List or Tuple Iteration in Python

 

List or Tuple Iterations:

We will travel on a entire list or tuple and extract each word. iteration takes one word from a list or tuple. It is used with a loop also. हम पूरी सूची या टपल पर यात्रा करेंगे और प्रत्येक शब्द को निकालेंगे। पुनरावृत्ति सूची या टपल से एक शब्द लेता है। इसका उपयोग लूप के साथ भी किया जाता है।

First of all, calculate the length of given list or tuple.

Example: -

I = [“Apple”, “Banana”,”Orange”]

T=len(i)

Print (t)

Output:

3

Iterate characters with length function

Example: -

I = [“Apple”,”Banana”,”Orange”]

T=len(i)

Print (t)

For a in range (t):

          Print (i[a])

Output:

Apple

Banana

Orange

Iterate reverse characters with length function

Example: -

i = [“Apple”,”Banana”,”Orange”]

i=i [-1::-1]

T=len(i)

Print (t)

For a in range (t):

          Print (i[a])

Output:

Orange

Banana

Apple

Example: -

i = [“Apple”,”Banana”,”Orange”]

T=len(i)

Print (t)

For a in range (t-1,-1,-1):

          Print (i[a])

Output:

Orange

Banana

Apple

Iterate characters without length function

Example: -

i = “Hello Friends”

For a in i:

          Print (a)

Output:

Apple

Banana

Orange

Iterate reverse characters without length function

Example: -

i = [“Apple”,”Banana”,”Orange”]

i=i [-1::-1]

For a in i:

          Print (a)

Output:

Orange

Banana

Apple

Iterators

An iterator is an object that contains a countable number of values.

An iterator is an object that can be iterated upon, meaning that you can traverse through all the values.

Technically, in Python, an iterator is an object which implements the iterator protocol, which consist of the methods __iter__() and __next__().

Iterator vs Iterable

Lists, tuples, dictionaries, and sets are all iterable objects. They are iterable containers which you can get an iterator from.

All these objects have a iter() method which is used to get an iterator:

Example

Return an iterator from a tuple, and print each value:

mytuple = ("apple", "banana", "cherry")
myit = iter(mytuple)

print(next(myit))
print(next(myit))
print(next(myit))
Even strings are iterable objects, and can return an iterator:

Example

Strings are also iterable objects, containing a sequence of characters:

mystr = "banana"
myit = iter(mystr)

print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
Looping Through an Iterator

We can also use a for loop to iterate through an iterable object:

Example

Iterate the values of a tuple:

mytuple = ("apple", "banana", "cherry")

for x in mytuple:
  print(x)

Example

Iterate the characters of a string:

mystr = "banana"

for x in mystr:
  print(x)

The for loop actually creates an iterator object and executes the next() method for each loop.

Create an Iterator

To create an object/class as an iterator you have to implement the methods __iter__() and __next__() to your object.

As you have learned in the Python Classes/Objects chapter, all classes have a function called __init__(), which allows you to do some initializing when the object is being created.

The __iter__() method acts similar, you can do operations (initializing etc.), but must always return the iterator object itself.

The __next__() method also allows you to do operations, and must return the next item in the sequence.

Example

Create an iterator that returns numbers, starting with 1, and each sequence will increase by one (returning 1,2,3,4,5 etc.):

class MyNumbers:
  def __iter__(self):
    self.a = 1
    return self

  def __next__(self):
    x = self.a
    self.a += 1
    return x

myclass = MyNumbers()
myiter = iter(myclass)

print(next(myiter))
print(next(myiter))
print(next(myiter))
print(next(myiter))
print(next(myiter))

StopIteration

The example above would continue forever if you had enough next() statements, or if it was used in a for loop.

To prevent the iteration to go on forever, we can use the StopIteration statement.

In the __next__() method, we can add a terminating condition to raise an error if the iteration is done a specified number of times:

Example

Stop after 20 iterations:

class MyNumbers:
  def __iter__(self):
    self.a = 1
    return self

  def __next__(self):
    if self.a <= 20:
      x = self.a
      self.a += 1
      return x
    else:
      raise StopIteration

myclass = MyNumbers()
myiter = iter(myclass)

for x in myiter:
  print(x)

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