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Learning Objectives

  • While loop
  • For loop
  • Enumerate
  • Range

Loops

  • A loop is a sequence of instructions continuously repeated until a specific condition is reached.
  • Think about a teacher taking attendance. The teacher calls out a name, and the student responds present.
  • The teacher is going through the list of students one by one and calling a name to get "present" if the student is present or "<<silence>>" if they are absent.
  • This process will only end when the whole list of students is completed. This will be the condition that will break out of the loop.
  • Let's look at the types of loops in Python!

For Loop

  • "For loop" is used for iterating over a sequence (that can be any data structure - list/tuples or even a string)
  • Iteration means performing an action repeatedly
  • Syntax:
1
2
for variable in sequence:
   expression
  • Which means "for each variable in sequence, execute the expression"
  • Python uses indentation as its method of grouping statements. So all the statements having the same indentation will be considered inside the for loop.

Looping through a List

  • Example:
    Let's say we wish to store the heights of our family members in a list and print them one by one.

    image.png

  • Internal Working:

    • First, we store all the heights in a list named fam_heights
    • Now, we'll go to each element and print it
    • This action will continue until all the list elements are printed in order.

Looping through a String

  • Even strings are iterable objects; they contain a sequence of characters:

image.png

  • As you can see, each string character is printed in a separate line.
  • We can even apply string methods in the for loop.

image.png

Enumerate

  • With the for loop, you could print the heights of your family members.
  • But what if you also want to access the index of each element of the list? Here is where the enumerate function comes into play.
  • The enumerate function iterates over the elements of a list and associates an index with them. You need to use two variables (index and height in this case) to store the values given by enumerate.

image.png

While Loop

  • With the while loop, we can execute a set of statements repeatedly as long as a condition is true.
  • Syntax:
1
2
while condition:
  statement(s)

  • All the statements indented by the same number of character spaces after a while condition are considered part of a single block of code. Python uses indentation as its method of grouping statements.

  • For example:

image.png

  • Let us understand what is happening in the above program.
    • Value of x is assigned as 1
    • The while loop starts with the condition that x must be less than 4
    • The subsequent two statements have the same indentation and will be considered a part of the while loop
    • The value of x is printed. Initially, x=1
    • The value of x is then increased by 1. So now, x=2
    • Control goes back to the condition line; x is less than 4 (2<4). This means that the following statements will be executed again
    • This continues until x is assigned a value of 4. Then, the condition fails as x is not less than 4
    • The next two statements will not be executed, and the while loop will end.

range()

  • range() allows the user to generate a series of numbers within a given range.
  • The user can decide where that series of numbers will begin and end and how big the difference will be between one number and the next.
  • range() takes mainly three arguments:
    • A start argument is a starting number of the sequence. i.e., lower limit. By default, it starts with 0 if not specified.
    • A stop argument is an upper limit. i.e., generate numbers up to this number. The range() doesn't include this number in the result.
    • The step is a difference between each number in the result. The default value of the step is one if not specified.
  • range() only works with the integers. You can not use float number or any other type in a start, stop and step argument of a range().

You can call the range function in three ways:

  • range(stop) takes one argument.
  • range(start, stop) takes two arguments.
  • range(start, stop, step) takes three arguments.

Example 1: Using only one argument

image.png

  • Output:
    0, 1, 2, 3, 4,
  • By default, print statement prints in different lines. You can use the end argument if you want to print the output in the same line. ',' specifies that a comma will separate each output.
  • Only a stop argument is passed to range(). So by default, it takes start = 0 and step = 1.

Example 2: Using two arguments (i.e., start and stop)

image.png

  • Output:
    5, 6, 7, 8, 9,
  • By default, it took the step value as 1.

Example 3: Using all three arguments

image.png

  • Output:
    Printing All even numbers between 2 and 10 using range()
    2, 4, 6, 8,
  • All three arguments are specified i.e., start = 2, stop = 10, step = 2. The step value is two, so the difference between each number is 2.

Additional Reading

Let's Practice!

  1. Print the First ten natural numbers using a while loop.
  2. Iterate over the following list and print the elements:
    list1 = [12, 15, 32, 42, 55, 75, 122, 132, 150, 180, 200]
  3. Accept a number n from the user and print its multiplication table
  4. Use the enumerate function to print the elements of this list along with the indices:
    grocery = ['bread', 'milk', 'butter']
  5. Take a number n from the user and find the sum of all numbers between 1 to n.
  6. Create a sequence of numbers from 3 to 5, and print each item in the sequence.
  7. Create a sequence of numbers from 3 to 19, but increment by 2 instead of 1.
  8. Print the letters of the string "Python" in the same line:
    • Using a simple for loop
    • Using the range function

The above questions are for self-practice and ungraded; you don't need to upload them.

You can download the slides for this topic from here.