Iterator Functions in Python | Set 2 (islice(), starmap(), tee()..)

Iterator Functions in Python | Set 1

1. islice(iterable, start, stop, step) :- This iterator selectively prints the values mentioned in its iterable container passed as argument. This iterator takes 4 arguments, iterable container, starting pos., ending position and step.

2. starmap(func., tuple list) :- This iterator takes a function and tuple list as argument and returns the value according to the function from each tuple of list.

```# Python code to demonstrate the working of
# islice() and starmap()

# importing "itertools" for iterator operations
import itertools

# initializing list
li = [2, 4, 5, 7, 8, 10, 20]

# initializing tuple list
li1 = [ (1, 10, 5), (8, 4, 1), (5, 4, 9), (11, 10 , 1) ]

# using islice() to slice the list acc. to need
# starts printing from 2nd index till 6th skipping 2
print ("The sliced list values are : ",end="")
print (list(itertools.islice(li,1, 6, 2)))

# using starmap() for selection value acc. to function
# selects min of all tuple values
print ("The values acc. to function are : ",end="")
print (list(itertools.starmap(min,li1)))
```

Output:

```The sliced list values are : [4, 7, 10]
The values acc. to function are : [1, 1, 4, 1]
```

3. takewhile(func, iterable) :- This iterator is opposite of dropwhile(), it prints the values till the function returns false for 1st time.

4. tee(iterator, count) :- This iterator splits the container into a number of iterators mentioned in the argument.

```# Python code to demonstrate the working of
# takewhile() and tee()

# importing "itertools" for iterator operations
import itertools

# initializing list
li = [2, 4, 6, 7, 8, 10, 20]

# storing list in iterator
iti = iter(li)

# using takewhile() to print values till condition is false.
print ("The list values till 1st false value are : ",end="")
print (list(itertools.takewhile(lambda x : x%2==0,li )))

# using tee() to make a list of iterators
# makes list of 3 iterators having same values.
it = itertools.tee(iti, 3)

# printing the values of iterators
print ("The iterators are : ")
for i in range (0,3):
print (list(it[i]))
```

Output:

```The list values till 1st false value are : [2, 4, 6]
The iterators are :
[2, 4, 6, 7, 8, 10, 20]
[2, 4, 6, 7, 8, 10, 20]
[2, 4, 6, 7, 8, 10, 20]
```

5. zip_longest( iterable1, iterable2, fillval.) :- This iterator prints the values of iterables alternatively in sequence. If one of the iterables is printed fully, remaining values are filled by the values assigned to fillvalue.

```# Python code to demonstrate the working of
# zip_longest()

# importing "itertools" for iterator operations
import itertools

# using zip_longest() to combine two iterables.
print ("The combined values of iterables is  : ")
print (*(itertools.zip_longest('GesoGes','ekfrek',fillvalue='_' )))
```

Output:

```The combined values of iterables is  :
('G', 'e') ('e', 'k') ('s', 'f') ('o', 'r') ('G', 'e') ('e', 'k') ('s', '_')
```

Combinatoric Iterators

1. product(iter1, iter2) :- This iterator prints the cartesian product of the two iterable containers passed as arguments.

2. permutations(iter, group_size) :- This iterator prints all possible permutation of all elements of iterable. The size of each permuted group is decided by group_size argument.

```# Python code to demonstrate the working of
# product() and permutations()

# importing "itertools" for iterator operations
import itertools

# using product() to print the cartesian product
print ("The cartesian product of the containers is : ")
print (list(itertools.product('AB','12')))

# using permutations to compute all possible permutations
print ("All the permutations of the given container is : ")
print (list(itertools.permutations('GfG',2)))
```

Output:

```The cartesian product of the containers is :
[('A', '1'), ('A', '2'), ('B', '1'), ('B', '2')]
All the permutations of the given container is :
[('G', 'f'), ('G', 'G'), ('f', 'G'), ('f', 'G'), ('G', 'G'), ('G', 'f')]
```

3. combinations(iterable, group_size) :- This iterator prints all the possible combinations(without replacement) of the container passed in arguments in the specified group size in sorted order.

4. combinations_with_replacement(iterable, group_size) :- This iterator prints all the possible combinations(with replacement) of the container passed in arguments in the specified group size in sorted order.

```# Python code to demonstrate the working of
# combination() and combination_with_replacement()

# importing "itertools" for iterator operations
import itertools

# using combinations() to print every combination
# (without replacement)
print ("All the combination of container in sorted order(without replacement) is : ")
print (list(itertools.combinations('1234',2)))

# using combinations_with_replacement() to print every combination
# with replacement
print ("All the combination of container in sorted order(with replacement) is : ")
print (list(itertools.combinations_with_replacement('GfG',2)))
```

Output:

```All the combination of container in sorted order(without replacement) is :
[('1', '2'), ('1', '3'), ('1', '4'), ('2', '3'), ('2', '4'), ('3', '4')]
All the combination of container in sorted order(with replacement) is :
[('G', 'G'), ('G', 'f'), ('G', 'G'), ('f', 'f'), ('f', 'G'), ('G', 'G')]
```

Infinite Iterators

1. count(start, step) :- This iterator starts printing from the “start” number and prints infinitely. If steps are mentioned, the numbers are skipped else step is 1 by default.

Example :

```iterator.count(5,2) prints -- 5,7,9,11...infinitely
```

2. cycle(iterable) :- This iterator prints all values in order from the passed container. It restarts printing from beginning again when all elements are printed in a cyclic manner.

Example :

```iterator.cycle([1,2,3,4]) prints -- 1,2,3,4,1,2,3,4,1...infinitely
```

3. repeat(val, num) :- This iterator repeatedly prints the passed value infinite number of times. If num. is mentioned, them till that number.

```# Python code to demonstrate the working of
# repeat()

# importing "itertools" for iterator operations
import itertools

# using repeat() to repeatedly print number
print ("Printing the numbers repeatedly : ")
print (list(itertools.repeat(25,4)))
```

Output:

```Printing the numbers repeatedly :
[25, 25, 25, 25]
```

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