For loops

A for loop iterates over elements of a sequence (tuple or list). A variable is created to represent the object in the sequence. For example,

x = [100,200,300]
for i in x:
      print (i)   #these parenthesis are needed for the code to get executed in higher versions of Python

This will output

100
200
300

The for loop loops over each of the elements of a list or iterator, assigning the current element to the variable name given. In the example above, each of the elements in x is assigned to i.

A built-in function called range exists to make creating sequential lists such as the one above easier. The loop above is equivalent to:

l = range(100, 301,100)
for i in l:
    print (i)

The next example uses a negative step (the third argument for the built-in range function):

for i in range(5, 0, -1):
    print (i)

This will output

5
4
3
2
1

The negative step can be -2:

for i in range(10, 0, -2):
    print (i)

This will output

10
8
6
4
2

For loops can have names for each element of a tuple, if it loops over a sequence of tuples:

l = [(1, 1), (2, 4), (3, 9), (4, 16), (5, 25)]
for x, xsquared in l:
    print x, ':', xsquared

This will output

1 : 1
2 : 4
3 : 9
4 : 16
5 : 25

Links:

• 4.2. for Statements, The Python Tutorial, docs.python.org
• 4.3. The range() Function, The Python Tutorial, docs.python.org

While loops

A while loop repeats a sequence of statements until some condition becomes false. For example:

x = 5
while x > 0:
    print (x)  #all the print statement must be in parenthesis for version 3.4.0
    x = x - 1  #the algebra need not be done within the parenthesis

Will output:

5
4
3
2
1

Python's while loops can also have an 'else' clause, which is a block of statements that is executed (once) when the while statement evaluates to false. The break statement inside the while loop will not direct the program flow to the else clause. For example:

x = 5
y = x
while y > 0:
    print (y)
    y = y - 1
else:
    print (x)

This will output:

5
4
3
2
1
5

Unlike some languages, there is no post-condition loop.

Links:

• 3.2. First Steps Towards Programming, The Python Tutorial, docs.python.org

Breaking and continuing

Python includes statements to exit a loop (either a for loop or a while loop) prematurely. To exit a loop, use the break statement:

x = 5
while x > 0:
    print x
    break
    x -= 1
    print x

This will output

5

The statement to begin the next iteration of the loop without waiting for the end of the current loop is 'continue'.

l = [5,6,7]
for x in l:
    continue
    print x

This will not produce any output.

Else clause of loops

The else clause of loops will be executed if no break statements are met in the loop.

l = range(1,100)
for x in l:
    if x == 100:
        print x
        break
    else:
        print x," is not 100"
else:
    print "100 not found in range"

Another example of a while loop using the break statement and the else statement:

expected_str = "melon"
received_str = "apple"
basket = ["banana", "grapes", "strawberry", "melon", "orange"]
x = 0
step = int(raw_input("Input iteration step: "))
 
while(received_str != expected_str):
    if(x >= len(basket)): print "No more fruits left on the basket."; break
    received_str = basket[x]
    x += step # Change this to 3 to make the while statement
              # evaluate to false, avoiding the break statement, using the else clause.
    if(received_str==basket[2]): print "I hate",basket[2],"!"; break
    if(received_str != expected_str): print "I am waiting for my ",expected_str,"."
else:
    print "Finally got what I wanted! my precious ",expected_str,"!"
print "Going back home now !"

This will output:

Input iteration step: 2
I am waiting for my  melon .
I hate strawberry !
Going back home now !

White Space

Python determines where a loop repeats itself by the indentation in the whitespace. Everything that is indented is part of the loop, the next entry that is not indented is not. For example, the code below prints "1 1 2 1 1 2"

for i in [0, 1]:
    for j in ["a","b"]:
        print("1")
    print("2")

On the other hand, the code below prints "1 2 1 2 1 2 1 2"

for i in [0, 1]:
    for j in ["a","b"]:
        print("1")
        print("2")