DAY 8: Loops¶

Today we will learn about control structure and loops. Control structure and loops are fundamental concept in programming. Hence they are implemented in every programming language. Actually these concepts make a programming language different from calculators.

1. Loops¶

Loops are used to run a block of code again and again based on some condition. There are two kinds of loops most programming language have.

• While Loop
• For Loops

1.1 While loop¶

In python while loop is used to run a block of code again and again till a test is true. A simple example of such code is the following.

i = 1

while i < 6: # code in this block run repeatedly
    print(i) 
    i += 1

   >>>1
      2
      3
      4
      5

1.2 For Loop¶

A for loop in python is used to iterate over a sequence (list ,tuple, string) or other iterable objects.

A simple for loop to print each element of the following

numbers = [6, 5, 3, 8, 4, 2, 5] # given list for iteration

for val in numbers: # this code block will run for 
    print(val)      # each element of iterable  

6
5
3
8
4
2
5

A common situation in which for loop is used to store the value of a computation for each element of a list. Suppose we want to find sum of each element of a list, then we can write the following code.

numbers = [6, 5, 3, 8, 4, 2, 5] # given iterable
sum = 0  # final value will be stored in sum

for val in numbers: # this code will run each element of list   
    sum = sum + val # we add each element in our sum variable

# after leaving for loop look at indentation
print (sum) # finally print the sum

33

1.2.1 Conditionally Controlling the loop¶

The for loop does not require any test to be performed in the code. But these are three important cases where for is used with a condition.

• break: It will terminates the loop based on a test.

  A common use case is suppose you want to search for some property of element of a list. Then we will break the loop once the condition is met. For example suppose we are looking for number 5 in the given list of numbers.

  numbers = [6, 5, 3, 8, 4, 2, 5] # given iterable
  
  for val in numbers: # this code will run each element of list   
      if val == 3:    # the for loop will run until this test is satisfied
          break
      print(val)
  

  6
  5
  

• continue: It is used to skip the rest of the code inside a loop for the current iteration only. Loop does not terminate but continues on with the next iteration.

A common situation is suppose we want to print odd integers only in a given list, then we can skip a code block for even integer as follows

numbers = [6, 5, 3, 8, 4, 2,5] # given iterable

for val in numbers:  # this code will run each element of list  
    if val % 2 == 0: # checks for even value and  
        continue     # leave the current iteration
    print(val)

5
3
5

• pass: It is a null statement. The statements results into no operation. when it is executed,nothing happens. It is useful as a placeholder,when a statement is required syntactically,but no codes need to be executed.

For example we can achieve the above task with the following code with pass.

numbers = [6, 5, 3, 8, 4, 2,5] # given iterable

for val in numbers:  # this code will run each element of list  
    if val % 2 == 0: # checks for even value and  
        pass         # leave the current iteration
    else:
        print(val)

5
3
5

1.3 Range Function¶

In loops, range is used to control how many times the loop will be repeated. It is used to generate a finite sequence of natural numbers based on three arguments. First and last argument is optional.

The range function looks like

range(start, stop, step)

• start states the integer value at which the sequence begins, if this is not included then start begins at 0

• stop is always required and is the integer that is counted up to but not included

• step sets how much to increase (or decrease in the case of negative numbers) the next iteration, if this is omitted then step defaults to 1

So the range function can be used in three different ways

1. Using Stop Only: We can use the range as only one argument as a positive natural number say n. It will make a iterable of n natural numbers starting with 0. For example

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

   0
   1
   2
   3
   4
   

2. Using start and stop: We can use range function with two arguments start and stop. It will create a iterable of stop - start natural numbers with start. For example

   for i in range(5, 10): # create iterable start with
       print(i)           # 5 of length 10 - 5 = 5
   

   5
   6
   7
   8
   9
   

3. Using start, stop and step: When all three arguments used, it generate a iterable that will began with natural number start to stop - 1 with a step. For example

   for i in range(1, 20, 3):
       print(i)
   

   1
   4
   7
   10
   13
   16
   19
   

sequence = range(1, 20, 3)
print(sequence)

range(1, 20, 3)

sequence = range(1, 20, 3)
list_sequence = list(sequence)
print(list_sequence)

[1, 4, 7, 10, 13, 16, 19]

2. References¶

1. W3School on While Loop
2. W3School on For Loop