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Friday, 3 April 2020

Learning Python Part-14: Python lists

Python lists are similar to arrays in programming languages like C, C++ or Java. Lists are a useful tool for preserving a sequence of data and further iterating over it. However, Python lists are more flexible compared to them. 

Python list:
  • Elements in a list do not require to be of the same type.  
  • It can be a mixture of elements like numbers, strings, other lists and so on. 
  • Allows duplicate members.
  • Lists can have sublists as elements. These sublists may contain more sublists as well
a = [1, 2.2, 'python’, [a,b,c]]
  • Lists can be updated while program is running, whereas in C programming language, the size of an array has to be fixed at compile time.
  • To declare a list, Items separated by commas are enclosed within brackets [ ].
a = [1, 2.2, 'python’, a, b, c, d]
  • List is an ordered sequence of items.

  • The elements can be accessed via indices using slicing operator i.e. []
  • Index starts form 0 in Python. The index must be an integer. We can't use float or other types, this will result into TypeError.

  • Python allows negative indexing for its sequences.
  • The index of -1 refers to the last item, -2 to the second last item and so on.

  • Lists are mutable, meaning, value of elements of a list can be altered. 
  • We can use assignment operator (=) to change an item or a range of items.

  • We can add one item to a list using append() method or add several items using extend() method.

  • We can also use + operator to combine two lists. This is also called concatenation.
a = [1, 3, 5]
print(a + [9, 7, 5])                 # Output: [1, 3, 5, 9, 7, 5]

  • We can insert one item at a desired location by using the method insert() or insert multiple items by squeezing it into an empty slice of a list.
a = [1, 9]
print(a)                                # Output: [1, 3, 9]

Python List Methods:
  • Methods that are available with list object in Python programming are listed below.
  • They are accessed as list.method().

List Comprehension: 
  • List comprehension is an elegant and concise way to create a new list from an existing list in Python.
  • List comprehension consists of an expression followed by for statement inside square brackets.
  • Here is an example to make a list with each item being increasing power of 2.

  • Above code is equivalent to
power_2 = []

for x in range(10):
      power_2.append(2 ** x)

  • A list comprehension can optionally contain more for/if statements as nested loops.
Other List Operations in Python:
  • List Membership Test: 
    • We can test if an item exists in a list or not, using the keyword in.

  • Iterating Through a List:
    • Using a for loop we can iterate though each item in a list.
for fruit in ['apple','banana','mango’]:
         print("I like",fruit)

Built-in functions with List:
  • reduce() 
Apply a particular function passed in argument to all elements in list, stores the intermediate result and only returns the final summation value
  • sum() 
Sums up the numbers in the list
  • ord() 
Returns an integer representing the Unicode code point of the given Unicode character
  • cmp() 
Compares lists. If first list is “greater” than second list, returns 1
  • max() 
return maximum element in given list
  • min() 
return minimum element in given list
  • all()
Returns true if all element are true or if list is empty
  • any() 
Returns true if any element of the list is true. if list is empty, returns false
  • len() 
Returns length of the list or size of the list
  • enumerate() 
Returns enumerate object of list
  • accumulate() 
Apply a particular function passed in its argument to all of the list elements returns a list containing the intermediate results
  • filter() 
Tests if each element of a list true or not
  • map() 
Returns a list of the results after applying the given function to each item of a given iterable.
  • lambda() 
This function can have any number of arguments but only one expression, which is evaluated and returned.

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