Python for loop
last modified January 29, 2024
Python for loop tutorial shows how to create loops in Python with
for statement.
Python loop definition
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.
Python 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.
Python for loop with string
The following example uses Python for statement
to go through a string.
#!/usr/bin/python
word = "cloud"
for let in word:
print(let)
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
Python for 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
Python for loops with range
Python range function generates a list 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
Python looping 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
Python looping 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
Python looping 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
Python 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
Python for 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
Python looping custom iterable
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
Python for 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
Python for 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
num = 0
for num in range(1000):
num = num + 1
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.
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