Day 8

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.

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

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


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


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


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


Range doesn’t make a list

The range doesn’t make a list. i.e., if you run this code the output will not be a sequence.

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

range(1, 20, 3)


In order to get the list. You have to pass this iterable in list function.

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

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