Lists

Lists are one of the most important data types in Python. They are flexible and easy to use. Lists are sequences of objects. Each element of the list can be of any type, including another list.
Lists have all the common properties of sequences: they are ordered which means that each element can be referenced by an integer index. As for all ordered types in Python, the indices start at 0.

Lists are dynamically sized and they are mutable. They must be declared in some manner before they can be used.

• Empty list
  • L=[]
• List with specified elements (the values of the elements must have been assigned)
  • L=[L1,L2,L3,L4,L5]
• List with N items, each having value V (both N and V must have been assigned and N must be an integer)
  • L=[V]*N
• Return a new list from a built-in function
  • L=list(range(12)) This returns a list of integers 0,1,..11

List Elements

To access a particular element by its index, we enclose the index value in square brackets. If a list L has been defined, we can access its third element with

print(L[2])

Lists are mutable so individual elements can be changed.

L[2]=42

Exercise In your choice of JupyterLab or Spyder, type and run

myL=[1,2,3,5,6,7]
myL[1]=4
print(myL)

Sublists are often called slices.

subL=myL[1:3]

Always remember that the upper bound is excluded, so this slice is elements 1 and 2, which are the second and third elements.

This extracts elements from the third to the last.

subL=myL[2:]

This extracts the elements from the beginning to the third element (index number 2).

subL=myL[:3]

An increment can also be specified:

subL=myL[1:7:2]

This extracts elements 1, 3, and 5.

Changing Lists

• Initialize an empty list
  • L=[]
• Append an element to a list
  • L1.append("Graham")
• Extend a list with another list
  • L1.extend(["Michael","Terry"])

It is important to understand the difference between appending and extending. Appending adds the argument as the new last element exactly as it appears. It takes any type. Extending requires a list as its argument. It joins the two lists sequentially. It is equivalent to

• L=[1,2,3]+[4,5,6]

• Insert an element

  • L.insert(i,item)
  • This inserts item before element i. To add an item at the beginning of the list, use
    • L.insert(0,item)

Shortening lists:

• Delete an element by its index
  • del L[i]
• Delete the first occurrence of a particular element
  • L.remove(item)
  • The item must match exactly or an error occurs.
• Remove and return an element
  • item=L.pop(<i>)
  • The angle brackets indicate an optional argument and are not typed. If the argument is absent the last element is returned. If it is present that value is returned.
    • lastVal=L.pop()
    • A_val=L.pop(2)
  • Keep in mind that pop shortens the list.

Much more can be done with lists.

• Length (number of elements)
  • lenL=len(L)
• Sum of values (if all elements are the same type and sum is defined for them)
  • sumL=sum(L)
• Maximum or minimum value of the items (if they are the same type and max and min are defined)
  • max(L) min(L)
• Membership test (returns Boolean)
  • item in list
• Index of first time item occurs
  • myIndex=L.index(item)
• Number of times item occurs
  • numItem=L.count(item)
• Sort a list (when possible) in place (overwrites the original)
  • L.sort()
• Return a sorted list to a new list
  • Lsorted=sorted(L)
• Reverse the list in place (overwrites)
  • L.reverse()
• There is no direct function to reverse and return into another list, so we use this handy trick
  • Lreversed=L[::-1]
• In Python 3, reversed(L) returns an iterator and not a list, but you may use the list constructor to convert it.
  • Lreversed=list(reversed(L))

Copying lists:

A=[1,2,3,4]
B=A
print(A)
B[2]=9
print(A)

B is just an alias (a “nickname”) for A. If B changes so does A. This is true for all mutable types. Slicing notation creates a view that can make a copy if the entire list is included.

C=A[:]
C[1]=11
print(A)
print(C)

An alternative is to explicitly use the list constructor function:

D=list(A)

Exercise

Type

numList=list(range(10))

Print the length of the list. Change the fourth element to 11. Extend the list with L=[20,30,40]. Print the index of the item that has the value 9. Remove that item from the list. Print the current length of the list. Compute the average of the list elements. Sort the list and then reverse the sorted version.