COMP 200 Resources

List Comprehensions

List comprehensions are a convenient way to declaratively define lists in terms of a process. List comprehensions are really nothing more than short-cuts to writing loops to define and fill lists with values.

The basic syntax for a list comprehension is

[ expression for expr in sequence]

Where

expression -- any Python expression that results in a value. In general, this expression would involve expr.
expr -- a variable name to indicate a single value from sequence.
sequence -- any Python expression the results in an iterable entity, e.g. list, tuple.

Examples:

# [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] can be made by
[x for x in range(10)]   

# [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] can be made by
[x**2 for x in range(10)]  

# [(-2, 0.25), (3, 27), (-1, -1.0), (-2, 0.25), (5, 3125)] can be made by
[(x, x**x) for x in [-2, 3, -1, -2, 5]]

For creating more complex lists, the general syntax for list comprehensions is

[ expression for expr in sequence1
             for expr2 in sequence2 ...
             for exprN in sequenceN
             if condition ]

Where

For every element in sequence1, every element in sequence2 is run and for every element in sequence2, every element in sequence3 is run, etc., etc..
expression is only calculated if condition is True.

Examples:

# [(0, -2), (0, -3), (1, -2), (1, -3), (2, -2), (2, -3), (3, -2), (3, -3), (4, -2), (4, -3)] can be made by
[(x,y) for x in range(5)  
    for y in [-2, -3]]   

# [(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)] can be made from
[(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]

COMP 200 Resources

List Comprehensions

References