Python Lists

The basics of working Python lists.

list1 = ['apple', 1]
list2 = ['banana', 2]
lists = [list1, list2]

print lists # [ ['apple', 1], ['banana', 2] ]
print lists[0] # ['apple', 1]
print lists[0][1] # 1

# Autofilling a list
list3 = 3 * ['cherry'] + ['danish']
print list3 # ['cherry', 'cherry', 'cherry', 'danish']


# Slicing
numbers = [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]
print numbers[3:11] # [3, 4, 5, 6, 7, 8, 9, 10] Note the first value is inclusive.
print numbers[9:] # [9, 10, 11, 12, 13, 14, 15, 16]
print numbers[:6] # [0, 1, 2, 3, 4, 5]
print numbers[12:20] # [12, 13, 14, 15, 16]
print numbers[-4:] # [13, 14, 15, 16] # Note again, the first index is inclusive.
print numbers[8:-3] # [8, 9, 10, 11, 12, 13]
print numbers[-7:-3] # [10, 11, 12, 13] 
print numbers[:] # Prints the entire sequence.
print numbers[1:11:2] # [1, 3, 5, 7, 9] Prints slice, incrementing by steps of two
print numbers[::3] # [0, 3, 6, 9, 12, 15]

# Concatenating sequences
list4 = list1 + list2 + list3
print list4 # ['apple', 1, 'banana', 2, 'cherry', 'cherry', 'cherry', 'danish']
# A list sequence cannot be concatenated with a string sequence
# list5 - list4 + "I am a string" # This would throw the error below.
'''
 list5 - list4 + "I am a string"
NameError: name 'list5' is not defined
'''

# Multiplying sequence
list6 = [None] * 5
print list6 # [None, None, None, None, None]

# Print the number of items in a sequence.
print len(numbers) # 17

# Print the smallest value.
print min(numbers) # 0
print max(numbers) # 16
print min(lists) # ['apple', 1]

# Checking for membership
print 'apple' in list4 # True
print 'Scandanavia' in list4 # False

# Check how many times an element appears in a sequence
print list3.count('cherry') # 3

# Get the index of an item
print list3.index('danish') # 3
print list3[3] # danish


aString = 'This is a string'
print 'i' in aString # True
print 'is' in aString # True in version 2.3+. Will throw an exception 
# in earlier versions.
# In works recursively as well
print 'apple' in list4 # True


# Convert a string to a list
aList = list(aString)
print aList 
# ['T', 'h', 'i', 's', ' ', 'i', 's', ' ', 'a', ' ', 's', 't', 'r', 'i', 'n', 'g']
# Join a list into a string
bString = ''.join(aList)
print bString # This is a string


# Updating lists
bList = ['a', 'b', 'c']
bList[2] = 3
print bList # ['a', 'b', 3]
del bList[1]
print bList # ['a', 3]

numbersB = numbers[:]
numbersB[10:] = ['a', 'b', 'c']
print numbersB # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 'a', 'b', 'c'
numbersB[5:10] = []
print numbersB # [0, 1, 2, 3, 4, 'a', 'b', 'c']
numbersB[5:5] = [5, 6, 7] 
print numbersB # [0, 1, 2, 3, 4, 5, 6, 7, 'a', 'b', 'c']
numbersB = numbers[:]
del numbersB[::3]
print numbersB # [1, 2, 4, 5, 7, 8, 10, 11, 13, 14, 16]

numbersB.append('appended')
print numbersB # [1, 2, 4, 5, 7, 8, 10, 11, 13, 14, 16, 'appended']
ext = ['a', 'b', 'c']
numbersB.extend(ext)
print numbersB # [1, 2, 4, 5, 7, 8, 10, 11, 13, 14, 16, 'appended', 'a', 'b', 'c']
numbersB = numbers[:]
del numbersB[5]
numbersB.insert(5,"five")
print numbersB # [0, 1, 2, 3, 4, 'five', 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]
print numbersB.pop() # 16
print numbersB.pop(10) # 10
print numbersB # [0, 1, 2, 3, 4, 'five', 6, 7, 8, 9, 11, 12, 13, 14, 15] 
# (10 and 16 are gone)

# Remove the first occurence
print list3 # ['cherry', 'cherry', 'cherry', 'danish']
list3.remove('cherry')
print list3 # ['cherry', 'cherry', 'danish']
list3.reverse()
print list3 # ['danish', 'cherry', 'cherry']
moreNumbers = [126, 8, 9832, 54]
print sorted(moreNumbers) # [8, 54, 126, 9832]
print moreNumbers # Original sequence order is unchanged.
moreNumbers.sort()
print moreNumbers # Original sequence has been altered. 

anotherList = ['run', 'whirlwind', 'sing', 'Alpha', 'aardvark', 9]
anotherList.sort()
print anotherList # [9, 'Alpha', 'aardvark', 'run', 'sing', 'whirlwind']
anotherList.sort(reverse=True) # Reverse order
print anotherList # ['whirlwind', 'sing', 'run', 'aardvark', 'Alpha', 9]
# Remove the 9 because our next sort will use the len() function. Integers don't 
# have a length, so this would cause an error if we left it in place.
anotherList.remove(9)
anotherList.sort(key=len) # Key can refer to any function
print anotherList # ['run', 'sing', 'Alpha', 'aardvark', 'whirlwind']

!!Categories
[[!python]], [[!data structures]]