Flow-control structures: Difference between revisions

[[Category:Less Than 20 Examples]]{{Task|Control Structures}}

{{Control Structures}}

 

=={{header|Ada}}==

===Loops===

Python supports ''break'' and ''continue'' to exit from a loop early or short circuit the rest of a loop's body and "continue" on to the next loop iteration.

# Search for an odd factor of a using brute force:

for i in range(n):

result = None

print "No odd factors found"

In addition, as shown in the foregoing example, Python loops support an ''else:'' suite which can be used to handle cases when the loop was intended to search for something, where the code would break out of the loop upon finding its target. In that situation the ''else:'' suite can be used to handle the failure. (In most other languages one is forced to use a "sentinel value" or a special flag variable ... typically set to "False" before the loop and conditionally set to "True" within the loop to handle situations for which the Python ''else:'' on loops is intended).

 

A custom Exception class is normally declared with the ''pass'' statement as no methods of the parent class are over-ridden, no additional functionality is defined and no attributes need be set. Example:

 

class MyException(Exception): pass

 

One normally would choose the most similar existing class. For example if MyException was going to be raised for some situation involving an invalid value it might be better to make it a subclass of ValueError; if it was somehow related to issues with inappropriate objects being passed around then one might make it a subclass of TypeError.

To create a "virtual base class" (one which is not intended to be directly instantiated, but exists solely to provide an inheritance to it's derived classes) one normally defines the requisite methods to raise "NotImplementedError" like so:

 

class MyVirtual(object):

def __init__(self):

raise NotImplementedError

 

It then becomes necessary for any descendants of this class to over-ride the ''__init__()'' method. Any attempt to instantiate a "MyVirtual" object directly will raise an exception.

 

====Case 1 - Try, Except====

try:

temp = 0/0

print "An error occurred."

# Output : "An error occurred"

====Case 2 - Try, Except====

try:

temp = 0/0

print "You've divided by zero!"

# Output : "You've divided by zero!"

====Case 3 - Try, Except, Finally====

try:

temp = 0/0

# An error occurred

# End of 'try' block...

 

Note: Prior to version 2.5 a ''try:'' statement could contain either series of ''except:'' clauses '''or''' a ''finally:'' clause but '''not both.''' It was thus necessary to nest the exception handling in an enclosing ''try:''...''finally:'' loop like so:

 

try:

try:

finally:

do_some_cleanup() # run in any case, whether any exceptions were thrown or not

====Case 4 - Try, Except, Else====

try:

temp = 1/1 # not a division by zero error

# Output :

# No apparent error occurred.

====Case 5 - Try, Except, break, continue====

i = 0

while 1: # infinite loop

# You've divided by zero. Decrementing i and continuing...

# Imaginary Number! Breaking out of loop

====Case 6 - Creating your own custom exceptions, raise====

# Let's call our custom error "StupidError"; it inherits from the Exception class

# Output :

# Something stupid occurred: Segfault

 

===Case 7 - continue, else in "for" loop===

i = 101

for i in range(4): # loop 4 times

if(__name__ == "__main__"):

main()

 

===The "with" statement===

See [[http://www.python.org/peps/pep-0343.html PEP 0343, The "with" statement]]

 

class Quitting(Exception): pass

max = 10

raise Quitting

print line,

 

The ''with'' statement allows classes to encapsulate "final" (clean-up) code which will automatically be executed regardless of exceptions that occur when working "with" these objects. Thus, for the foregoing example, the file will be closed regardless of whether it's more than 10 lines long. Many built-in and standard library classes have "context managers" which facilitate their use in ''with:'' code. In addition it's possible to define special __enter__() and __exit__() methods in one's own classes which will be implicitly called by the interpreter when an object is used within a ''with:'' statement.