10 advance list comprehensions

  • https://www.youtube.com/watch?v=twxE0dEp3qQ

  • General syntax of list comprehension: 

     [ f(x)  for  x  in  list 
         if cond1
         if cond2
	 ...
 ]

    Note: f(x) can be a conditional expression : 

      val1(x) if cond(x) else val2(x)     # The else clause is mandatory

  This is equivalent to C's:  cond(x) ? val1(x) : val2(x)

Simple list comprehension

  • Get a list of values: 

    values = []

for x in range(10):
    values.append(x)

  • Solution with list comprehension: 

    values = [ x for x in range(10) ]

    Output: 

    [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

    You can write a for-loop inside of a list.

DEMO: progs/append01.py + append01-c.py

Generalized list comprehension

  • Simplest function with list comprehension: 

    values = [ x for x in range(10) ]

  • In general, we can use any function of x: 

    values = [ f(x) for x in range(10) ]

    Example: 

    def square(x):
     return x**2

values = [ square(x) for x in range(10) ]

DEMO: progs/append01-c2.py

Comprehension condition

  • Get the list of even values: 

    evens = []

for x in range(10):
    isEven = x % 2 == 0
    if isEven:
        evens.append(x)

  • Solution with list comprehension: 

    evens = [ x for x in range(10) if (x%2 == 0) ]

    Output: 

    0, 2, 4, 6, 8]

DEMO: progs/append02.py + append02-c.py

Comprehension with multiple conditions

  • Filter a list of strings and output only the ones that start with "a" and end in "y": 

    input = ["any", "albany", "apple", "baby", ""]

output = []       # Initialize output

for x in input:
   if len(x) == 0:   # Protect illegal access of x[0] and x[-1]
      continue 
   if x[0] != "a":
      continue
   if x[-1] != "y":
      continue
   output.append(x)

  • Solution with list comprehension: 

    output = [x 
            for x in input  # You can use a list instead of a for-loop
            if len(x) != 0
            if x[0] == "a"
            if x[-1] == "y"
         ]

DEMO: progs/filter01.py + filter01-c.py

Nested list comprehension

  • Flatten a matrix (= list of lists) 

    matrix = [ [1, 2, 3], [4, 5, 6], [7, 8, 9] ]

out = []

for row in matrix:
    for num in row:
        out.append(num)

  • Solution with list comprehension: 

    out = [num
           for row in matrix      # No  ":" !!!
               for num in row
      ]

DEMO: progs/nested01.py + nested01-c.py

IF/ELSE inside a comprehension

  • Categorize a list of numbers as even or odd: 

    input = [1, 2, 3, 4, 6]

out = []

for x in input:
    if x % 2 == 0:
        out.append("even")
    else:
        out.append("odd")

  • Solution with list comprehension: 

    out = [ "even" if x % 2 == 0 else "odd"     # else-clause is required
               for x in input
      ]

DEMO: progs/if-else01.py + if-else01-c.py + if-else01-c2.py

IF/ELSE inside a comprehension

  • Add 1 to all odd numbers in a list 

    input = [1, 2, 3, 4, 6]

out = []

for x in input:
    if x % 2 == 0:
        out.append(x)
    else:
        out.append(x+1)    ### Add 1 when x is odd

  • Solution with list comprehension: 

    out = [ x if x % 2 == 0 else x+1     # else-clause is required
          for x in input
      ]

DEMO: progs/if-else02-c.py

Nested list comprehension

  • Build a 3-dimensional list with list comprehension: 

    out = []

for a in range(3):
    l1 = []
    for b in range(4):
        l2 = []
        for x in range(2):
            l2.append(x)       ## [0 1]
        l1.append(l2)          ## [ [0 1]  [0 1]  [0 1] ]
    out.append(l1)

  • Solution with list comprehension: 

    out = [ [ [ 
            x  for x in range(2)
          ]
          for b in range(4)         ### We can use anonimous index var...
        ]
        for a in range(3)
      ]

How to read a nested list comprehension

  • Nested list comprehension: 

    out = [ [ [ 
            x  for x in range(2)
          ]
          for b in range(4)         ### We can use anonimous index var...
        ]
        for a in range(3)
      ]

  • The most outer for-loop in the list comprehension is at the right: 

    out = [ [ [                       
            x  for x in range(2)  
          ]                       
          for b in range(4)       
        ]                         
        for a in range(3)  Outer for-loop
      ]

  • The left-hand side of the for-loop will get added to the list (and recurse inside)

Dictionary comprehension

  • Syntax of a dictionary comprehension: 

      {
     key: value    for  key, value  in  list
  }

  • Difference:

    • Use   { .... }   to enclose the for-loop

    • The elements before the for-loop is: 

        f(k,v): g(k,v)

    • The for-loop will use 2 indexes in a dictionary comprehension

Example of dictionary comprehension

  • Making a dictionary from a list of pairs: 

     List of pairs:

    pairs = [ ("a", 1),  ("b", 2),  ("c", 3) ]
              ^^^^^^^    ^^^^^^^    ^^^^^^^
               pair 1     pair 2     pair 3

  • How to create a dictionary using list comprehension: 

    pairs = [ ("a", 1),  ("b", 2),  ("c", 3) ]   # List of pairs

myDict  = { k: v   for  k,v in pairs }

myDict2 = { k+str(v): v*v   for  k,v in pairs }

DEMO: progs/list2Dict01.py

Set comprehension

  • Syntax of a set comprehension: 

      {
     value    for  value  in  list
  }

  • Difference:

    • Use   { .... }   to enclose the for-loop

    • The elements before the for-loop is: 

        f(value)

    • The for-loop will use 1 index in a set comprehension

Example of set comprehension

  • Making a set (no duplicates) from a list: 

     List of numbers:

    nums = [ 1, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4 ]    # List has duplicates

  • How to create a set using set comprehension: 

    nums = [ 1, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4 ]

mySet  = { x   for  x in nums }

DEMO: progs/list2Set01.py

Generators and lists

  • The expression: 

        f(x)  for  x  in  range(1000000)

    is a generator (that returns a value when needed)

  • The expression: 

        [ f(x)  for  x  in  range(1000000) ]

    is a list (object) (and all its values are stored)

DEMO: progs/genVSlist01.py + progs/genVSlist01b.py