a collection data type 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 ID: 636377
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Slide1
Strings
The BasicsSlide2
Strings
a collection data type
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”Slide3
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 valueSlide4
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 beSlide5
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
D
o 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
()
Slide6
The slice operator
The “substring” operator in Python
See separate set of slides