Strings and Lists

what is a string

a sequence of characters

• some characters = special such as /n = new line

what are characters

• upper and lowercase letters

• numbers

• symbols

• spaces

what matters in strings…

order!

escaping characters

either use ““ and inside use ‘

OR

before apostrophe use \

what does \t do

tabs = useful for formatting

what to use for long strings

can split using \

• “I’m sorry to have kept you, “\

• “but I have been rather busy”

OR use triple quotations “““

len()

string length

• counts all characters, including spaces, new lines and tabs to get length

• EG

  • ni = “Ni!”

  • print(“Length of string is: “, len(ni))

  • Length of string is: 3

Style

PEP-8

readability counts

print(witches[9]) = d

for assigned string “witches", the character for the index of 9 (technically the 10th character because we start at 0) is d

for index in range(12, len(witches)):

indent print(witches[index])

vertically down it will print the string of witches from the character of index 12 to the end of the string/string length

for index in range(12, len(witches)-1, -1, -1):

indent print(witches[index])

flips the way that the code is written so that it is readable bottom to top

for index in range(0, len(witches), 2):

indent print(witches[index])

prints every second character (1, 3, 5…)

for index in range(len(witches)-1, -1, -3):

indent print(witches[index])

prints every third character in reverse order from the end to the beginning.

negative indexing

every string also has corresponding negative numbers, the last character of the string would be referred to as -1

what would the output of ‘John’ + ‘Cleese’ be?

JohnCleese

print(John, Terry, Graham, sep=’*’)

John Paul*Terry Cruise*Graham Maine

print(John, Terry, Graham, sep=’,’)

John Paul,Terry Cruise,Graham Maine

ends

if we don’t want a character printed at the end of each line we use…

end=

example of end=

pa = “pineapples are great”

for index in range(len(pa))

indent print(pa[index], end=’ ‘)

OUTPUT: p i n e a p p l e s a r e g r e a t

what does concatenation refer to

using + to combine strings

min(s)

max(s)

s.count(x)

smallest item of s

biggest item of s

total number of occurrences of x in s

ASKII table gives….

numbered order of all characters

print(menuitem2.index(‘spam))

returns the index of the first occurrence of 'spam' in the list 'menuitem2'.

s.upper()

s.lower()

s.strip()

s.replace(‘_’, ‘_’)

• converts characters to uppercase

• converts characters to lowercase

• returns a copy of the string with leading and trailing whitespace removed such as tabs, spaces, etc

• replaces instance of first thing with second thing

when to use lists

when you need to keep a collection of data

what can lists contain

numbers, strings, other lists, combinations

difference between lists and strings

lists can hold combinations of data, string cannot

benefits of lists

• flexible

• dynamic

• can change and delete anytime

• can access using an index

python[2] = ‘__’

updating index 2 in list called python to contain __

del python[5]

deletes index of 5 in list python

x = [1, 2, 3]

y = [4, 5, 6]

z = x + y

[1, 2, 3, 4, 5, 6]

if you have a series of lists you can…

print(menu[4])

print(menu[4][2])

• prints list 4

• prints element 2 of list 4

ingredients = menuitem2.split(',’)

print(ingredients)

splits all of menuitem2 with commas into a list of ingredients.

slicing

you can slice strings and lists to access their parts

name = “John Cleese”

name[5:10]

returns 'Clee', a substring from the original string. REMEMBER the 10th index is not counted it is UP TO BUT NOT INCLUDING 10

name = “John Cleese”

name[:4]

returns 'John', the substring from the beginning of the string up to but not including index 4.

name = “John Cleese”

name[4:]

returns ' Cleese', the substring from index 4 to the end of the string.

name = “John Cleese”

name[:-2]

returns 'John Clee' = doesn’t include -2 which would be s

import random

for i in range(5):

• print(random.random(), end=” “)

print()

prints 5 random numbers which are separated by spaces which are between 0 and 1

print(random.randrange(0,101,5)

returns values between 0 and 100 which are divisible by 5

WHAT IS HAPPENING?:

sides = [“heads”, “tails”]

outcomes = [0, 0]

for i in range (10000):

• toss = random.choice(sides)

• if toss == “heads”:

  • outcomes[0] += 1

• else:

  • outcomes[1] += 1

• list called sides contains a string called heads and a string called tails

• the outcomes are defined as both being 0

• for 10 000 tries

• make a random choice of sides, in which there are 2 elements, 0 which is heads and 1 which is tails

• if the choice is “heads”

• the element 0, heads, has its total increase by 1

• if this is not the case

• element 1, tails, has its total increase by 1

NOTE: this continues to run for 10 000 tries

Pseudo random number generator (PRNG)

long peruod

uniformly distributed

reproducible

quick and easy to compute

Linear Congruential Generator (LCG)

short period

not uniformly distributed

reproducible

poor quality

Monte Carlo techniques

• model systems by simulating via random sampling

• powerful, flexible, direct

• sometimes the only feasible way to solve something

how to add an element into a list

list.append()

how would you make a string uppercase and then duplicate it?

mystring = “abcd”

mystring.upper()*2

OUTPUT = ABCDABCD