List Comprehensions

Author: OmkarPathak

Programs/P50_ListComprehensions.py

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+# Author: OMKAR PATHAK
+# In this example we will see how to write list comprehensions to make our tasks easier
+
+# Python.org says:
+# List comprehensions provide a concise way to create lists.
+# Common applications are to make new lists where each element is
+# the result of some operations applied to each member of another sequence
+# or iterable, or to create a subsequence of those elements that satisfy a certain condition.
+
+numbers = []
+for i in range(10):
+    numbers.append(i)
+print(numbers)              # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+
+# Side Effect of above operation:It creates a variable(or overwrites) named 'x'
+# that still exists after the loop completes. To get rid of this Side Effect we use List comprehensions.
+
+# List comprehension:
+numbers = [i for i in range(10)]
+print(numbers)              # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+
+# Let us see few more examples
+squares = [i * i for i in range(10)]
+print(squares)              # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
+
+# This is same as:
+squares = []
+for i in range(10):
+    squares.append(i * i)
+
+# Some more:
+odds = [i for i in numbers if i % 2 != 0]
+print(odds)                 # [1, 3, 5, 7, 9]
+
+# This is same as:
+odds = []
+for i in numbers:
+    if i % 2 != 0:
+        odds.append(i)
+
+# We can also use functions in comprehensions
+def isSqaure(x):
+    import math
+    sqrt = int(math.sqrt(x))
+    return x == sqrt * sqrt
+
+squares = [x for x in range(100) if isSqaure(x) == True]
+print(squares)             # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
+
+# Some Complex comprehensions:
+pairs = [[x, x * x] for x in numbers]
+print(pairs)               # [[0, 0], [1, 1], [2, 4], [3, 9], [4, 16], [5, 25], [6, 36], [7, 49], [8, 64], [9, 81]]