Repetition Structures

A repetition structure causes a statement or set of statements to execute
repeatedly. Repetition structure, which is more commonly known as a loop
is used to repeat a set of statements.

Two broad categories of
loops are: condition-controlled and count-controlled.

A condition-controlled
loop uses a true/false condition to control the number of times that it repeats.

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

In Python the while
statement is used to write a condition-controlled loop, and the for statement is
used to write a count-controlled loop.

1. While loop:

• Loops are either infinite or conditional. Python while loop keeps reiterating a
  block of code defined inside it until the desired condition is met.
• The while loop contains a boolean expression and the code inside the loop is repeatedly
  executed as long as the boolean expression is true.
• The statements that are executed inside while can be a single line of code or a
  block of multiple statements.

Syntax:

while(expression):

    Statement(s)

Example:

i=1

while i<=6:

print("TTWRDC")

i=i+1

2. For loop:

Python for loop is
used for repeated execution of a group of statements for the desired number of
times. It iterates over the items of lists, tuples, strings, the dictionaries
and other iterable objects.

Syntax:

for var in sequence:

Statement(s)

Example 1:

numbers = [1, 2, 4, 6,
11, 20]

seq=0

for val in numbers:

seq=val*val

print(seq)

Example 2: Iterating
over a Tuple:

tuple = (2,3,5,7)

print ('These are the
first four prime numbers ')

#Iterating over the tuple

for a in tuple:

print
(a)

Example 3: Iterating
over a dictionary:

#creating a dictionary

college =
{"MPCs":"block1","BZC":"block2","BCom":"block3"}

#Iterating over the
dictionary to print keys

print ('Keys are:')

for keys in college:

print
(keys)

#Iterating over the
dictionary to print values

print ('Values are:')

for blocks in college.values():

print(blocks)

Example 4: Iterating
over a String:

#declare a string to
iterate over

college = 'TTWRDC'

#Iterating over the
string

for name in college:

print
(name)

3. Nested For loop:

When one Loop defined
within another Loop is called Nested Loops.

Syntax:

for val in sequence:

for
val in sequence:

statements

statements

Example:

for i in range(1,6):

for
j in range(0,i):

print(i,
end=" ")