CMPE150 - Fall2021

CMPE 150

Introduction to Computing

Fall 2021

### Lists * Created by placing elements inside square brackets [], separated by commas. * A list can have arbitrary number of items. * The items may be of different types (integer, float, string, etc.). ```python my_list = [] # empty list my_list = [1, 2, 3] # list of integers my_list = [1, "Hello", 3.4] # list with mixed data types ``` * A list can also have another list as an item. ```python # nested list my_list = ["mouse", [8, 4, 6], ['a']] ```

### List Index: To access elements * The index operator [] is used to access an item in a list. * Indices start at 0. So, a list having 5 elements will have an index from 0 to 4. * The index must be an integer. ```python my_list = ['p', 'r', 'o', 'b', 'e'] print(my_list[0]) # p print(my_list[2]) # o print(my_list[4]) # e # Nested List n_list = ["Happy", [2, 0, 1, 5]] print(n_list[0][1]) print(n_list[1][3]) # Error! Only integer can be used for indexing print(my_list[4.0]) ```

Negative indexing

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.

my_list = ['p','r','o','b','e']
print(my_list[-1]) # last item
print(my_list[-5]) # fifth last item

### List Slicing * A range of items can be accessed using the slicing operator : ```python my_list = ['p','r','o','g','r','a','m','i','z'] print(my_list[2:5]) # elements from index 2 to index 4 print(my_list[5:]) # elements from index 5 to end print(my_list[:]) # elements beginning to end ``` Output ``` ['o', 'g', 'r'] ['a', 'm', 'i', 'z'] ['p', 'r', 'o', 'g', 'r', 'a', 'm', 'i', 'z'] ```

### Add/Change Elements * Lists are mutable, meaning their elements can be changed. * Assignment operator = for changing an item or a range of items. ```python odd = [2, 4, 6, 8] odd[0] = 1 # change the 1st item print(odd) odd[1:4] = [3, 5, 7] # change 2nd to 4th items print(odd) ``` Output ``` [1, 4, 6, 8] [1, 3, 5, 7] ```

### Add/Change Elements We can also use `append()` and `extend()` methods ```python odd = [1, 3, 5] odd.append(7) print(odd) odd.extend([9, 11, 13]) print(odd) ``` Output ``` [1, 3, 5, 7] [1, 3, 5, 7, 9, 11, 13] ```

### Add/Change Elements * `+` operator can be used to combine two lists. This is called concatenation. * `*` operator repeats a list for the given number of times. ```python odd = [1, 3, 5] print(odd + [9, 7, 5]) print(["re"] * 3) ``` Output ``` [1, 3, 5, 9, 7, 5] ['re', 're', 're'] ```

### Add/Change Elements * It is possible to insert one item at a desired location by using the method `insert()` * Multiple items can be inserted by squeezing it into an empty slice of a list. ```python odd = [1, 9] odd.insert(1,3) print(odd) odd[2:2] = [5, 7] print(odd) ``` Output ``` [1, 3, 9] [1, 3, 5, 7, 9] ```

### Remove Elements * remove() to remove the given item * pop() to remove an item at the given index. ```python my_list = ['p','r','o','b','l','e','m'] my_list.remove('p') # Output: ['r', 'o', 'b', 'l', 'e', 'm'] print(my_list) # Output: 'o' print(my_list.pop(1)) # Output: ['r', 'b', 'l', 'e', 'm'] print(my_list) ```

### Useful Methods * index() returns the index of the first matched item * count() returns the count of the number of items passed as an argument * sort() sort items in a list in ascending order * reverse() reverse the order of items in the list

### Useful Methods ```python my_list = [3, 8, 1, 6, 8, 8, 4] # Add 'a' to the end my_list.append('a') # Output: [3, 8, 1, 6, 0, 8, 8, 4, 'a'] print(my_list) # Index of first occurrence of 8 print(my_list.index(8)) # Output: 1 # Count of 8 in the list print(my_list.count(8)) # Output: 3 ```

### List Comprehension * A list comprehension consists of an expression followed by for statement inside square brackets. ```python pow2 = [2 ** x for x in range(10)] print(pow2) ``` Output ``` [1, 2, 4, 8, 16, 32, 64, 128, 256, 512] ``` This code is equivalent to: ``` pow2 = [] for x in range(10): pow2.append(2 ** x) ```

### Membership Test * Test if an item exists in a list or not, using the keyword in. ```python my_list = ['p', 'r', 'o', 'b', 'l', 'e', 'm'] print('p' in my_list) # Output: True print('a' in my_list) # Output: False print('c' not in my_list) # Output: True ```

### Iterating through a list * We can iterate through each item in a list. ```python for fruit in ['apple','banana','mango']: print("I like",fruit) ``` Output ``` I like apple I like banana I like mango ```

### Tuples * Ordered collection of objects * Equivalent to Lists except: * Tuples are created by enclosing the elements in parentheses (()) instead of square brackets ([]). * Tuples are immutable, meaning that the elements cannot be modified. ```python t = ('foo', 'bar', 'baz', 'qux', 'quux', 'corge') print(t[0]) # indexing, Output = 'foo' print(t[1:4]) # slicing, Output = ('bar', 'baz', 'qux') ```

### Tuples are immutable ```python t = ('foo', 'bar', 'baz', 'qux', 'quux', 'corge') t[2] = 'Bark!' Traceback (most recent call last): File "", line 1, in t[2] = 'Bark!' TypeError: 'tuple' object does not support item assignment ```

### Packing and Unpacking ```python t = ('foo', 'bar', 'baz', 'qux', 'quux', 'corge') (s1, s2, s3, s4, s5, s6) = t print(s1) print(s2) print(s3) print(s4) print(s5) print(s6) ``` Note that the number of variables on the left must match the number of values in the tuple while unpacking.

CMPE 150

Introduction to Computing

Fall 2021

Negative indexing

References