Loops in python

source: robogardenchina.com

In programming a loop is a structure that repeats a sequence of instructions until a specific condition is met. It is used to iterate over a sequence (list, tuple, string) or other iterable objects. Iterating over a sequence is called traversal.

For loop

for loops iterate over a collection of items and run a block of code with each element from the collection.

numbers_list = [1, 2, 3, 4, 5]
for i in numbers_list:
    print(i)

The above for loop iterates over a list named numbers_list. Each iteration sets the value of i to the next element of the list. So first it will be 1, then 2 etc. The output will be as follow:

for loop’s output

range is a function that returns a series of numbers under an iterable form-

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

gives the exact same result as the first for loop. 5 is not printed as the range here is the first five numbers starting from 0.

To iterate through a list you can use for :

animal_list = ['Cat', 'Dog', 'Cow', 'Fox', 'Tiger', 'Lion']
for animal in animal_list:
    print(animal)

iterate through a lsit

The same result can be obtained using range-

for animal in range(len(animal_list)):
    print(animal_list[animal])

The len() is a built-in function returns the number of items in an object.

If you want to loop through both the elements and the index for the elements, you can use Python’s enumerate function:

for index, item in enumerate(animal_list):
    print(index, ':', item)

enumerate will generate tuples, which are unpacked into index and item (the actual value from the list). The above loop will print

use of enumerate function

To iterate through a dictionary you can use for :

Considering the following dictionary and to iterate through its keys, you can use:

customer = {'Name': 'Anna', 'Mobile': '12345678', 'Age': 23, 'City': 'Dhaka'}
for key in customer:
    print(key)v

Output:

keys of a dictionary

This is equivalent to :

for key in customer.keys():
    print(key)

To iterate through its keys and values, use:

for key, value in customer.items():
    print(key, ' : ', value)

Output:

key value of a dictionary

While Loop

A while loop will cause the loop statements to be executed till the loop condition remains true . The following code will execute the loop statements a total of 4 times-

i = 0 
while i < 4:
    i = i + 1

while loops are useful for checking if some condition has been met. The following loop will continue to execute until myObject is ready.

myObject = anObject()
while myObject.IsNotReady():
     myObject.TrytoGetReady()

If the condition is always true the while loop will run forever (infinite loop), if it is not terminated by a break or return statement or an exception.

while True:
    print(10)

Python doesn’t have a do or a do-while loop (this will allow code to be executed once before the condition is tested). However, you can combine a while True with a break to achieve the same purpose.

a = 20
while True:
    a = a-2
    print(a)
    if a<10:
        break
print('Done.')

This will show:

If you want to loop over a list of tuples for example:

collection = [('a', 'b', 'c'), ('x', 'y', 'z'), ('1', '2', '3')]

Instead of doing this:

for item in collection:
    i1 = item[0]
    i2 = item[1]
    i3 = item[2]
    # logic

You can simply do-

for i1, i2, i3 in collection:
    # logic

This will also work for most types of iterables, not just tuples.

Break and Continue in Loops

When a break statement executes inside a loop, control flow “breaks” out of the loop immediately:

i = 0
while i < 7:
    print(i)
    if i == 4:
        print("Breaking from loop")
        break
    i += 1

The loop conditional will not be evaluated after the break statement is executed.

break statement

break statements can also be used inside for loops.

A continue statement will skip to the next iteration of the loop but continuing the loop. As with break, continue can only appear inside loops:

for i in (0, 1, 2, 3, 4, 5):
    if i == 2 or i == 4:
        continue
    print(i)

Output:

Note that 2 and 4 aren’t printed, this is because continue goes to the next iteration instead of continuing when i ==2 or i == 4 occurred.

You can use return statement as a break ( note that any code after return will not be executed either)-

def break_loop():
    for i in range(1, 5):
        if i == 2:
            return i
        print(i)
    return 5

break_loop()
Output: 1

Pass Statement

pass is a null statement for when a statement is required by Python syntax , but no action is required or desired by the programmer. This can be useful for code that is yet to be written.

for x in range(10):
    pass #we don't want to do anything right now , so we'll pass