Python Programming/Flow control

As with most imperative languages, there are three main categories of program flow control:

• loops
• branches
• function calls

Function calls are covered in the next section.

Generators and list comprehensions are advanced forms of program flow control, but they are not covered here.

Loops

In Python, there are two kinds of loops, 'for' loops and 'while' loops.

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 = [100,200,300,400]
for i in x:
    print i

This will output

100
200
300
400

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 first example above, each of the elements in l is assigned to i.

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

l = range(100, 401,100)
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(10, 0, -1):
    print i

This will output

10
9
8
7
6
5
4
3
2
1

or

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. For instance

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

will output

1 : 1
2 : 4
3 : 9
4 : 16
5 : 25

While loops

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

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

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.

Breaking, continuing and the else clause of loops

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.

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 !

Branches

There is basically only one kind of branch in Python, the 'if' statement. The simplest form of the if statement simple executes a block of code only if a given predicate is true, and skips over it if the predicate is false

For instance,

>>> x = 10
>>> if x > 0:
...    print "Positive"
...
Positive
>>> if x < 0:
...    print "Negative"
...

You can also add "elif" (short for "else if") branches onto the if statement. If the predicate on the first “if” is false, it will test the predicate on the first elif, and run that branch if it’s true. If the first elif is false, it tries the second one, and so on. Note, however, that it will stop checking branches as soon as it finds a true predicate, and skip the rest of the if statement. You can also end your if statements with an "else" branch. If none of the other branches are executed, then python will run this branch.

>>> x = -6
>>> if x > 0:
...    print "Positive"
... elif x == 0:
...    print "Zero"
... else:
...    print "Negative"
...
'Negative'

Conclusion

Any of these loops, branches, and function calls can be nested in any way desired. A loop can loop over a loop, a branch can branch again, and a function can call other functions, or even call itself.