Python – Iteration(for/while loops)

For Loop

for loop is used for iterating over a sequence which can be a list, tuple, dictionary, set, or a string. Iterating is also known as traversing. Loop continues until we reach the last item in the sequence. The body of for loop is separated from the rest of the code using indentation.

Syntax

for val in sequence:
    # do something with val

Example

# list
numbers = range(1,10, 2)

for number in numbers:
    print(number)

# string
message = "Hello World"

for character in message:
    print(f'char: {character}')

# tuple
""" id, person info """
person = tuple(('12345',{"name":"Folau","grade":"3.5"}))
for p in person:
    print(p)

# set
names = set(('Folau','Lisa','Mele'))

for name in names:
    print(name)

#dictionary
profile = dict(name="Folau",age=30,job="SWE")
for attr in profile:
    print(attr)

 

The for loop does not require an indexing variable to set beforehand.

With the break statement we can stop the loop before it has looped through all the items. The example below stops its execution when count is 3.

numbers = range(1,10, 2)

count = 0;
for number in numbers:
    print(number)
    if count == 3:
       break
    count++

With the continue statement we can stop the current iteration of the loop, and continue with the next. The example below shows that when number is 0 the program will throw an error but instead of terminating the program it should keep going til the end of the list.

numbers = range(1,10, 2)

for number in numbers:
    print(number)
    try:
      result = 12 / number
    except:
      continue

 

for loops cannot be empty, but if you for some reason have a for loop with no content, put in the pass statement to avoid getting an error.

numbers = range(1,10, 2)

for number in numbers:
    pass

else statement with for loop

We can use else statement with for loop to execute some code when the for loop is finished. It’s useful in logging or sending a notification when the processing of a sequence is successfully completed.

teams = ['Lakers','Jazz','Suns']
for team in teams:
    print(team)
else:
    print("done looping through teams")

Reverse iteration using reversed function

The for loop iterates through the sequence elements in the order of its occurrence. Sometimes we have to iterate through the elements in the reverse order. We can use reversed() function with the for loop to achieve this.

numbers = (1, 2, 3, 4, 5)

for num in reversed(numbers):
    print(num)

 

While Loop

while loop is used to repeat a block of code until the specified condition is False. It is used when we don’t know the number of times the code block has to execute. We should take proper care in writing while loop condition if the condition never returns False, the while loop will go into the infinite loop. Every object in Python has a boolean value. If the value is 0 or None, then the boolean value is False. Otherwise, the boolean value is True. We can define an object boolean value by implementing __bool__() function. We use the  reserved keyword  – while – to implement the while loop in Python. We can terminate the while loop using the break statement. We can use continue statement inside while loop to skip the code block execution. Python supports nested while loops.

Syntax

while condition:
    # while body
else:
    # else body

Example

count = 0
while count < 10:
    print(count)
    count+=1
else:
    print("done counting!")

Python for loop and while loops are enough to create any type of loops. We can use the break and continue statements to control the loop execution flow. You can also use the “else” block for logging successful execution of the loops.

 

Source code on Github




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