python for loops

Python For Loops

A for loop is used for iterating over a sequence (that is either a list, a tuple,

a dictionary, a set, or a string).

This is less like the for keyword in other programming languages, and works more like an iterator method as found in other object-orientated programming languages.

With the for loop we can execute a set of statements, once for each item in a list, tuple, set etc.

Example

Print each fruit in a fruit list:



fruits = ["apple", "banana", "cherry"]for

x in fruits:

 

print(x)



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Example

Print each fruit in a fruit list:

The for loop does not require an indexing variable to set beforehand.

Looping Through a String

Even strings are iterable objects, they contain a sequence of characters:

Example

Loop through the letters in the word "banana":



for x in "banana":  print(x)



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Example

Loop through the letters in the word "banana":

The break Statement

With the break statement we can stop the

loop before it has looped through all the items:

Example

Exit the loop when x is "banana":



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

  if x ==

"banana":    break



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Example

Exit the loop when x is "banana":

Example

Exit the loop when x is "banana",

but this time the break comes before the print:



fruits = ["apple", "banana", "cherry"]for x in fruits:  if x ==

"banana":    break  print(x)



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Example

Exit the loop when x is "banana",

but this time the break comes before the print:

The continue Statement

With the continue statement we can stop the

current iteration of the loop, and continue with the next:

Example

Do not print banana:



fruits = ["apple", "banana", "cherry"]for x in fruits:  if x ==

"banana":    continue  print(x)



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Example

Do not print banana:

The range() Function

The range() function returns a sequence of numbers, starting from 0 by default, and increments by 1 (by default), and ends at a specified number.

Example

Using the range() function:



for x in range(6):

 

print(x)



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Example

Using the range() function:

Note that range(6) is not the values of 0 to 6, but the values 0 to 5.

The range() function defaults to 0 as a starting value, however it is possible to specify the starting value by adding a parameter: range(2, 6), which

means values from 2 to 6 (but not including 6):

Example

Using the start parameter:



for x in range(2, 6):

 

print(x)



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Example

Using the start parameter:

The range() function defaults to increment the sequence by 1,

however it is possible to specify the increment value by adding a third parameter: range(2, 30, 3):

Example

Increment the sequence with 3 (default is 1):



for x in range(2, 30, 3):

 

print(x)



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Example

Increment the sequence with 3 (default is 1):

Else in For Loop

The else keyword in a

for loop specifies a block of code to be

executed when the loop is finished:

Example

Print all numbers from 0 to 5, and print a message when the loop has ended:



for x in range(6): 

print(x)else: 

print("Finally finished!")

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Example

Print all numbers from 0 to 5, and print a message when the loop has ended:

Note: The else block will NOT be executed if the loop is stopped by a break statement.

Example

Break the loop when x is 3, and see what happens with the

else block:



for x in range(6):  if x == 3: break 

print(x)else: 

print("Finally finished!")

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Example

Break the loop when x is 3, and see what happens with the

else block:

Nested Loops

A nested loop is a loop inside a loop.

The "inner loop" will be executed one time for each iteration of the "outer

loop":

Example

Print each adjective for every fruit:



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

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

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Example

Print each adjective for every fruit:

The pass Statement

for loops cannot be empty, but if you for

some reason have a for loop with no content, put in the pass statement to avoid getting an error.

Example



for x in [0, 1, 2]:  pass

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Example

Test Yourself With Exercises

Exercise:

Loop through the items in the fruits list.

Start the Exercise