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DAY 5

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

1. Boolean

Since we have seen that control structure depends on test. Test is nothing but a statement in pyton whose value is either True or False but not together. For this we have a data type in python to represent this, known as boolean data type.

1.1 Assigning Boolean Data

test1 = True
test2 = False
print(test1)
print(test2)

True
False

Many relational operation result in boolean in pyton. For example if we ask following questions?

test1 = 5 >= 0     # True
test2 = 5 < 0      # False
test3 = 6 == 3     # False
test4 = (6/2 == 3) # Ture, use bracket for better looks
print(test1)
print(test2)
print(test3)
print(test4)

True
False
False
True

We can also check some variable has a particular value or not using this idea. 

f_name = "Sandeep"
l_name = "Suman"
full_name = f_name + " " + l_name
test1 = (full_name == "Sandeep Suman") 
print(test1)

True

1.2 Boolean Operations

Boolean data has three main operation. Others are combination of them

  • not: It interchange the value of boolean between True and False.
  • or: It takes the logical or operation between two boolean statements.
  • and: It takes the logical and operation between two boolean statements.

The result can be summarized in a table as follows

A B not A not B A == B A =! B A or B A and B
T F F T F T T F
F T T F F T T F
T T F F T F T T
F F T T T F F F 
f_name = "Sandeep"
l_name = "Suman"
full_name = f_name + " " + l_name
test1 = (full_name == "Sandeep Suman") # True 
test2 = not test1                      # not True = False
test3 = (full_name == "Sandeep Suman") or (5 <= 0)  # True OR  False = True
test4 = (full_name == "Sandeep Suman") and (5 <= 0) # True AND False = False
print(test1)
print(test2)
print(test3)
print(test4)

True
False
True
False

2. Conditional Statements

Till now all the lines of a program run one by one. The control structure will enable us to run a line based on a test.

2.1 General Structure

The structure of general control struructure in python(or in any programming language) will look like the following:

if <test1>:
    <statement1>
elif <test2>:
    <statement2>
    .
    .
    .
else:
    <final statement>

The situation can graphically represented as the following diagram

General If Statement

If we look at another perspective we can think this as the following sequnce of tests

sequencial if statement

When the i-th test fails, then program check for (i+1)-th test and so on. If any test say k-th one will pass then the program will run the k-th statement will run and program will exit this control structure. If all the statement fails the program will run the final statement nested under else.

2.2 Implementation

Except the first if statement all other are optional. We can combine as many test as we want.

  • One Test: The smallest possible program with if statement can written as following

    number = 77
if number > 60:  # colon represent a block in python
    print("1st Devision") # each block need to be indented by 4 whitespace

    1st Devision

    The output is expected. But even if the number , we will not get any output because the code has no else statement.

    number = 55
if number > 60:
    print("1st Division")

  • If...else: We can combine else to catch the program when test fails as follows

    number = 55
if number > 60:
    print("1st Division") 
else:
    print("Not 1st Division")

    Not 1st Division

  • Elif: More than one test can be used using elif statement. 

    number = 37
if number >= 60:
    print("1st Division") 
elif number >= 45:
    print("2nd Division")
elif number >= 35:
    print("Third Division")
else:
    print("Fail")

    Third Division

    Ex: Change the number to get each type of output.

Ex: Write a program that will give the grade of a student based on marks. You can use any criteria for grade.

3. 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

3.1 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

3.2 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

3.2.1 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

Important

You can see that the code will run until the test is satisfied for the first time.

  • 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

3.3 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

Warning

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]

4. References

  1. W3School on If Statements
  2. W3School on While Loop
  3. W3School on For Loop