Python - Control Structures

Python - Control Structures
Lasith Niroshan
University of Colombo School of Computing
lasithniro.online@gmail.com
July 17, 2018
Lasith Niroshan (UCSC) python-III July 17, 2018 1 / 24

Overview
1 Introduction to Control Structure
2 Conditional Statements
the if statement
if-else statement
if-elif-else statement
3 Loops
while
break and continue
for
4 Usage of loops

Learning Outcomes
Brieﬂy describes control structures
Lists and brieﬂy describes the types of control structures
Uses control structures appropriately in programming
Applies nested control structures in programs

Introduction
Sometimes we want to execute statements,
only under certain conditions (if)
repeatedly: loops (for, while)
repeated part of the code in another part of the program (functions)

The if statement
For an if statement, a condition (e.g., x > 0) is evaluated to either True
(run the code) or False (dont run it, or move on to the else block).
Example (if.py)
if (x>0):
print("x is greater than zero")
print(’number checking program in over.’)
Note:
Conditions (e.g., x ¡ 4) can be put in parentheses, but do not need to
be.
0 and ”” (the empty string) evaluate to False.
A block consists of one or more statements

The if-else statement
The else part will execute, when if condition is failed.
Example (if else.py)
if (x>0):
else:
print("x is lower than 0)

The if-else-elif statement
If we have more than one conditions to check we have to use elif
command.
else + if = elif
Example (if elif.py)
if (x>0):
elif(x<0):
print("x is lower than zero")
else:
print("x must equal to 0")

Tests
x == y equals y
x < y is less than y
x > y is greater than y
x >= y is greater than or equal to y
x <= y is less than or equal to y
x! = y is not equal to y
x is y is the same object as y
x is not y is not the same object as y
x in y is a member of y
x not in y is not a member of y
Caution: = and == are diﬀerent.

Indentation
Spaces are important.
Indentation shows structure of code.

Exercise 1: if, else and Blocks
What are the values of a, b and c after executing the following piece of
code?
Example (while.py)
a = b = 2;c = False
if not c:
if b < a:
b += 5
a = b-1
elif a < b:
c = True
else:
if a+b < 4:
c = False
a = 11
b = 2.2
print(a, b, c)

Loops: while
A while loop does a similar condition check, but then runs a block of code
as long as that condition evaluates to True.
Example (while.py)
i = 1
while i < 10:
print (str(i) + " is my friend.")
i += 1
Run the block of indented code 10 times.
Each time, the variable i has a new value.

Output

Loops: break and continue Statements
Example (get text.py)
#!/usr/bin/python3
while expr_1:
if expr_2:
block_1
continue
block_2
if expr_3:
break
block_3
break : break out of the loop.
continue : continue with the next iteration.

Loops: Example
continue.py
for i in range(5):
if (i == 3):
continue
print(i)
break.py
for i in range(5):
if (i > 2):
break
print(i)

Loops: Exercise 2 - continue & break with while loop
What is the output after executing the following piece of code?
exercise2.py
#!/usr/bin/python3
i=0
while ( i < 5 ):
i = i + 1
if (i == 3):
continue
if (i > 4):
break
print(i)

Loops: for
We can use for loops when we know the limitations.
Example (for.py)
for i in range(0, 10):
print (str(i) + " is my friend.")
For now, you can imagine that range(0,5) creates a list:
[0, 1, 2, 3, 4]
range(start, end, step) : All arguments must be integers.
range(0,10,2) returns [0, 2, 4, 6, 8]
range(10,0,-2) returns [10, 8, 6, 4, 2]
range() does not actually return lists - if you want get lists (e.g. for
printing):
x = list(range(0,2)) print(x)

Loops: for
We can use for loops when we know the limitations.
Example (for.py)
weekdays = [’Mon’, ’Tues’, ’Fri’]
for day in weekdays:
print("Today is a..."+day)
Python executes the block of the loop once per item of the list.
The list item is assigned to the variable, here day .

Loops: Exercise 3 - nested for loop
What is the output of the following program?
exercise3.py
#!/usr/bin/python3
fruits = [”apple”, ”banana”, ”melon”]
for i in range(2, 6, 2):
for f in fruits:
print(str(i) + ” ” + f + ”s”)

Loops: Exercise 4 - Factorial
The factorial of a number n is deﬁned as follows: n! = Πn
k=1k
Example
0! = 1
1! = 1
2! = 2 ∗ 1 = 2
3! = 3 ∗ 2 ∗ 1 = 6
4! = 4 ∗ 3 ∗ 2 ∗ 1 = 24
...
Write a Python program that computes the factorial of 14.
The result should be 87178291200.

Use #1: iteration
As weve just seen, while loops can be used to iterate over a sequence.
This is most commonly done by iterating over integers, because
integers easily count how many times you do something.
You can change the way you iterate-e.g., i += 2 or i -= 1 or
whatever.

Use #2: until
Another, subtly diﬀerent use is to perform the same actions until a certain
condition is reached.
Example (get text.py)
#!/usr/bin/python3
user_input = ""
while len(user_input) < 10:
user_input = input("Please enter a long string: ")
print ("Thank you for entering a long enough string!")

A Common Mistake : inﬁnite loops
One common mistake when using while loops is to forget to iterate a
variable, such as i;
Example (inﬁnite loop.py)
i = 0
while (i < 10):
print i
# infinite loop b/c i is not iterated
If your code doesnt stop:
Kill it (Ctrl-C on Unix, Ctrl-Z on Windows)
Examine your while loop and make sure the value is changing as you
expect it to
In general, this is a good way to debug your code

Learning Outcomes
Brieﬂy describes control structures
Lists and brieﬂy describes the types of control structures
Uses control structures appropriately in programming
Applies nested control structures in programs

The End

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