Week 4.pptx computational thinking and programming

Computational Thinking with
Programming
Fall 2019
@csebennett
@cse_bennett

Control
Structures
• Control flow is the order that instructions are executed in a program.
• A control statement is a statement that determines the control flow of
a set of instructions.
• Types of Control:
• Sequential control: Instructions are executed in the order that
they are written
• Selection control: Selectively executes the instructions.
E.g. Decision Control
• Iterative control: Repeatedly executes the instructions. E.g. Loops.

Selection Control or Decisions
(It is a control statement providing selective execution of instructions)
• if statements
• if else statements
• elif statements
• nested if conditions
Decisions
in a
Python
program

If Statement
It is a selection control statement based on the
value of a given Boolean expression
if statement in python takes an expression with it.
If the expression results to True
• then the block of statements under it is executed.
If it results to False
• then the block is skipped and control transfers to the statements
after the block.
Remember to indent the statements in a block equally
Expression’s value can be True or False.
We may want to do something only when a certain condition is true.
Syntax:
if test expression:
statement(s)

Example: What will be the output ?
1.
2.
3.
4.
5.

Example: What will be the output ?
1.
2.
3.
4.
5.
Output: Smaller
Output: Finish
Output: yay
Output: 3 is a positive number.
This is always printed.
Output: This is also always printed.

if...else Statement
a.k.a. Two way decisions
What happens when the
condition is untrue or false?
We can mention that it in the
block after the else statement.
An ‘else’ statement comes
right after the block after ‘if’.

Example
Syntax:
if test expression:
Body of if
else:
Body of else

Header, Suite
and Indentation
• One unique aspect of Python is that the amount of
indentation of each program line is significant.

Nested if statements (multi-way selection)
You can put an if
statement in the
block under another
if statement.
This is to implement
further checks.

Example: What will be the output?

if...elif...else Statement
Replacement to
the else-if
statements
More than one
condition to
check
If condition 1 isn’t
True, condition 2
is checked.
If it isn’t true,
condition 3 is
checked.

if...elif...else Statement: Example Use
Syntax:
if test expression:
Body of if
elif test expression:
Body of elif
else:
Body of else

Example:
What will be
the output?

Single Statement Condition
Write a single statement under if

Exercise
Find out the Max of three numbers

MCQs
1. All if statements must contain either an
else or elif header.
a) TRUE
b) FALSE
2. Which of the following statements are
true regarding headers in Python?
a) Headers begin with a keyword and
end with a colon.
b) Headers always occur in pairs.
c) All headers of the same compound
statement must be indented the
same amount.
3. Which of the following statements is true?
a) Statements within a suite can be indented a
different amount.
b) Statements within a suite can be indented a
different amount as long as all headers in
the statement that it occurs in are indented
the same amount.
c) All headers must be indented the same
amount as all other headers in the same
statement, and all statements in a given
suite must be indented the same amount.
4. The elif header allows for,
a) Multi-way selection that cannot be
accomplished otherwise
b) Multi-way selection as a single if statement
c) The use of a “catch-all” case in multi-way
selection

MCQs: Answers
1. All if statements must contain either an
else or elif header.
a) TRUE
b) FALSE
2. Which of the following statements are
true regarding headers in Python?
a) Headers begin with a keyword and
end with a colon.
b) Headers always occur in pairs.
c) All headers of the same compound
statement must be indented the
same amount.
3. Which of the following statements is true?
a) Statements within a suite can be indented a
different amount.
b) Statements within a suite can be indented a
different amount as long as all headers in
the statement that it occurs in are indented
the same amount.
c) All headers must be indented the same
amount as all other headers in the same
statement, and all statements in a given
suite must be indented the same amount.
4. The elif header allows for,
a) Multi-way selection that cannot be
accomplished otherwise
b) Multi-way selection as a single if statement
c) The use of a “catch-all” case in multi-way
selection

Iterative Control (Loop)
• An iterative control statement is a control
statement that allows for the repeated
execution of a set of statements.
• Due to their repeated execution, iterative
control structures are commonly referred to as
“loops.
• Loop Statements:
• While
• For
• Nested loop

While Statement (indefinite loop)
• A while statement is an iterative control statement that repeatedly executes a set of
statements based on a provided Boolean expression (condition).

Example 1
• Find all even numbers from 0 to n. where, n is given by user.

Example 2
• Print all even numbers between n to m. m should be greater than n.

Example 3
Write a program to take numbers from the user until he enter 0 as
input. then print sum of all entered number.

Example 4
• Write an efficient program to determine sum of N natural numbers
where N is given by user.

For Loop
(definite loop)
• A for loop is used for iterating over a
sequence (that is either a list, a tuple,
a dictionary, a set, or a string). Can
execute a set of statements, once for
each item in a list, tuple, set etc.
• Example:
Syntax:
for iterating_var in sequence:
statements(s)
Loop to read
sequence

Sequences
• Sequence of character - 'QWERTYUIOPASDFGHJKL’
• Sequence of words - ['abc','def','efg','ijk’]
• Sequence of numbers - [1,2,3,4,5,6,7,8,9]
• Sequence of mix data – [‘Suvi’, 4, “LKG”, “Bennett University”, 98.5
Sequence of numbers can also be generated as:
• range(start, end, difference)
• range(3) = (0,1,2)
• range(1,5) = (1,2,3,4)
• range(3,9,2) = (3, 5, 7)
• range(9,2,-1) = (9,8,7,6,5,4,3)
• range(9,2,1) = []

For Loop: What will be the output?
1.
2.
3.
4.
5.

For Loop: Answers to Previous Questions
1 2 3 4 5

•Let one grain of wheat be placed on the first square
of a chessboard, two grains on the second square,
four grains on the third square, eight grains on the
fourth square, and so on until all square are filled in
chessboard. what will be the total weight in ton of
grains on whole 8×8 chessboard? If 15432 grains in
one kg and 907.18 kg in one ton then.
Problem
Exercise 2

Week 4.pptx computational thinking and programming

Nested Loop
• Loop inside a loop a is called nested loop.
• Example:

Nested Loop: What will be the output?
1.
2.
3.
4.
5.

Find all prime numbers between given two numbers

Find first 100 prime numbers start from 2.

Find number is Strong or not
If the sum of the factorial of the digits in a number is equal to the original number, the
number is a strong number.

Accept the limit and print the strong numbers from 1 to
the given limit.

Infinite loop
• An infinite loop is an iterative control structure
that never terminates (or eventually terminates
with a system error).
• Infinite loops are generally the result of
programming errors.
• For example: if the condition of a while loop can
never be false, an infinite loop will result when
executed.

Loop Control Statements
• Break Statement: Terminates the loop statement and transfers execution to the
statement immediately following the loop.
• Continue Statement: Causes the loop to skip the remainder of its body and
immediately retest its condition prior to reiterating.
• Pass Statement:The pass statement in Python is used when a statement is required
syntactically but you do not want any command or code to execute.
Use of pass in if:
a = 33
b = 200
if b > a:
pass
Example:
for letter in 'Python’:
if letter == 'h’:
pass
print('This is pass block’)
print('Current Letter :', letter)
print(“Loop Ended!“)
Output:
Current Letter : P
Current Letter : y
Current Letter : t
This is pass block
Current Letter : h
Current Letter : o
Current Letter : n
Loop Ended

Break and Continue: Examples
• Break Statement:
• Continue Statement:
Ex.1: fruits =
["apple", "banana", "cherry"]
for x in fruits:
print(x)
if x == "banana":
break
Output:
apple
banana
Ex.2: fruits =
for x in fruits:
if x == "banana":
break
print(x)
fruits =
for x in fruits:
if x == "banana":
continue
print(x)
Output:
apple
cherry

MCQs
1. A while loop continues to iterate until its
condition becomes false.
a) TRUE
b) FALSE
2. A while loop executes zero or more times.
a) TRUE
b) FALSE
3. All iteration can be achieved by a while loop.
a) TRUE
b) FALSE
4. An infinite loop is an iterative control
structures that,
a) Loops forever and must be forced to
terminate
b) Loops until the program terminates with a
system error
c) Both of the above
5. The terms definite loop and indefinite loop
are used to indicate whether,
a) A given loop executes at least once
b) The number of times that a loop is
executed can be determined before the
loop is executed.
6. A Boolean flag is,
a) A variable
b) Has the value True or False
c) Is used as a condition for control
statements
d) All of the above

MCQs: Answers
1. A while loop continues to iterate until its
condition becomes false.
a) TRUE
b) FALSE
2. A while loop executes zero or more times.
a) TRUE
b) FALSE
3. All iteration can be achieved by a while loop.
a) TRUE
b) FALSE
4. An infinite loop is an iterative control
structures that,
a) Loops forever and must be forced to
terminate
b) Loops until the program terminates with a
system error
5. The terms definite loop and indefinite loop
are used to indicate whether,
a) A given loop executes at least once
b) The number of times that a loop is
executed can be determined before
the loop is executed.
6. A Boolean flag is,
a) A variable
b) Has the value True or False
c) Is used as a condition for control
statements
d) All of the above

Week 4.pptx computational thinking and programming

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