MEIN 50010: Python Strings

Transcription

1 : Python Strings Fabian Sievers Higgins Lab, Conway Institute University College Dublin Wednesday,

2 Lecture Basic string manipulation Converting between different variable types strings Command-line (sys module)

3 Strings and Characters Strings in Python can be viewed as lists of characters. We can apply many standard list operations and functions to strings >>>s[1] E >>>s[1:4] ELF >>>len(s) Indexing Slicing String length 8 Strings can be concatenated together using + operator >>>"Hello" + " " + "World" Hello World

4 Strings have associated functions to perform basic operations Syntax: <string>.<function>(arg1, arg2,...) String Modification: >>>s = "Hello World " Make Uppercase >>>s.upper() HELLO WORLD Make Lowercase >>>s.lower() hello world Remove trailing white-space >>>s.strip() Hello World These functions make a copy, don t change original >>>s Hello World

5 Find & Replace Strings have simple functions for finding and replacing characters or sub-strings >>>s = "Hello World" Find position of sub-string in longer string >>>s.find("world") 6 >>>s.find("good-bye") -1-1 returned if sub-string not found Count number of times sub-string occurs >>>s.count( o ) 2 Replace characters or sub-strings >>>s.replace( ll, zz ) Hezzo World replace() makes copy, does not change string

6 Splitting and Joining Use split() function to separate a string into multiple parts >>>names = "john;jane;mark" Split string on separator (e.g., ; ) >>>nl=names.split(";") >>>nl [ john, jane, mark ] Output is a list containing multiple String values Use the join() function to join together a list of strings >>>"/".join(nl) john/jane/mark

7 Converting between Types Mixing incompatible types is not permitted in Python >>>1 + 2 Traceback (most recent call last): File <stdin>, line 1, in <module> TypeError: unsupported operand type(s) for +: int and str Use conversion to change value between basic types To a string >>>str(27) 27 >>>str(0.45) 0.45 From a string >>>int( 27 ) 27 >>>float( 0.45 ) 0.45

8 Converting between Types Often use split() and type conversion when parsing data files >>>data = "0.19,1.3,4.5" Split original string into list of strings >>>parts = data.split(, ) >>>parts [ 0.19, 1.3, 4.5 ] Convert list of strings, create new list of numeric values >>>list(map(float, parts)) [0.19, 1.3, 4.5] >>>[float(i) for i in parts] [0.19, 1.3, 4.5] (# list comprehension) >>>values = [] >>>for p in parts:... values.append( float(p) ) >>>values [0.19, 1.3, 4.5]

9 String Basic "<format string">.format(var1, var2,..., varn) format() method allows variable substitution format() inserts formatted variables into place holders in format string Place holders in format string are {} >>> {} {}.format( one, two ) one two >>> {} {}.format(1,2) 1 2 You can change the order in which place holders are filled >>> {1} {0}.format( one, two ) two one

10 Padding By default variables only take up as many characters as needed One can align strings right (spaces part of 2nd string) >>> {}{:>10}.format( one, two ) one two (the signify invisible spaces) or align them right (spaces part of 1st string) >>> {:10}{}.format( one, two ) one two One can choose the padding character >>> {}{:X<10}.format( one, two ) onexxxxxxxtwo One can center a string >>> {:X^10}.format( four ) XXXfourXXX

11 Truncating Truncating is opposite of padding, cut down long strings or floats to specific number of characters Number behind. in format specifies precision of output >>> {:.7}.format( stewardesses ) steward Can truncate floating point numbers >>> {}.format( ) >>> {:1.4f}.format( ) Can zero-pad numbers >>> {:06.2f}.format( )

12 Arguments To get data into your Python script can Hard-code variable a = 3 Need to edit script every time a changes input() variable from keyboard a = input( input variable: ) Requires manual intervention, person sitting at keyboard Use the command line import sys a = int(sys.argv[1]) This is the way to go Most (Python) scripts not called by humans but by other scripts

13 sys.argv Call script in terminal & add command line arguments C:\Users\Fabian> pythagoras.py 3 4 Running script pythagoras.py in my Windows home directory with 2 command line arguments 3 and 4 Can also specify command line arguments in IDE Unfortunately, does not work with IDLE In script need to import sys sys module part of Python core Command line arrives in script as sys.argv sys.argv is a list of string objects [ pythagoras.py, 3, 4 ] First argument (sys.argv[0]) name of script You must know what to expect and cast Accessing command-line argument that does not exist causes IndexError Attempting to convert command-line argument to wrong format causes ValueError

14 Belfield string: Colm Ryan