Lists

CSCI 1012

Announcements

  • Homework 7 grace days
    • Today (17 Mar) - 1 grace day
    • Tomorrow (18 Mar) - 2 grace days
  • Homework 8 (lists) is due Sunday, 23 March 11:55 PM

In-Class Exercises

  • File names:
    • ex1.py
    • ex2.py
    • ex3.py
  • Turn in on the submit server before the end of lecture for credit
  • Turn in one .zip with all the .py files
    • Unlimited attempts during lecture
  • We’ll provide solutions – pay attention!

Common Errors

  • Incorrect file names:
    • check_even.py vs. checkEven.py
  • Incorrect function name
    • def check_even(x): vs. def check_evens(x):
  • Spacing errors
    • 1 2 3 vs. 1 2 3

Read your autograder output!

Today

  • Introducing lists
  • Similarities between lists and strings
  • Differences between lists and strings
  • List methods
  • Lists and functions

Warmup

Recall with strings:

  • Strings can be compared with < and >
    • Checks alphabetical order within case
    • 'a' is “less than” 'b'
    • 'b' is “less than” 'c'
    • 'b' is not less than 'C'
      • One character is uppercase, one is lowercase

Exercise 1

Write a function ex1 that takes a string and finds the “lowest” character, alphabetically, in the string, and returns that character:

  • ex1('hello') returns 'e'
  • ex1('cat') returns 'a'
  • ex1('pool') returns 'l'

Assume that every character in the string will be a lower case letter. Submit as ex1.py.

Lists

  • Strings are an ordered collection of characters
  • Lists are a general ordered collection
    • They can contain anything
L = [1, 'a', 2.0, False]
  • List elements can be individually accessed
L_final = L[len(L)-1]
print(L)
print(L_final)
[1, 'a', 2.0, False]
False

Indexing, Slicing

Similar to strings:

L = ["a", 1, "the", 2.0, True]
print(L[0])
print(L[2:4])
a
['the', 2.0]

Looping:

L = ["a", 1, "the", 2.0, True]
for i in range(len(L)):
  print(i, L[i])

In the visualizer

Lists Are “Mutable”

Strings can’t be changed in place:

s = 'fortify'
s[0] = 'm'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment


Lists can be changed in place:

S = ['f', 'o', 'r', 't', 'i', 'f', 'y']
S[0] = 'm'
print(S)
['m', 'o', 'r', 't', 'i', 'f', 'y']

List Methods

  • A method is a function that is ‘part of’ an object
L = [1, 3, 2]
print(L)
L.append(4)
print(L)
[1, 3, 2]
[1, 3, 2, 4]
  • .append changes the list
  • There was no reassignment with =

Methods vs. Functions

  • Functions are called on the list
  • Methods are “internal” to the list
L = [1, 3, 2, 4]
L.append("b")
print(L)
a = len(L)
print(a)
[1, 3, 2, 4, 'b']
5

“Building” Lists

s = "" # empty string
L = [] # empty list

We saw how to ‘build’ strings:

s = ""
for j in range(1, 8):
  if j % 3 == 0:
    s = s + "🐱"
  else:
    s = s + str(j)
  if j != 7:
    s = s + "_"
print(s)
1_2_🐱_4_5_🐱_7

“Building” Lists

We can ‘build’ lists, too:

A = []
j = 0
while len(A) < 10:
  A.append(j + 1)
  j = j + 2
print(A)
[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]

  • No reassignment to A

  • Lists: append with .append method

  • Strings: concatenate with +=

List Concatenation

Lists concatenate with + operator:

L = ["a", 2]
K = [3, "b"]
J = L + K
print(J)
J = J + [0]
print(J)
['a', 2, 3, 'b']
['a', 2, 3, 'b', 0]
  • Both operands must be lists!
L = [1, 2, 3]
L = L + 4
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate list (not "int") to list

Exercise 2

Write a function ex2 that takes one argument, a list of numbers, and returns the smallest number in the list. Don’t use built-in min, max, or sorting.

  • ex2([1, 2, 3]) returns 1
  • ex2([1, 0, 2, 0]) returns 0
  • ex2([5, 4, 3, -2, 1]) returns -2

Submit as ex2.py

Built-Ins

E = [1, 5, 4, -3, 2, 8]

print("min:", min(E))
print("max:", max(E))
print("sorted:", sorted(E))
print("3 in E?", 3 in E)
print("7 in E?", 7 in E)
print("E dot sort:", E.sort())
min: -3
max: 8
sorted: [-3, 1, 2, 4, 5, 8]
3 in E? False
7 in E? False
E dot sort: None
  • None 😐😐😐

Horrors Beyond Our Comprehension

def another(L):
  L = L + 4
  return L

K = 5
M = another(K)
print("M:", M)
print("K:", K)
M: 9
K: 5

Horrors Beyond Our Comprehension

def another(L):
  L = L + L[-1] + L[-1]
  return L

K = "strings are immutable"
M = another(K)
print("M:", M)
print("K:", K)
M: strings are immutableee
K: strings are immutable

Horrors Beyond Our Comprehension

def another(L):
  L.append(L[-1])
  return L

K = ["first", "second", "last"]
M = another(K)
print("M:", M)
print("K:", K)
M: ['first', 'second', 'last', 'last']
K: ['first', 'second', 'last', 'last']

What just happened?

Lists Are Fun

s = "trivial"
print("first:", s[3:7]) # slicing a string

B = [1, 2, 3]
print("second:", B[1:]) # slicing a list

C = ["house", "boat"]
print("third:", C[1][1:4]) # multiple indexing

D = [[1, 2, 3], [4, 5, 6], [7, 8, 9, [10, 11, 12]]] # nested lists
print("fourth:", D[2][3][1:][0])
first: vial
second [2, 3]
third: oat
fourth: 11

Exercise 3

Write a function ex3 that takes a list of numbers and returns a new list, identical to the old list, but with the smallest number in the original list appended to the end. Do not modify the original list.

Examples:

  • ex3([0, 1, 2]) returns [0, 1, 2, 0]
  • ex3([4, 1, 5, 1]) returns [4, 1, 5, 1, 1]
  • ex3([2, 2]) returns [2, 2, 2]

Submit as ex3.py