Skip to content

Latest commit

 

History

History
424 lines (282 loc) · 6.88 KB

forloop.md

File metadata and controls

424 lines (282 loc) · 6.88 KB

Python for loop

A loop is a sequence of instructions that is continually repeated until
a certain condition is reached. For instance, we have a collection of items
and we create a loop to go through all elements of the collection.
Loops in Python can be created with for or while statements.

The for statement

Python for statement iterates over the items of any sequence (such as a list or a string), in the order
that they appear in the sequence.

for var in sequence:
   do_statement(s)

The above is the general syntax of the Python for statement.

Loop with string

The following example uses Python for statement to go through a string.

#!/usr/bin/python

word = "cloud"

for e in word:
    
    print(e)

We have a string defined. With the for loop, we print the letters of the word
one by one to the terminal.

$ ./for_loop_string.py 
c
l
o
u
d

Syntax sugar

The for loop is a syntax sugar for the following while loop:

vals = [1, 2, 3, 4, 5, 6]


it = iter(vals)

while True:
    try:
        e = next(it)
    except StopIteration:
        break
    else:
        print(e)

The code is equivalent to:

for e in vals:
  print(e)

Loop & else

The for loop has an optional else statement which is executed when the looping has finished.

#!/usr/bin/python

words = ["cup", "star", "monkey", "bottle", "paper", "door"]

for word in words:
    
    print(word)
else:

    print("Finished looping")

We go over the list of words with a for loop. When the iteration is over, we print the Finished looping
message which is located in the body following the else keyword.

$ ./for_loop_else.py 
cup
star
monkey
bottle
paper
door
Finished looping

Loop with range

Python range function generates a sequence of numbers.

range(n)

The function generates numbers 0...n-1.

range(start, stop, [step])

The function generates a sequence of numbers; it begins with start and ends with stop, which is not
included in the sequence. The step is the increment and defaults to 1 if not provided.

With the help of the range function, we can repeat a code block n times.

#!/usr/bin/python

for i in range(1, 6):
    
    print(f"Statement executed {i}")

The code example executes the code block five times.

$ ./repeating_statement.py 
Statement executed 1
Statement executed 2
Statement executed 3
Statement executed 4
Statement executed 5

In the next example we generate two sequences of integers with for loop.

#!/usr/bin/python

for n in range(1, 11):
    print(n, end=' ')
    
print()

for n in range(0, 11, 2):
    
    print(n, end=' ')    
    
print()

The example prints two sequences of integers: 1, 2, ...10 and 0, 2, ...10.

$ ./for_loop_range.py 
1 2 3 4 5 6 7 8 9 10 
0 2 4 6 8 10

Loop over a tuple and list

With Python for loop, we can easily traverse Python tuples and lists.

#!/usr/bin/python

nums = (1, 2, 3, 4, 5, 6)   
words = ["cup", "star", "monkey", "bottle"]

for n in nums:
    
    print(n, end=' ')

print()

for word in words:
    
    print(word, end=' ')
    
print()

The code example prints the elements of a tuple and a list.

$ ./for_loop_tuple_list.py 
1 2 3 4 5 6 
cup star monkey bottle 

Loop with index

Sometimes we need to get the index of the element as well; for this we can use the enumerate function.

#!/usr/bin/python

words = ("cup", "star", "monkey", "bottle", "paper", "door")

for idx, word in enumerate(words):
    
    print(f"{idx}: {word}")

With the help of the enumerate function, we print the element of the list with its index.

$ ./for_loop_index.py 
0: cup
1: star
2: monkey
3: bottle
4: paper
5: door

Loop over a dictionary

In the following example, we loop over a Python dictionary.

#!/usr/bin/python

data = { "de": "Germany", "sk": "Slovakia", "hu": "Hungary", "ru": "Russia" }    

for k, v in data.items():
    
    print(f"{k} is an abbreviation for {v}")

The code example prints the keys and the values of the Python dictionary.

$ ./for_loop_dictionary.py 
sk is an abbreviation for Slovakia
ru is an abbreviation for Russia
hu is an abbreviation for Hungary
de is an abbreviation for Germany

Nested for loop

It is possible to nest a for loop into another loop.

#!/usr/bin/python

nums = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

for i in nums:
    
    for e in i:
        
        print(e, end=' ')
    
    print()

We have a two-dimensional list of integers. We loop over the elements with two for loops.

$ ./for_loop_nested.py 
1 2 3 
4 5 6 
7 8 9

Loop with zip

The zip function creates an iterator from the given iterables.

#!/usr/bin/python

words1 = ["cup", "bottle", "table", "rock", "apple"]
words2 = ["trousers", "nail", "head", "water", "pen"]

for w1, w2 in zip(words1, words2):
    
    print(w1, w2)

In the example, we iterate over two lists in one for loop.

$ ./for_loop_zip.py 
cup trousers
bottle nail
table head
rock water
apple pen

Looping custom iterables

In the next example we loop over a custom iterable.

#!/usr/bin/python

import random

def myrandom(x):
    
    i = 0
    
    while i < x:
        
        r = random.randint(0, 100)
        
        yield r
        
        i = i + 1

for r in myrandom(5):
    
    print(r)

The code example creates a generator function that yields random integers. With the for loop
we generate five random integers.

$ ./for_loop_custom_iterable.py 
14
43
53
44
70

Loop with break

The break statement terminates the for loop.

#!/usr/bin/python

import random
import itertools 

for i in itertools.count():
    
   val = random.randint(1, 30)
   print(val)

   if val == 22:
      break

In the example, we create an endless for loop. We generate and print random numbers from 1...29.
If the generated number equals to 22, the for loop is ended with the break keyword.

$ ./for_loop_break.py 
7
27
2
27
7
9
3
25
15
22

Loop with continue

The continue keyword is used to interrupt the current cycle, without jumping out of the
whole loop. It initiates a new cycle.

#!/usr/bin/python

for num in range(1000):
       
   if num % 2 == 0:
      continue
      
   print(num, end=' ')
   
print()

We print all numbers smaller than 1000 that cannot be divided by number 2 without a remainder.

Declarative iteration

The following Java code uses a declarative loop using forEach.

import java.util.List;

void main() {

    var vals = List.of("sky", "blue", "nice");

    vals.forEach(e -> System.out.println(e));
}

There are equivalent of Java's forEach loop in Python, but they are awkward.

vals = [1, 2, 3, 4, 5]

mv = map(lambda e: print(e), vals)

for _ in mv:
    pass

[print(e) for e in vals]

list(map(lambda e: print(e), vals))