time: 13:55)
everything from i onwards then I can say s i colon and this will go up to the position length of s minus 1, but if I write explicitly as a slice, I will only write length of s. So, essentially this is the main reason that python has this convention that whenever I write something like a range of 1 to m plus 1 then I have this extra plus 1 here. So, the main reason for this plus 1 here is to avoid having to write minus 1. If I had to include the last character and if I start numbering at 0, then every time I wanted to go to the end of the string I would have to say length of s minus 1. It is much more convenient to just say length of s, and implicitly assume that it knows that it should not go to length of s, but length of s minus 1. So, this whole confusion if you would like to call that in python about that fact that all ranges end one short of the right hand side of the range, stems from the fact that you very often want to run from something to the length of it in a list or a sequence or a string and when you say that you do not want have keep remembering to say minus 1. (Refer Slide Time: 16:45) Let us play with the second in the python interpreter. So, if I say s is equal to hello then we saw that if I do 1 to 4, I get 'ell'. If I say colon 3 then I get 'hel' that is 0 1 2. If I say 2 colon, I get 'llo' that is 2 3 4. What if I say 3 2 1, so this says: start at position 3 and go up to position 1 minus 1 which is 0. So, python does not give you an error, it takes all these
invalid ranges, anything where for example, the starting point to the ending point does not define a valid range, and it says this is the empty string. On the other hand, if I say something like go from 0 to 7, where there is no 7th position in the string, here python will not give an error instead, it will just go up to the last position which actually exists in the string below 7. So, in general these range values are treated in a sensible way, if you give values which do not make sense. As far as possible python tries to do something sensible with the slice definition. (Refer Slide Time: 17:57) Though we have access to individual positions or individual slices or sequences within a string, we cannot take a part of a string and change it as it stands. So, we cannot update a string in place. Suppose, we want to take our string “hello” and change it to the string “help!” it would be nice if we could take the third and the fourth position. So, remember 0, 1, 2, 3, 4, 5, so 0, 1, 2, 3, 4, so it would be nice if we could say make this into a p and make this into an exclamation mark, so that I could get help instead of hello.
Here we have the string hello defined in four, and if I now try to say s 3 is equal to p, then it says this does not support item assignment, which is what we are trying to say you cannot change parts of a string as it stands. (Refer Slide Time: 19:12) Instead of doing this, instead of trying to take a string and change the part of it as its stands what you need to do is actually construct a new string effectively using the notion of slices and concatenation. Here what we want to do is we want to take the first part of the string as it is. These are the first three characters, and then we want to change this to p exclamation mark. So, what we can say is update s by taking 0, 1, 2 which is slice 0 to 3 and concatenating it with the new string p exclamation mark. So, this is how you modify strings in python, but important thing is this is a new s we are not claiming that this s is same as old s. There we build a new string from the old string and perhaps store it back in the same name, it is partly like when we say j is equal to j plus 5, we are actually saying that we have created a new value for j and stored it back in j. Here again we are creating a new string and putting it back, but we are not modifying it. Now this distinction between modifying and creating a new value may not seem very
important at this moment, but it will become important as we go along. So, strings are what are called immutable values, you cannot change them without actually creating a fresh value; whereas, lists as we will see which are more general type of sequence can be changed in place you can take one part of a list and then replace it by something else. So, we will see more about this later, this is a fairly important concept. Remember for now that strings cannot be changed in place. (Refer Slide Time: 20:42) To summarize what we have seen is that text values are important for computation, and python has the types - string or str, which is a sequence of characters to denote text values. And there is no distinction for a separate type for a single character; there is no single character type in python, a single character is just a string of a length 1. We can extract individual characters by index positions, we can use slices to extract sub strings, and we can glue strings together using the concatenation operator plus, but strings are immutable. We cannot take a value assigned to a string name and update it in place. We can create a new value by manipulating it using slices and concatenation, but we cannot directly update it, because strings are immutable.
