2.2. Control Constructs¶
Keeping with the ‘keep it simple‘ philosophy of Python, there are three primary control constructs (aside from functions and classes) that determine the sequence of instructions executed by a Python program. (The if Statement, The while Loop, The for Loop)
if statement is pretty much the same as it is with other
programming languages. It is the primary selection statement that
provides the semantics for determining conditional execution of blocks of
The simplest form works to determine conditional execution of a single block of code:
if x < 5: statement_1 statement_2 ...
Add the else clause to pick between two code blocks:
if x < 5: alt_statement_a1 alt_statement_a2 ... else: alt_statement_b1 alt_statement_b2 ...
The elif clause lets us pick from several code blocks to execute:
if x < 5: alt_statement_a1 alt_statement_a2 ... elif x < 10: # 5 <= x < 10 alt_statement_b1 alt_statement_b2 ... elif x < 15: # 10 <= x < 15 alt_statement_c1 alt_statement_c2 ... else: # x >= 15 alt_statement_d1 alt_statement_d2 ...
while loop is the primary looping construct to use when the
number executions of the loop depend on the data being processed or how the
user interacts with our program. Python
while loops work the
same as they do with other languages.
while something_is_true: do_this_code_block
Sometimes we have to execute part of the code in a loop before we know that we
are are ready to exit the loop, or that we want to skip the rest of the code
block and process the next value in the sequence (if one exists). This is
continue come to the rescue.
break statement will get us out of the last loop we entered.
continue statement will skip the remainder of the code block so
that the loop condition may be re-evaluated and possibly start the loop over
again or stop executing the loop.
126.96.36.199. Infinite loops¶
Sometimes we want to loop forever, or until told to stop. This is a case
where, you might actually want to write an infinite loop, usually with a
break somewhere in the loop. Remember that any value that is
not one of False, 0 or None evaluates to True. Thus we usually see
infinite loops expressed as either:
while 1: ... if exit_condition: ... break ...
while True: ... if exit_condition: ... break ...
188.8.131.52. Nested Loops¶
It is also quite common to have nested loops. For these loops, we often need a mechanism to get out of just the inner loop or out of booth loops on certain conditions.
Using a boolean variable as the test condition for one of the loops, usually the outer loop, seems to work most of the time for these situations:
more = True while more: ... while True: # your inner loop ... if a_conditon: # exit just the inner loop ... break if another_condition: # exit both inner and outer loops ... more = False break
As with other programming languages, the
for loop in Python is
considered a definite, or counted loop because unless a
statement is executed, the number of times that the loop will execute is
known.  However, the
for loop does not look or work the
same way in all languages. The Python
for loop is similar the
same in Unix shell, perl and other scripting programming languages, which
is quite different than the
for in Java or C/C++. A Python
for loop iterates over the elements in a sequence rather than
using three operators (initialize, test and increment).
>>> a = [1, 2]: >>> for i in a: ... print i ... 1 2
Iterating over a data sequence is the most common usage of
loops in any programming language. For those times when you just want a
sequence of numbers, the built-in
range() function comes in handy.
range() may be called with one, two or three parameters.
If one parameter is supplied, it is assumed to be the stop value, which
means that it starts at 0, as do the indices of lists and tuples. When two
parameters are supplied, they are assumed to be for start and stop.
Here are some examples of using
for loops with the
>>> for i in range(2): ... print i ... 0 1 >>> x = range(1,3) >>> for i in x: ... print i ... 1 2
range([start, ]stop[, step])
Return a list of integers.
- start (integer) – first value in the list
- stop (integer) – one greater than the last value in the list
- step (integer) – the difference between consecutive values in the list
Note that if stop is the only supplied parameter, it is also the length of the returned list.
A couple examples of
>>> range(5) [0, 1, 2, 3, 4] >>> range(2,5) [2, 3, 4] >>> range(1,5,2) [1, 3] >>> range(0,6,2) [0, 2, 4]
|||For some iterable objects, it is not quite so obvious how many
items are contained in the sequence. However, it should be determinate.
One example of a not-so-obvious case is the generator function