As one of the most basic functions in programming, loops are an important piece to nearly every programming language. Loops enable developers to set certain portions of their code to repeat through a number of loops which are referred to as iterations. This topic covers using multiple types of loops and applications of loops in Python.
- while <boolean expression>:
- for <variable> in <iterable>:
- for <variable> in range(<number>):
- for <variable> in range(<start_number>, <end_number>):
- for <variable> in range(<start_number>, <end_number>, <step_size>):
- for i, <variable> in enumerate(<iterable>): # with index i
- for <variable1>, <variable2> in zip(<iterable1>, <iterable2>):
|boolean expression||expression that can be evaluated in a boolean context, e.g. |
|variable||variable name for the current element from the |
|iterable||anything that implements iterations|
Break and Continue in Loops
break statement executes inside a loop, control flow "breaks" out of the loop immediately:
The loop conditional will not be evaluated after the
break statement is executed. Note that
break statements are only allowed inside loops, syntactically. A
break statement inside a function cannot be used to terminate loops that called that function.
Executing the following prints every digit until number
4 when the
break statement is met and the loop stops:
break statements can also be used inside
for loops, the other looping construct provided by Python:
Executing this loop now prints:
Note that 3 and 4 are not printed since the loop has ended.
If a loop has an
else clause, it does not execute when the loop is terminated through a
continue statement will skip to the next iteration of the loop bypassing the rest of the current block but continuing the loop. As with
continue can only appear inside loops:
4 aren't printed, this is because
continue goes to the next iteration instead of continuing on to
i == 2 or
i == 4.
continue only operate on a single level of loop. The following example will only break out of the inner
for loop, not the outer
Python doesn't have the ability to break out of multiple levels of loop at once -- if this behavior is desired, refactoring one or more loops into a function and replacing
return may be the way to go.
return from within a function as a
return statement exits from a function, without executing the code that comes after it.
If you have a loop inside a function, using
return from inside that loop is equivalent to having a
break as the rest of the code of the loop is not executed (note that any code after the loop is not executed either):
If you have nested loops, the
return statement will break all loops:
for loops iterate over a collection of items, such as
dict, and run a block of code with each element from the collection.
for loop iterates over a list of numbers.
Each iteration sets the value of
i to the next element of the list. So first it will be
2, etc. The output will be as follow:
range is a function that returns a series of numbers under an iterable form, thus it can be used in
gives the exact same result as the first
for loop. Note that
5 is not printed as the range here is the first five numbers counting from
for loop can iterate on any iterable object which is an object which defines a
__getitem__ or a
__iter__ function returns an iterator, which is an object with a
next function that is used to access the next element of the iterable.
Iterating different portion of a list with different step size
Suppose you have a long list of elements and you are only interested in every other element of the list. Perhaps you only want to examine the first or last elements, or a specific range of entries in your list. Python has strong indexing built-in capabilities. Here are some examples of how to achieve these scenarios.
Here's a simple list that will be used throughout the examples:
Iteration over the whole list
To iterate over each element in the list, a
for loop like below can be used:
for loop assigns s for each element of
lst. This will print:
Often you need both the element and the index of that element. The
enumerate keyword performs that task.
idx will start with zero and increment for each iteration, while the
s will contain the element being processed. The previous snippet will output:
Iterate over sub-list
If we want to iterate over a range (remembering that Python uses zero-based indexing), use the
This would output:
The list may also be sliced. The following slice notation goes from element at index 1 to the end with a step of 2. The two
for loops give the same result.
The above snippet outputs:
Indexing and slicing is a topic of its own.
Iterating over dictionaries
Considering the following dictionary:
To iterate through its keys, you can use:
This is equivalent to:
or in Python 2:
To iterate through its values, use:
To iterate through its keys and values, use:
Note that in Python 2,
.items() return a
list object. If you simply need to iterate trough the result, you can use the equivalent
The difference between
.iteritems() is that the
iter* methods are generators. Thus, the elements within the dictionary are yielded one by one as they are evaluated. When a
list object is returned, all of the elements are packed into a list and then returned for further evaluation.
Note also that in Python 3, Order of items printed in the above manner does not follow any order.
Iterating over lists
To iterate through a list you can use
This will print out the elements of the list:
range function generates numbers which are also often used in a for loop.
The result will be a special range sequence type in python >=3 and a list in python <=2. Both can be looped through using the for loop.
If you want to loop though both the elements of a list and have an index for the elements as well, you can use Python's
enumerate will generate tuples, which are unpacked into
index (an integer) and
item (the actual value from the list). The above loop will print
Iterate over a list with value manipulation using
lambda, i.e. apply lambda function on each element in the list:
NB: in Python 3.x
map returns an iterator instead of a list so you in case you need a list you have to cast the result
print(list(x)) (see http://www.riptutorial.com/python/example/8186/map-- in http://www.riptutorial.com/python/topic/809/incompatibilities-moving-from-python-2-to-python-3 ).
Looping and Unpacking
If you want to loop over a list of tuples for example:
instead of doing something like this:
or something like this:
You can simply do this:
This will also work for most types of iterables, not just tuples.
Loops with an "else" clause
while compound statements (loops) can optionally have an
else clause (in practice, this usage is fairly rare).
else clause only executes after a
for loop terminates by iterating to completion, or after a
while loop terminates by its conditional expression becoming false.
else clause does not execute if the loop terminates some other way (through a
break statement or by raising an exception):
Most other programming languages lack this optional
else clause of loops. The use of the keyword
else in particular is often considered confusing.
The original concept for such a clause dates back to Donald Knuth and the meaning of the
else keyword becomes clear if we rewrite a loop in terms of
if statements and
goto statements from earlier days before structured programming or from a lower-level assembly language.
is equivalent to:
These remain equivalent if we attach an
else clause to each of them.
is equivalent to:
for loop with an
else clause can be understood the same way. Conceptually, there is a loop condition that remains True as long as the iterable object or sequence still has some remaining elements.
Why would one use this strange construct?
The main use case for the
for...else construct is a concise implementation of search as for instance:
To make the
else in this construct less confusing one can think of it as "if not break" or "if not found".
Some discussions on this can be found in [Python-ideas] Summary of for...else threads, Why does python use 'else' after for and while loops? , and Else Clauses on Loop Statements
The "half loop" do-while
Unlike other languages, Python doesn't have a do-until or a do-while construct (this will allow code to be executed once before the condition is tested). However, you can combine a
while True with a
break to achieve the same purpose.
This will print:
The Pass Statement
pass is a null statement for when a statement is required by Python syntax (such as within the body of a
while loop), but no action is required or desired by the programmer. This can be useful as a placeholder for code that is yet to be written.
In this example, nothing will happen. The
for loop will complete without error, but no commands or code will be actioned.
pass allows us to run our code successfully without having all commands and action fully implemented.
pass can be used in
while loops, as well as in selections and function definitions etc.
while loop will cause the loop statements to be executed until the loop condition is falsey. The following code will execute the loop statements a total of 4 times.
While the above loop can easily be translated into a more elegant
while loops are useful for checking if some condition has been met. The following loop will continue to execute until
myObject is ready.
while loops can also run without a condition by using numbers (complex or real) or
If the condition is always true the while loop will run forever (infinite loop) if it is not terminated by a break or return statement or an exception.