1. Strings The Basics

2. Strings can refer to a string variable as one variable or as many different components (characters) string values are delimited by either single quotes or double quotes operators + and * (overloaded operators) + “concatenation” sticks two strings together to get a new string * “replication” repeats a string a given number of times to give a new string precedence is * above + order of concatenation matters! “a” + “b” is not the same as “b” + “a”

3. Indexing Also called subscripts A way to tell individual characters of a string apart notation like that used for lists [ ] with an integer (constant or variable) inside Strings are numbered from 0 on left end and increasing Strings are also numbered from -1 on right end and decreasing You can use an expression as an index also, like str[k + 1], as long as the expression has an integer value

4. The len function Note this is a function, not a method (do not call it with the dot notation) len operates on one argument, either a string or a list returns an integer result, tells how many characters are in the string or how many elements are in the list lowest return value is zero for the empty string Python claims “no upper limit on string length”, literally it depends on your environment – how much RAM you have, which OS you are running, on a typical PC today with Windows it is around 2 billion characters s[len(s)] would give an error! Remember that len tells you how many characters, not what the last subscript in the string would be

5. chr and ord functions Sometimes you need to work with the ASCII codes of individual characters chr(integer) will return the character corresponding to the ASCII code argument, example chr(65) will return “A” ord(char) will return the ASCII code (as an integer) of the single character that you send, example ord(“A”) will return 65 In general, use the characters instead of their ASCII values in coding, it will be much more obvious what you are doing Do NOT say “if ch >= 65 and ch <= 90” when you are checking for upper case it is MUCH better to say if ch >= “A” and ch <=“Z” or even if ch in ascii_uppercase or if ch.isupper()

6. Strings and the slice operator

7. The slice operator This is the “substring” method which most languages have Given a string, you can slice pieces of the string the syntax of the operator uses the square brackets s[5:7] means the part of the string s which starts at position 5 and includes everything up to but not including position 7, in other words, the characters at position 5 and at position 6 Omitting an argument before the colon means to start at the beginning of the string s[:3] means s[0], s[1] and s[2] Omitting an argument after the colon means to include the rest of the string s[4:] means s[4], s[5]… up to and including the end of the string

8. slice returns a new string Suppose s were “abcdef” and you wrote an expression like s[1:4] Its value is “bcd”, a new string (the original string is not changed at all) If the expression were on a line by itself, it does nothing! You need to use the expression IN some statement, like an assignment statement, an if statement, a while statement, a print statement print(s[1:4]) makes sense t = s[1:4] makes a copy of the 3 characters as a string and puts it into t s = s[1:4] this does the same as the statement above, but the previous value of s is also discarded if s[1:4] == “bcd”: would result in True

9. Be careful of your range str[i:i+1] is the same as saying str[i] It is possible to go out of range with slice – don’t go past the end of the string! You can leave off both the starting and ending points, as in s[:]. This produces a copy of the whole string.

10. Strings and Lists – the split method

11. The split method for strings This method is very powerful and is unique to Python It only applies to strings – remember this! The syntax is source.split(delimiter) or source.split() What does it do? it breaks the string into smaller pieces based on the delimiter string (if given) or using any whitespace character (if no delimiter given) It returns a list of strings

12. With no delimiters Suppose that str is “ abc \t def\t xyz “ str has lots of whitespace in it If you give the expression str.split() you get the result [“abc”, “def”, “xyz”] Note that the elements of the list do NOT have any whitespace in them at all Note that something like “ \t “ is used as one delimiter, not used individually (which would give you lots of empty strings in the list)

13. With delimiters Suppose str contained “abbabbadefbb” and you wrote the expression str.split(“bb”) You get the result [“a”,”a”,”adef”,””] Note that there is an empty string at the end of the list because the delimiter was found at the very end of the string (same thing happens if the delimiter is at the start of the string also) You are only allowed one string as a delimiter, it can be as long as you wish

14. Strings in Python String Traversal

15. Traversing a string Traversing just means to process every character in a string, usually from left end to right end Python allows for 2 ways to do this – both useful but not identical if all you need is the value of each character in the string for ch in stringvar: will work fine. ch is given the value of each character from left to right, and can be used in the for loop as you need The other way uses the location of the characters in the string for ct in range(len(stringvar)): makes the variable ct an integer which has all the values of the subscripts of the characters in the string. If it is important that you know where a character is, this is the form you use.

16. Strings in Python String Methods

17. String methods You do not have to include the string library to use these! Since strings are objects, you use the dot notation (like in graphics) Note that in the methods mentioned which return string results, they create a new string; they do NOT change the old one! upper(), lower() return a new string with the alphabetic characters in a string changed to the other case, they don’t change any other characters at all replace(old, new) will return a brand new string with the first pattern, “old”, replaced by the second pattern, “new”, wherever it is in the original string

18. Strings are immutable Strings are immutable – meaning they cannot be changed once they are created They can be discarded; the variable name can be reassigned to another different string But individual characters cannot be changed in a string If name = “Mary” you cannot say name[0]= “S” To change the first letter of the string you would have to do something like name = ‘S’ + name[1:] To make a string all upper case, you would say something like name = name.upper() - that is, upper returns a string and you assign that new string to the same variable, the old string is discarded

19. Strings and whitespace whitespace is a specific name for a set of characters, namely the blank or space character ‘ ‘, the tab character ‘\t’, and the newline character ‘\n’ They are very often used as delimiters (markers between) in various string methods (strip, split) str.strip() removes all leading and trailing whitespace characters from a string. It does NOT alter any whitespace characters in the middle of the string. Example: “ ab\tcd\n”.strip() will “ab\tcd” str.split() will use whitespace characters as delimiters to break the string into a list of strings

20. The find method, the index method, the count method and the in operator All of these methods and operators involve finding something in a string They could all be written with loops and if statements but it’s much easier to use prewritten functions in is the simplest – syntax a in b It returns a Bool if it finds the first string a, somewhere in the string b. It does not matter if there are multiple occurrences of the string a, does not matter where the string a is positioned in the string b. It does have to be an exact match, as far as case, spacing, etc.

21. The find method and index method find(target) – this is called with the dot notation on the source string, so a call like s.find(“me”) is looking in the string s for the string “me” It works from the left end of the string every time It returns the location of the first occurrence of the target string It returns -1 if the target string does not occur at all in the source The index method works the same as the find method, except that if the search fails, the index method causes an exception and the program crashes! Safety rule: use the “in” operator first, before using the index method

22. The count method The count method is similar to the find and index methods count(“m”) will return an integer from 0 up to length of the string It is a method so it’s called with the dot notation s.count(“m”) would return an integer that shows how many m’s appear in the string

23. Summary of location/membership the in operator gives you just True or False, tells you the target string is in the source string or not, NO location find and index return the location of the left-most occurrence of the target string in the source string (find returns -1 for failure, index crashes the program for failure) count returns the number of occurrences of the target string in the source string

24. Strings in Python Creating a string

25. There are many different ways to create a string The simplest way is to hard code it into your program mystring = “Hello” Remember individual characters of mystring cannot be changed afterward (strings are immutable) Many of the string methods return new strings newstring = mystring.upper() The typecast str will create a string from another data type mynumstring = str(num)

26. Getting a string from input When you read from the keyboard, the input function will always return a string myname = input(“What’s your name? “) When you learn about external files, one thing you do is read from them (see Chapter 10) The input from a file always comes in as a string or list of strings

27. Other ways to create a string You can build up a string one character at a time The pattern is very similar to an accumulator You initialize a variable to an empty string new_one = “” Then in a loop you concatenate the new character onto the variable new_one = new_one + ch