Repetition Structure

Acknowledgement : the contents of this notebook are partially based on the book "Starting Out with Python (3rd Edition)".

A repetition structure causes a statement or set of statements to execute repeatedly.

In [3]:
# calculate an average of some numbers 

number1 = float(input('number 1 : '))
number2 = float(input('number 2 : '))
number3 = float(input('number 3 : '))
number4 = float(input('number 4 : '))
number5 = float(input('number 5 : ')) # we need to call input function 5 times!

average = (number1+number2+number3+number4+number5)/5

print('Average = ',average)
number 1 : 1
number 2 : 2
number 3 : 3
number 4 : 4
number 5 : 5
Average =  3.0
In [3]:
# calculate an average of some numbers using the repetition structure

num = int(input('How many number? '))

average = 0.0
for i in range(num):
    number = float(input('Number : ')) # here we only write this line only once!
    average = average + number
    
average = average / num
print('Average = ',average)
How many number? 5
Number : 1
Number : 2
Number : 3
Number : 4
Number : 5
Average =  3.0

The while Loop : A Condition-Controlled Loop

A condition-controlled loop causes a statement or set of statements to execute repeatedly as long as a condition is true.


while condition:
      statement
      statement
      etc.

In the code above, the block of statements inside the while loop is executed repeatedly until the condition becomes false. The while loop is a pretest loop.

In [8]:
# while-loop example

passwd = input('Password? ')

while passwd != 'password':
    print('Incorrect password. Please try again.')
    passwd = input('Password? ')

print('Congratulation! you entered a correct password.')
Password? 123456
Incorrect password. Please try again.
Password? abcdef
Incorrect password. Please try again.
Password? mnhgfdr
Incorrect password. Please try again.
Password? kjhljkhkjl
Incorrect password. Please try again.
Password? Password
Incorrect password. Please try again.
Password? password 
Incorrect password. Please try again.
Password? password
Congratulation! you entered a correct password.
In [3]:
# an infinite loop

while True:
    print('hahaha')

The for loop : A Count-Controlled Loop

A count-controlled loop iterates a specific number of times.


for var in [value1,value2,etc]:
      statement
      statement
      etc.

In the code above, the block of statements is executed repeatedly and the number of times it gets executed equals to length of the list [value1,value2,etc]. Moreover, at each iteration, the value of the corresponding element in the list will be assigned to the variable var.

In Python, a comma-separated sequence of data items that are enclosed in a set of brackets is called a list.

In [4]:
# for-loop example

for i in [1,2,3,4,5,6,7,8,9,10]:
    print(i)
1
2
3
4
5
6
7
8
9
10
In [5]:
# using the range function with the for loop (an iterable object contains a sequence of values
# that can be iterated over)

for i in range(15,0,-2):
    print(i)
15
13
11
9
7
5
3
1
In [7]:
# example : the total sum of numbers

start = int(input('start: '))
stop = int(input('stop: '))

# swap the values of start and stop if start is greater than stop
if start > stop:
    tmp = start
    start = stop
    stop = tmp

sum_value = 0
for i in range(start,stop+1):
    sum_value += i
    
print(sum_value)
start: 30
stop: 5
455
In [13]:
# the augmented assignment operators
number = 2
number /= 2 # number = number / 2
print(number)

number += 10 # number = number + 10
print(number)

number -= 5 # number = number - 5
print(number)

number *= 2 # number = number * 2
print(number)
1.0
11.0
6.0
12.0

Nested Loop

A nested loop refers to a structure of a loop inside another loop.

In [16]:
# print patterns

width = int(input('Width : '))
height = int(input('Height : '))

for h in range(height):
    for w in range(width):
        print('*',end='')
    print('')
Width : 20
Height : 10
********************
********************
********************
********************
********************
********************
********************
********************
********************
********************
In [18]:
width = int(input('Width : '))

for w in range(width):
    for i in range(w+1):
        print('*',end='')
    print('')
Width : 20
*
**
***
****
*****
******
*******
********
*********
**********
***********
************
*************
**************
***************
****************
*****************
******************
*******************
********************
In [ ]: