For Loop

source: Python Tutorial/For Loop.md

The for statement in Python is a bit different from what you usually use in other programming languages.

Rather than iterating over a numeric progression, Python's for statement iterates over the items of any iterable (list, tuple, dictionary, set, or string). The items are iterated in the order that they appear in the iterable.

For Loop

# Iterate through a list
colors = ['red', 'green', 'blue', 'yellow']
for x in colors:
    print(x)
# Prints red green blue yellow

# Iterate through a string
S = 'python'
for x in S:
    print(x)
# Prints p y t h o n

  

Break in for Loop

Python break statement is used to exit the loop immediately. It simply jumps out of the loop altogether, and the program continues after the loop.

# Break the loop at 'blue'
colors = ['red', 'green', 'blue', 'yellow']
for x in colors:
    if x == 'blue':
        break
    print(x)
# Prints red green
  

Continue in for Loop

The continue statement skips the current iteration of a loop and continues with the next iteration.

# Skip 'blue' and execute other colors in list
colors = ['red', 'green', 'blue', 'yellow']
for x in colors:
    if x == 'blue':
        continue
    print(x)
# Prints red green yellow
  

Else in for Loop

Python allows an optional else clause at the end of a for loop. The else clause will be executed if the loop terminates naturally (through exhaustion).

colors = ['red', 'green', 'blue', 'yellow']
for x in colors:
    print(x)
else:
    print('Done!')
# Prints red green blue yellow
# Prints Done!

## If the loop terminates prematurely with break, the else clause won't be executed.

colors = ['red', 'green', 'blue', 'yellow']
for x in colors:
    if x == 'blue':
        break
    print(x)
else:
    print('Done!')
# Prints red green

  

range() function in for loop

If you need to execute a group of statements for a specified number of times, use built-in function range().

The range(start,stop,step) function generates a sequence of numbers from 0 up to (but not including) specified number.

# Generate a sequence of numbers from 0 6
for x in range(7):
    print(x)
# Prints 0 1 2 3 4 5 6

range() provides a simple way to repeat an action a specific number of times.

# Print 'Hello!' three times
for x in range(3):
    print('Hello!')
# Prints Hello!
# Prints Hello!
# Prints Hello!

The range starts from 0 by default. But, you can start the range at another number by specifying start parameter.

# Generate a sequence of numbers from 2 to 6
for x in range(2, 7):
    print(x)
# Prints 2 3 4 5 6

# Increment the range with 2
for x in range(2, 7, 2):
    print(x)
# Prints 2 4 6
  

Nested for Loop

A loop inside another loop is called a nested loop.

# Flatten a nested list
list = [[1, 2, 3],[4, 5, 6],[7, 8, 9]]   
for sublist in list:
    for number in sublist:
        print(number)
# Prints 1 2 3 4 5 6 7 8 9
  

Access Index in for Loop

To iterate over the indices of a sequence, you can combine range() and len() as follows:

colors = ['red', 'green', 'blue']
for index in range(len(colors)):
    print(index, colors[index])
# Prints 0 red
# Prints 1 green
# Prints 2 blue

However, in most such cases it is convenient to use the enumerate() function.

colors = ['red', 'green', 'blue']
for index, value in enumerate(colors):
    print(index, value)
# Prints 0 red
# Prints 1 green
# Prints 2 blue
  

Unpacking in a for loop

Below for loop does a multiple assignment (unpack the current tuple) each time through the loop.

# Tuple unpacking
T = [(1, 2), (3, 4), (5, 6)]
for (a, b) in T:
    print(a, b)
# Prints 1 2
# Prints 3 4
# Prints 5 6

Likewise, you can iterate through both keys and values in a dictionary.

# Dictionary unpacking
D = {'name': 'Bob', 'age': 25}
for x, y in D.items():
	print(x, y)
# Prints age 25
# Prints name Bob
  

Modify a List While Iterating

Don't alter mutable objects while looping on them. It may create an infinite loop.

# infinite loop
colors = ['red', 'green', 'blue']
for x in colors:
    if x == 'red':
        colors.insert(0, 'orange')
        print(colors)
  

It is recommended that you first make a copy. The slicing operator makes this especially convenient.

colors = ['red', 'green', 'blue']
for x in colors[:]:
	if x == 'red':
		colors.insert(0, 'orange')
print(colors)
# Prints ['orange', 'red', 'green', 'blue']
  

Looping Through Multiple Lists

Using built-in zip() function you can loop through multiple lists at once.

# Loop through two lists at once
name = ['Bob', 'Sam', 'Max']
age = [25, 35, 30]
for x, y in zip(name, age):
    print(x, y)
# Prints Bob 25
# Prints Sam 35
# Prints Max 30
  