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

1. islice (iterable, start, stop, step) : — this iterator selectively prints the values ​​referenced in its iterable container passed as an argument. This iterator takes 4 arguments, an iterative container, start position, end position, and stride.

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

# Python code to demonstrate how it works
# islice () and starmap ()

 
# import & quot; itertools & quot; for iterator operations

import itertools

 
# initializing list

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

  
# initialize the list of tuples

li1 = [( 1 , 10 , 5 ), ( 8 , 4 , 1 ), ( 5 , 4 , 9 ), ( 11 , 10 , 1 )]

  

  
# using islice () to slice the list according to ... need
# starts printing from 2nd index to 6th skip 2

print ( "The sliced ​​list values ​​are:" , end = "")

print ( list (itertools.islice (li, 1 , 6 , 2 )))

 
# using starmap () to select a value acc. function
# selects the minimum 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 the opposite of drop while (), it prints the values ​​ until the function returns false the first time.

4. tee (iterator, count) : — This iterator splits the container into multiple iterators mentioned in the argument.

# Python code to demonstrate how it works
# takewhile () and tee ()

 
# import & quot; itertools & quot; for iterator operations

import itertools

 
# initializing list

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

  
# save the list in an iterator

iti = iter (li)

  
# using takewhile () to print values ​​until the 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 list iterators
# makes a list of 3 iterators that have the same value.

it = itertools.tee (iti, 3 )

 
# printing iterator values ​​

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 the iterables one at a time . If one of the iterations is printed completely, the remaining values ​​are filled with the values ​​assigned to fillvalue .

# Python code to demonstrate how it works
# zip_longest ()

 
# import & quot; itertools & quot; for iterator operations

import itertools

 
# using zip_longest () to combine the two iterations.

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`, `_`) 

Combinatorial 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 permutations of all elements of the iterable.  The size of each permutation group is determined by the group_size argument.

# Python- demo code
# product () and permutation ()

 
# import & quot; itertools & quot; for iterator operations

import itertools

 
# using product () to print a Cartesian product

print ( "The cartesian product of the containers is:" )

print ( list (itertools.product ( `AB` , ` 12` )))

 
# use 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 possible combinations (no 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 possible combinations (with replacement) of the container passed in arguments in the specified group size in sorted order.

# Python code to demonstrate how it works
# combination () and combination_with_replacement ()

 
# import & quot; itertools & quot; for iterator operations

import itertools

 
# use combinations () to print each combination
# (without replacements)

print ( "All the combination of container in sorted order (without replacement) is: " )

print ( list (itertools.combinations ( `1234` , 2 )))

 
# using combination_with_replacement () to print each 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 at "start" and prints indefinitely . If steps are mentioned, numbers are skipped, otherwise step is 1 by default.

Example:

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

2. loop (iterable) : — this iterator prints all values ​​in order from the passed container. It resumes printing from the beginning again when all items are cycled .

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 an infinite number of times. If a number is mentioned, they are up to that number.

# Python code to demonstrate how it works
# repeat ()

 
# import & quot; itertools & quot; for iterator operations

import itertools

 
# using repeat () to repeatedly print the 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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