7.5. List Slicing#

names=['Sahil','Sonia','Sourav','Deepak','Vishal']
age=[1,2,3]

7.5.1. Syntax#

  • names[starting index:ending index:step]

    • starting index means where to start from

      • is inclusive

      • if omitted,python assumes you want to start from begining

    • ending index

      • is exclusive if specified

        • if specified,python goes till this index-1

        • if not specified,python goes till this index(means till end)

      • if omitted,python assumes you want to go till end

    • step

      • Be default 1

        • 1 means go from left to right (forwards)

        • -1 means go from right to left (backwards)

        • 2 or n means steps,skipping -used to get elements at even/odd positions or every n’th position

  • INTUITION: Start/End at given position,going from left to right(or opp) and move till start/end/(last/begining if nothing given)

7.5.1.1. Get Indexes based on element#

names.index('Deepak')

3

7.5.2. Traversing from left to right#

names[:]
# or names[0:len(names)]
# or [ i for i in names]    
# Note 1:we generally prefer comprehension for iterations of lists
# Note 2:we generally don't use list comprehensions for strings

['Sahil', 'Sonia', 'Sourav', 'Deepak', 'Vishal']

7.5.3. Traversing from right to left#

names[::-1]
# or [i for i in names[::-1]]

['Vishal', 'Deepak', 'Sourav', 'Sonia', 'Sahil']

7.5.4. Getting First Element#

names[0]

'Sahil'

7.5.5. Getting Second element#

names[1]

'Sonia'

7.5.6. Getting first 3 elements#

names[0:3]

['Sahil', 'Sonia', 'Sourav']

7.5.7. Getting last element#

names[-1]

'Vishal'

7.5.8. Getting second last element#

names[-2]

'Deepak'

7.5.9. Getting last 3 elements#

names[-3:]

['Sourav', 'Deepak', 'Vishal']

7.5.10. Getting Last elements index#

len(names)-1

4

7.5.11. Give me elements based on index#

# eg,give me sonia,sourav,deepak
names=['Sahil','Sonia','Sourav','Deepak','Vishal']
names[names.index('Sonia'):names.index('Vishal')]  # last index is exclusive so I wrote visal

['Sonia', 'Sourav', 'Deepak']

7.5.12. Give me all even indexes#

names[::2]

['Sahil', 'Sourav', 'Vishal']

7.5.13. Give me all old indexes#

names[::3]

['Sahil', 'Deepak']

Give me the nth elements

  Input In [15]
    Give me the nth elements
         ^
SyntaxError: invalid syntax

names[::2] # 2 can be replaced by N

7.6. Summary#

#   0 1 2 3 4 5 6 7 8  (these are also indexes but positive) positive means from start
a= [1,2,3,4,5,6,7,8,9]
#  -9-8-7-6-5-4-3-2-1  (these are also indexes but negative) negative means from end

# ---

#INTUITION:
# Start/End at given position,going from left to right(or opp) and move till start/end/(last/begining if nothing given)

# ---

# Using positive indexes
a[2] #3
a[2:4] # start from 2,end at 4 and go left to right ,[3,4] since last index is exclusive
a[2:] # start from 2 and go left to right  ,[3,4,5,6,7,8,9]

# Using negative indexes
a[-2] # 8
a[-4:-2] # start from -4,go left to right(because step=1 by default) and end at -2 and  # [6,7]
a[-2:-4] # start from -2,go from left to right(because step=1 by default) and end at -4 and,not possible,so wont give anything
a[-2:] # start from -2,go from left to right(because step=1 by default) #[8,9]
a[:-2] # end at -2 going from left to right, [1, 2, 3, 4, 5, 6, 7],but 8 isn't included since stop index isn't included

# Using steps (means two columns will come into picture)
# if two colons aren't specified,means stepis one means go forward
a[::1] # start from begining, go from left to right ,go till end , [1, 2, 3, 4, 5, 6, 7, 8, 9]
a[::-1] # start from ending, go from right to left,go till end, [9, 8, 7, 6, 5, 4, 3, 2, 1]
a[1::-1] # start from 1st index,go from right to left and end at last ,[2, 1]
a[:-3:-1] # end at -3,go from right to left , [9,8]
a[-3::-1] # start at -3,go from right to left , [7, 6, 5, 4, 3, 2, 1]