loops

1. introduction
2. while loops
3. for loops
4. for _ in range() loops
5. break and continue
6. nested loops

1. introduction

in programming, loops are used when you want to repeat a specific block of code.
- for loops, indentation matters in Python.
some examples include:
- finding the average.
- counting the number of times a specific value appears in a data structure.
- finding the minimum or maximum value.
two main types of loops in python include while and for loops. let’s briefly go over them.

2. while loops

executes while a a specific condition is True.
- once the condition is False, the loop ends/terminates.
the condition is checked before each loop iteration.
- if the condition is initially False, then the code block will not execute (AKA skipped over).
- if the condition never becomes False, then an infinite loop occurs.
  - this can be prevented by changing a variable within the loop body that is within the loop’s condition.
use a while loop when:
- the number of iterations required is not known beforehand.
- condition-based rather than numerical-based.
while loops also support an optional else block.
- executes once only if the loop’s condition becomes False and the loop terminates normally (without the use of a break statement).
sentinel value = value that causes a loop to end.
here are a few while loop examples:

# while loop
# if i never incremented, this loop would enter an infinite loop because
# i would never be greater than 5.
# this example closely emulates what a for loop does:
# - i is set to an initial value, 1 (start)
# - the while loop stops when i is greater than 5 AKA becomes equal to 6 (stop)
# - the while loop increments by 1 during each iteration (step)
i = 1
while i <= 5:
    print(i)
    i += 1 # i is incremented during each iteration.

# prints out numbers 1-5.

# while loop with else block
# since the while loop terminates normally, the code in the else block is executed.
j = 0
while j < 3:
    print(j)
    j += 1
else:
    print("no break statement was used.")

# prints out numbers 0-2 and "no break statement was used."

# since the while loop terminates using a break statement,
# the code in the else block is not executed.
k = 1
while k < 4:
    print(k)
    k += 1
    if k == 3:
        break
else:
    print("no break statement was used.")

# prints out numbers 1-2.

# in this case, "q" is the sentinel value because it causes the while loop to end.
sentinel_value = "q"
user_input = input("quit out of this while loop by entering 'q': ")

while user_input != sentinel_value:
    print("still in the loop! ")
    user_input = input("quit out of this while loop by entering 'q': ")

print("no longer in the loop")

# let input be:
# no
# q

# prints out:
# quit out of this while loop by entering 'q':
# still in the loop!
# quit out of this while loop by entering 'q':
# no longer in the loop

3. for loops

typically used to iterate over iterable objects, such as lists, tuples, strings, etc.
typically used in combination with the range() function, which returns an iterable object.
allows for processing elements in a container one by one.
- ex: appending a specific number of items to a list based on user input.
the loop automatically terminates when all items in the iterable have been processed.
two types of looping:
- looping by index
- looping by value
use a for loop when:
- you know the number of times you want to iterate through a code block.
- you want to iterate over the indices or values of some data structure.
typically preferred over while loops when possible so you don’t have to deal with infinite loops.
similar to while loops, for loops also have an optional else block.
- the else code block only executes if the loop terminates normally without encountering a break statement.
here are a few for loop examples:

print("simple for loop:")

# for loop
for i in range(5):
    print(i)

# prints out numbers 0-4.

print()
print("looping by index:")

# for loop using an iterable data structure (works for lists, tuples, strings, etc.)
a_list = [1, 2, 3, 4, 5]

# looping by index
for i in range(len(a_list)):
    print(i)

# prints out numbers 0-4.

print()
print("looping by index to access elements:")

# can use looping by index to to access list elements
for i in range(len(a_list)):
    print(a_list[i])

# prints out numbers 1-5.

print()
print("looping by value:")

# looping by value
# use this when you only need the values and not the indices of an iterable data structure.
for element in a_list:
    print(element)

# prints out numbers 1-5.

print()
print("looping by value using a list copy:")

# can iterate over a copy of a list using [:], which creates a shallow copy of the list.
# useful when you want to modify values in the list without the list size changing.
for element in a_list[:]:
    print(element)

# prints out numbers 1-5.

print()
print("looping by index and value using enumerate() function:")

# can use the enumerate() function to access both the index and its respective value.
for index, value in enumerate(a_list):
    print(str(index) + ", " + str(value))

# prints out numbers 1-5 and their corresponding index.

simple for loop:
0
1
2
3
4

looping by index:
0
1
2
3
4

looping by index to access elements:
1
2
3
4
5

looping by value:
1
2
3
4
5

looping by value using a list copy:
1
2
3
4
5

looping by index and value using enumerate() function:
0, 1
1, 2
2, 3
3, 4
4, 5

since dictionaries are unordered, key-value pairs are accessed rather than elements via index.
here are a few dictionary for loop examples:

print("iterating over keys:")
# new dictionary object
a_dict = {"a": 1, "b": 2, "c": 3}

# when a_dict alone is used as the iterable in a for loop,
# the keys are being iterated rather than the values.
for key in a_dict:
    print(key)

# prints out characters a-c.

print()
print("iterating over keys using keys() method:")

# alternatively, you can also use the keys() method,
# which returns an object containing the dictionary's keys.
# this is typically preferred over the previous example for readability purposes.
for key in a_dict.keys():
    print(key)

# prints out characters a-c.

print()
print("iterating over values using values() method:")

# if you want to iterate over the dictionary's values, you can use the values() method.
for value in a_dict.values():
    print(value)

# prints out numbers 1-3.

print()
print("iterating over keys and values using items() method:")

# if you want to have access to both the dictionary's keys and corresponding values,
# you can use the items() method.
for key, value in a_dict.items():
    print(key + ", " + str(value))

# prints out keys a-c and their corresponding values.

iterating over keys:
a
b
c

iterating over keys using keys() method:
a
b
c

iterating over values using values() method:
1
2
3

iterating over keys and values using items() method:
a, 1
b, 2
c, 3

4. for _ in range() loops

lets go into more detail about the range() function’s capabilities.
can take up to 3 arguments (in this order):
- start
  - defaults to 0 if not specified.
  - inclusive.
- stop
  - exclusive.
- step
  - defaults to 1 if not specified.
if one argument is specified, then:
- start = 0
- stop = given argument
- step = 1
if two arguments are specified, then:
- start = argument 1
- stop = argument 2
- step = 1
if three arguments are specified, then:
- start = argument 1
- stop = argument 2
- step = argument 3
here are a few for _ in range() examples:

print("for loop with range(4):")

# the stop value is always excluded
for i in range(4):
    print(i)

# prints out numbers 0-3.

print()
print("for loop with range(3, 7):")

for i in range(3, 7):
    print(i)

# prints out numbers 3-6.

print()
print("for loop with range(0, 5, 2):")

for i in range(0, 5, 2):
    print(i)

# prints out all even numbers from 0-4.

print()
print("for loop with range(5, 0, 2):")

# if the stop is less than or equal to the start and the step is positive,
# range() will return an empty sequence.
for i in range(5, 0, 2):
    print(i)

# won't print anything.

print("won't print anything.")
print()
print("for loop with range(5, 0, -1):")

# supports negative step
for i in range(5, 0, -1):
    print(i)

# prints out numbers 5-1 in descending order.

print()
print("for loop with range(0, 5, -1):")

# if the stop is greater than or equal to the start and the step is negative,
# range() will return an empty sequence.
for i in range(0, 5, -1):
    print(i)

# won't print anything.

print("won't print anything.")
print()
print("for loop with _ and range(6):")

# can optionally use _ instead of a variable if the value of range()
# during each iteration is not needed for your code to execute.
for _ in range(6):
    print("hi")

# prints out "hi" 6 times.

for loop with range(4):
0
1
2
3

for loop with range(3, 7):
3
4
5
6

for loop with range(0, 5, 2):
0
2
4

for loop with range(5, 0, 2):
won't print anything.

for loop with range(5, 0, -1):
5
4
3
2
1

for loop with range(0, 5, -1):
won't print anything.

for loop with _ and range(6):
hi
hi
hi
hi
hi
hi

5. break and continue

statements used to change the normal flow of execution within both for and while loops.
break statement immediately terminates the loop.
- program resumes at the statement following the loop.
- typically used when a specific condition is met within the loop.
continue statement skips the rest of the current iteration of the loop.
- program execution continues to the next iteration, reevaluating the loop’s condition (for while loops) or moving on to the sequence’s next item (for for loops).
- typically used when you want to skip parts of the loop’s code but want the loop to continue with following iterations.
here are a few loop examples using break and continue:

print("for loop with continue:")

# for loop with continue
for i in range(10):
    if i % 2 == 1:
        continue
    print(i)
print("printing even integers between 0-8 completed")

# prints out all even numbers from 0-8 and "printing even integers between 0-8 completed".

print()
print("for loop with break:")

# for loop with break
for i in range(1, 10):
    if i == 6:
        print("broken loop at:", i)
        break
    print(i)
print("printing integers 1-5 completed")

# prints out numbers 1-5, "broken loop at: 6", and "printing integers 1-5 completed".

print()
print("while loop with break:")

# while loop with break
count = 0
while count < 10:
    print(count)
    if count == 4:
        print("broken loop at count:", count)
        break
    count += 1
print("printing integers 0-4 completed")

# prints out numbers 0-4, "broken loop at count: 4", and "printing integers 0-4 completed".

print()
print("while loop with continue:")

# while loop with continue
number = 0
while number < 5:
    number += 1
    if number == 3:
        print("skipping number:", number)
        continue
    print(number)
print("printing integers 1-5 excluding 3 completed")

# prints out numbers 1-2, "skipping number: 3", numbers 4-5,
# and "printing integers 1-5 excluding 3 completed".

for loop with continue:
0
2
4
6
8
printing even integers between 0-8 completed

for loop with break:
1
2
3
4
5
broken loop at: 6
printing integers 1-5 completed

while loop with break:
0
1
2
3
4
broken loop at count: 4
printing integers 0-4 completed

while loop with continue:
1
2
skipping number: 3
4
5
printing integers 1-5 excluding 3 completed

6. nested loops

placing one loop inside of another loop.
the inner loop completes all of its iterations for every iteration of the outer loop.
the inner loop’s clause must be indented in relation to the outer loop.
- the inner loop’s statements must be further indented.
use nested loops when:
- iterating over multi-dimensional data structures (ex: nested lists).
- executing repeated action that involve multiple iterations at once.
here are a few nested loop examples:

print("nested for loops: ")

# nested for loops
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} * {j} = {i * j}")
    print("-" * 10) # separator

# prints all multiplication equations where the factors are numbers 1-3.

print()
print("nested while loops: ")

# nested while loops
rows = 3
i = 1
while i <= rows:
    j = 1
    while j <= i:
        print("*", end="")
        j += 1
    print() # new line after each row
    i += 1

# prints out a 3 by 3 right triangle using asterisks.

print()
print("iterating through a nested list: ")

# iterating through a nested list
condition = True
count = 0
nested_list = [["apple", "banana"], ["carrot", "date"], ["eggplant", "fig"]]

while condition:
    print(f"outer loop: {count}")
    for row in nested_list:
        for item in row:
            print(f" - {item}")
    count += 1
    if count >= 2:
        condition = False

# prints out all elements in the nested list twice,
# keeping track of the outer loop's variable value.

nested for loops: 
1 * 1 = 1
1 * 2 = 2
1 * 3 = 3
----------
2 * 1 = 2
2 * 2 = 4
2 * 3 = 6
----------
3 * 1 = 3
3 * 2 = 6
3 * 3 = 9
----------

nested while loops: 
*
**
***

iterating through a nested list: 
outer loop: 0
 - apple
 - banana
 - carrot
 - date
 - eggplant
 - fig
outer loop: 1
 - apple
 - banana
 - carrot
 - date
 - eggplant
 - fig

loops

Table of Contents

1. introduction

2. while loops

3. for loops

4. for _ in range() loops

5. break and continue

6. nested loops