Python Programming/Control Flow

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

• loops
• branches
• function calls

Function calls are covered in the next section.

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

Overview

Control flow in Python at a glance:

x = -6                              # Branching
if x > 0:                           # If
  print "Positive"
elif x == 0:                        # Else if AKA elseif
  print "Zero"
else:                               # Else
  print "Negative"
list1 = [100, 200, 300]
for i in list1: print i             # A for loop
for i in range(0, 5): print i       # A for loop from 0 to 4
for i in range(5, 0, -1): print i   # A for loop from 5 to 1
for i in range(0, 5, 2): print i    # A for loop from 0 to 4, step 2
list2 = [(1, 1), (2, 4), (3, 9)]
for x, xsq in list2: print x, xsq   # A for loop with two iterators
l1 = [1, 2]; l2 = ['a', 'b']
for i1, i2 in zip(l1, l2): print i1, i2 # A for loop iterating two lists at once
i = 5
while i > 0:                        # A while loop
  i -= 1
list1 = ["cat", "dog", "mouse"]
i = -1 # -1 if not found
for item in list1:
  i += 1
  if item=="dog":
    break                           # Break; also usable with while loop
print "Index of dog:",i             
for i in range(1,6):
  if i <= 4:
    continue                        # Continue; also usable with while loop
  print "Greater than 4:", i

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]
for i in x:
    print i

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 first example above, each of the elements in l 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, 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(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
    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.

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 !

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'

Links:

• 4.1. if Statements, The Python Tutorial, docs.python.org

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.

External links

• 4. More Control Flow Tools, The Python Tutorial, docs.python.org