Python Beginner's Tutorial (18) Python's For loop

Python For Loops

The for loop is used to iterate over sequences (i.e. lists, tuples, dictionaries, collections, or strings).

This is less similar to the for keyword in other programming languages and more like the iterator methods in other object-oriented programming languages.

By using a for loop, we can execute a set of statements for a list, tuple, each item in a collection, etc.

an actual example

Prints each fruit in the fruits list:

fruits = ["apple", "banana", "cherry"]
for x in fruits:
  print(x)

running example

apple
banana
cherry

Tip: For loops do not require index variables to be set in advance.

Loop over strings

Even strings are iterable objects that contain a series of characters:

an actual example

Loop over the letters in the word "banana":

for x in "banana":
  print(x)

running example

banana

break statement

By using the break statement, we can stop the loop before it traverses all the items:

an actual example

If x is "banana", exit the loop:

fruits = ["apple", "banana", "cherry"]
for x in fruits:
  print(x) 
  if x == "banana":
    break

running example

apple 
banana

an actual example

Exits the loop when x is "banana", but this time breaks before printing:

fruits = ["apple", "banana", "cherry"]
for x in fruits:
  if x == "banana":
    break
  print(x)

running example

continue statement

By using the continue statement, we can stop the current iteration of the loop and move on to the next:

an actual example

No bananas are printed:

fruits = ["apple", "banana", "cherry"]
for x in fruits:
  if x == "banana":
    continue
  print(x)

running example

apple
cherry

range() function

To loop through a set of code a specified number of times, we can use the range() function.

The range() function returns a sequence of numbers, starting at 0 by default and incrementing by 1 (by default), and ending with the specified number.

an actual example

Use the range() function:

for x in range(10):
  print(x)

running example

0
1
2
3
4
5
6
7
8
9

Note: range(10) is not a value from 0 to 10, but a value from 0 to 9.

The range() function defaults to a starting value of 0, but you can specify a starting value by adding a parameter: range(3, 10), which means values from 3 to 10 (but not including 10):

an actual example

Use the start parameter:

for x in range(3, 10):
  print(x)

running example

3
4
5
6
7
8
9

The range() function defaults to incrementing the sequence by 1, but you can specify the increment by adding a third argument: range(2, 30, 3):

an actual example

Use an incremental sequence of 3 (default value is 1):

for x in range(3, 50, 6):
  print(x)

running example

Else in a For loop

The else keyword in a for loop specifies the block of code to be executed at the end of the loop:

an actual example

Prints all numbers from 0 to 9 and prints a message at the end of the loop:

for x in range(10): 
    print(x)
else: 
    print("Finally finished!")

running example

0
1
2
3
4
5
6
7
8
9
Finally finished!

nested loop

Nested loops are loops within loops.

For each iteration of the "outer loop", the "inner loop" will be executed once:

an actual example Print each adjective for each fruit:

adj = ["red", "big", "tasty"]
fruits = ["apple", "banana", "cherry"]

for x in adj:
  for y in fruits:
    print(x, y)

running example

red apple
red banana
red cherry
big apple
big banana
big cherry
tasty apple
tasty banana
tasty cherry

pass statement

The for statement cannot be empty, but if for some reason you write an empty for statement, use a pass statement to avoid errors.

an actual example

for x in [0, 1, 2]: 
    pass

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