List indexing, list slicing
The indexing and slice operation we saw with strings also work on lists.
list index, from left to right, index number starts from 0; from right to left(counting backward), index starts with negative number -1.
Access an element, similar to a string, Python uses square brackets to enclose the index number to access a item lst[i].
A list is mutable, means you can change its contents. We can assign new value to an element using list index:
lst[i] = new value
Example:
dates = ['Monday', 'Tuesday', 'Wed', 'Th', 'Fri', 'Sat', 'Sun']
# access to an element
print( dates[1] ) # Tuesday
print( dates[0] ) # Monday
# change the value of an element
dates[0] = 'First day'
print( dates[0] ) # First day
# the last item
print( dates[6] ) # Sun (the last item)
print( dates[-1] ) # Sun (also the last item)
print( dates[len(dates)-1] ) # Sun (the last item again)
In previous example, dates[6], dates[-1], and dates[ len(dates)-1 ] all points to the same item: the last element in the list.
Built-in function len()
Insert, similar to string,
len( )function reports on the size of an list.
Slice of a list
Remember range ( start, end, step) function? Lists 'understand' start, end, step too.
list[start:end:step] gives a list slice:
The first number is the list index that the slice starts, if omitted, default value is 0, means the slice starts from the beginning of the list;
The second number after : is the end index(not included in the slice), if omitted, the slice goes to the end of the list;
The last number is the step, default value is 1.
# list slice
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
print(numbers[8:]) # [9, 10, 11, 12]
print(numbers[:4]) # [1, 2, 3, 4]
print(numbers[1:6]) # [2,3,4,5,6]
print(numbers [: : 3]) # [1, 4, 7, 10] (every third number)
We can use list index to reverse a list:
# reverse list
num = [0,1,2,3,4,5,6]
numR = num[ : : -1] # reverse a list, from end to start, going backward
print (numR) # [6,5,4,3,2,1,0]
num2 = num[4:1:-1]
print(num2) # [4, 3, 2] (a piece of backward list