For each loop in python

For loops

There are two ways to create loops in Python: with the for-loop and the while-loop.

When do I use for loops

for loops are used when you have a block of code which you want to repeat a fixed number of times. The for-loop is always used in combination with an iterable object, like a list or a range. The Python for statement iterates over the members of a sequence in order, executing the block each time. Contrast the for statement with the »while» loop, used when a condition needs to be checked each iteration or to repeat a block of code forever. For example:

For loop from 0 to 2, therefore running 3 times.

for x in range(0, 3): print("We're on time %d" % (x))

While loop from 1 to infinity, therefore running forever.

x = 1 while True: print("To infinity and beyond! We're getting close, on %d now!" % (x)) x += 1

When running the above example, you can stop the program by pressing ctrl+c at the same time. As you can see, these loop constructs serve different purposes. The for loop runs for a fixed amount of times, while the while loop runs until the loop condition changes. In this example, the condition is the boolean True which will never change, so it will run forever.

How do they work?

If you’ve done any programming before, you have undoubtedly come across a for loop or an equivalent to it. Many languages have conditions in the syntax of their for loop, such as a relational expression to determine if the loop is done, and an increment expression to determine the next loop value. In Python, this is controlled instead by generating the appropriate sequence. Basically, any object with an iterable method can be used in a for loop. Even strings, despite not having an iterable method — but we’ll not get on to that here. Having an iterable method basically means that the data can be presented in list form, where there are multiple values in an orderly fashion. You can define your own iterables by creating an object with next() and iter() methods. This means that you’ll rarely be dealing with raw numbers when it comes to for loops in Python — great for just about anyone!

Nested loops

When you have a block of code you want to run x number of times, then a block of code within that code which you want to run y number of times, you use what is known as a «nested loop». In Python, these are heavily used whenever someone has a list of lists — an iterable object within an iterable object.

for x in range(1, 11): for y in range(1, 11): print('%d * %d = %d' % (x, y, x*y))

Like the while loop, the for loop can be made to exit before the given object is finished. This is done using the break statement, which will immediately drop out of the loop and continue execution at the first statement after the block. You can also have an optional else clause, which will run should the for loop exit cleanly — that is, without breaking.

for x in range(3): if x == 1: break

Examples

for x in range(3): print(x) else: print('Final x = %d' % (x))

string = "Hello World" for x in string: print(x)

collection = ['hey', 5, 'd'] for x in collection: print(x)

list_of_lists = [ [1, 2, 3], [4, 5, 6], [7, 8, 9]] for list in list_of_lists: for x in list: print(x)

Creating your own iterable

class Iterable(object): def __init__(self,values): self.values = values self.location = 0 def __iter__(self): return self def next(self): if self.location == len(self.values): raise StopIteration value = self.values[self.location] self.location += 1 return value

Your own range generator using yield

def my_range(start, end, step): while start yield start start += step for x in my_range(1, 10, 0.5): print(x)

A note on `range`

The »range» function is seen so often in for statements that you might think range is part of the for syntax. It is not: it is a Python built-in function that returns a sequence following a specific pattern (most often sequential integers), which thus meets the requirement of providing a sequence for the for statement to iterate over. Since for can operate directly on sequences, there is often no need to count. This is a common beginner construct (if they are coming from another language with different loop syntax):

mylist = ['a', 'b', 'c', 'd'] for i in range(len(mylist)): # do something with mylist[i]

It can be replaced with this:

mylist = ['a', 'b', 'c', 'd'] for v in mylist: # do something with v

Consider for var in range(len(something)): to be a flag for possibly non-optimal Python coding.

More resources

ForLoop (last edited 2022-01-23 09:18:40 by eriky )

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Do You Need A Foreach in Python? [Alternatives]

Java 5 introduced the foreach method for iterating arrays similarly to the for loop, while loop , and do-while loop. Unfortunately, there is no exact foreach keyword in Python. There are different ways through which you can use it.

Like a normal for-loop, it begins with the keyword for. Rather than declaring and initializing loop counter variables, you declare a variable with the same type as the array, followed by a colon and then the array name. You can use your loop variable instead of an indexed array element in the loop body.

What’s the Difference between For and Foreach Loop?

For Loop	Foreach Loop
The most traditional way of iterating over arrays	A modern approach to iterating over sequence data types.
It does not pass any call-back functions while iterating.	The foreach loop passes a call-back function for each element of an array.
Works with an iterator, counter, and incrementor.	Works with the iterator, index of an element, and sequence type to iterate through.

Is there a foreach looping statement in Python?

As of Python 3.10, there isn’t a dedicated foreach syntax available. The standard Python for loop has similar functionality to a so-called “foreach loop”. Look at the below example:

for item in sequence: print(item)

The above loop iterates through each “item” in the sequence structure. On every iteration, the loop performs a print operation on the “item”. This, as you can see, is very similar to a Foreach statement in Java. A call-back, in this case – print, is being requested.

Writing a User Defined Foreach loop in Python

The following example shows how a standard foreach loop works. You can try this implementation on your browser here.

def myForEach(aFunction, anIterable): for eachElement in anIterable: aFunction(eachElement)

Explanation:

myForEach Is a function that takes 2 arguments. One is a function to allow call-backs for each eachElement . The other is a sequence data or an iterable type. Upon each iteration, a call back is performed aFunction on each item in the iterable.

Writing a Traditional For Loop in Python

The example below illustrates how traditional for-loops work.

myArray = ['a', 'b', 'c'] for index in range(len(myArray)): print("index: %s | value: %s" % (index, myArray[index]))

index: 0 | value: a index: 1 | value: b index: 2 | value: c

Explanation:

We iterate through the array myArray consisting of 3 elements in our “for loop”. Instead of iterating through the array itself, we’re keeping track of the index values. The iterations proceed till the end of the array. Unlike the foreach loop, the for loop iterates based on the size of the iterable and keeps track of the index position.

Iterating Through a Dictionary in Python using foreach

For dictionaries and mappings, __iter()__ is used to iterate over keys. In other words, if you directly put a dictionary into a for loop, Python will automatically call __iter()__ on that dictionary, giving you an iterator over its keys.

Python understands that a_dict is a dictionary and that it implements __iter()__ .

myDict = for key in myDict: print(key)

Upon each iteration, the loop is making a call-back to the “print” function.

Parallelizing the for loop In Python

Using Python’s multiprocessing package, we can send a request to another process and create a child process, parallelizing the loop. For each element of the iterable, the multiprocessing module could be substituted for the for loop. Due to the Global Interpreter Lock, using multiple threads in Python would not provide better results . multiprocessing.pool() function can be used. Refer to the following implementation:

import multiprocessing def sumAll(value): return sum(range(1, value + 1)) pool_obj = multiprocessing.Pool() result = pool_obj.map(sumAll,range(0,5)) print(result)

Foreach Loops Using Django Templates

A Django template is a text document or a Python string mark-up using the Django template language. These templates allow data from viewable to the template and provide features of programming such as variables, loops, and comments.

Here’s the Django template syntax for a for loop.

Skipping the First or Last Element Using Python For Loop

To skip the first element in an iterable, pass:

for item in myIterable[1:]: print(item)

Similarly, to skip the last element:

for item in myIterable[:-1]: print(item)

FAQs on Python Foreach

foreach loops are usually faster because the local variable that stores the element’s value in the array is faster to access than an element in the array. However, the for loop can be more efficient if the iterable has to be accessed every iteration.

As discussed, Python’s for-loop has behaviors similar to a standard foreach loop. Python for loop iterates through each “item” in the sequence structure. On every iteration, the loop performs a print operation on the “item”. Thereby functioning similarly to a traditional foreach.

Conclusion

We have discussed the foreach loop and its relevancy in Python 3. The differences between the classic for and for each loop have been established. We have also discussed how the Python for loop behaves like a foreach loop in functionality.

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