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 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.
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
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)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 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
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
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
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
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
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
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
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:
breakIn 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
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.
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))