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Strings

Learn about string variables, concatenation, replication, indexing, the len function, ASCII codes, and the slice operator in Python.

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Strings

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  1. Strings The Basics

  2. 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”

  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. The slice operator • The “substring” operator in Python • See separate set of slides

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