Python enumerate list with index

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Python enumerate list with index

How do I get the index of an element while I am iterating over a list? If you are coming from other programming languages (like C), most likely you are used to the idea of iterating over the length of an array by an index and then using this index to get the value at that location.

Python gives you the luxury of iterating directly over the values of

the list which is most of the time what you need. However, there are times when you actually need the index of the item as well. Python has a built-in function called enumerate that allows you to do just that. In this article, I will show you how to iterate over different types of python objects and get back both the index and the value of each item. Jump

directly to a specific section: Enumerate a List Let's see what to do if we want to enumerate a python list. You can iterate over the index and value of an item in a list by using a basic for loop L = ['apples', 'bananas', 'oranges'] for idx, val in enumerate(L): print("index is %d and value is %s" % (idx, val)) The code above will have this output: index is 0

and value is apples index is 1 and value is bananas index is 2 and value is oranges How easy was that! Now let's see how to enumerate a tuple. Enumerate a Tuple Enumerating a tuple isn't at all different from enumerating a list. t = ('apples', 'bananas', 'oranges') for idx, val in enumerate(t): print("index is %d and value is %s" % (idx, val)) And as you

expect, the output will be: index is 0 and value is apples index is 1 and value is bananas index is 2 and value is oranges Now that you know how to enumerate a list and a tuple, how can you enumerate a list of tuples? Enumerate a List of Tuples (The Neat Way) Say you have a list of tuples where each tuple is a name-age pair. L = [('Matt', 20),

('Karim', 30), ('Maya', 40)] Of course one way to enumerate the list is like this: for idx, val in enumerate(L): name = val[0] age = val[1] print("index is %d, name is %s, and age is %d" \ % (idx, name, age)) The above code will definitely work and it will print out this output. index is 0, name is Matt, and age is 20 index is 1, name is Karim, and age is 30

index is 2, name is Maya, and age is 40 However, a cleaner way to achieve this is through tuple unpacking With tuple unpacking, we can do something like this for idx, (name, age) in enumerate(L): print("index is %d, name is %s, and age is %d" \ % (idx, name, age)) Enumerate a String So what happens when you use the enumerate function on a

string object? An item in a string is a single character. So if you enumerate over a string, you will get back the index and value of each character in the string. Let's take an example: str = "Python" for idx, ch in enumerate(str): print("index is %d and character is %s" \ % (idx, ch)) And the output will be: index is 0 and character is P index is 1 and

character is y index is 2 and character is t index is 3 and character is h index is 4 and character is o index is 5 and character is n Enumerate with a Different Starting Index So as you know, indices in python start at 0. This means that when you use the enumerate function, the index returned for the first item will be 0. However, in some cases, you

want the loop counter to start at a different number. enumerate allows you to do that through an optional start parameter. For example, let's say we want to enumerate over a list starting at 1. The code will look like this: L = ['apples', 'bananas', 'oranges'] for idx, s in enumerate(L, start = 1): print("index is %d and value is %s" \ % (idx, s)) The above

code will result in this output. index is 1 and value is apples index is 2 and value is bananas index is 3 and value is oranges Needless to say, you can also start with a negative number. Why It doesn't Make Sense to Enumerate Dictionaries and Sets So does it make sense to use the enumerate function for dictionaries and sets? Absolutely not! Think

about it, the only reason you would use enumerate is when you actually care about the index of the item. Dictionaries and Sets are not sequences. Their items do not have an index, and they don't, by definition, need one. If you want to iterate over the keys and values of a dictionary instead (a very common operation), then you can do that using the

following code: d = {'a': 1, 'b': 2, 'c': 3} for k, v in d.items(): # k is now the key # v is the value print(k, v) And if you want to iterate over a set, then just use a regular for loop. s = {'a', 'b', 'c'} for v in s: print(v) Advanced: Enumerate Deep Dive In Python, the enumerate function returns a Python object of type enumerate Yes, there is an enumerate

built-in function and an enumerate object >>> type(enumerate([1, 2, 3])) Now let's go to Github and check how the enumerate object is implemented. As you can see, the enumerate object stores an index en_index, an iterator en_sit, and a result tuple en_result en_sit is actually the input parameter that we passed to the enumerate function. It must

be an iterable object. At a given index, the result is a tuple of two elements. The first element is the index and the second one is the item in en_sit with that index. enumerate objects themselves are also iterables with each item being the mentioned result tuple. >>> list(enumerate(['a', 'b', 'c'])) [(0, 'a'), (1, 'b'), (2, 'c')] That's why when we iterate over

the enumerate object with a for loop like this: >>> for idx, val in enumerate(['a', 'b']): ... print(idx, val) ... 0 a 1 b We are effectively unpacking these tuples to an index and a value. But there is nothing that prevents you from doing this (but don't do it :)) >>> for i in enumerate(['a', 'b']): ... print(i[0], i[1]) ... 0 a 1 b Finally, have fun enumerating

Learning Python? Check out the Courses section! Featured Posts Python tips for beginners, intermediate, and advanced levels. CS Career tips and advice. Special discounts on my premium courses when they launch. And so much more... Subscribe now. It's Free. In Python, you can get the element and index (count) from iterable objects such as lists

and tuples in for loop by the built-in function enumerate(). Built-in Functions - enumerate() -- Python 3.8.5 documentation This article describes the basics of enumerate(). How to use enumerate() Normal for loop for loop with enumerate() Start index from 1 with enumerate() Set step with enumerate() See the following articles for more information

about for loop and how to use enumerate() and zip() together. How to use enumerate() Normal for loop l = ['Alice', 'Bob', 'Charlie'] for name in l: print(name) # Alice # Bob # Charlie source: enumerate_start.py for loop with enumerate() By passing an iterable object in the argument of enumerate(), you can get index, element. for i, name in

enumerate(l): print(i, name) # 0 Alice # 1 Bob # 2 Charlie source: enumerate_start.py Start index from 1 with enumerate() As in the example above, by default, the index of the enumerate() starts from 0. If you want to start from another number, pass it in the second argument of the enumerate(). Example starting from 1: for i, name in enumerate(l,

1): print(i, name) # 1 Alice # 2 Bob # 3 Charlie source: enumerate_start.py Example starting from the other number: for i, name in enumerate(l, 42): print(i, name) # 42 Alice # 43 Bob # 44 Charlie source: enumerate_start.py For example, this is useful when generating a serial number string. It is smarter to pass the starting number in the second

argument of the enumerate() than to calculate i + 1. for i, name in enumerate(l, 1): print('{:03}_{}'.format(i, name)) # 001_Alice # 002_Bob # 003_Charlie source: enumerate_start.py Set step with enumerate() There is no argument like step to specify increment to enumerate(), but it can be done as follows. step = 3 for i, name in enumerate(l): print(i

* step, name) # 0 Alice # 3 Bob # 6 Charlie source: enumerate_start.py If you're moving to Python from C or Java, you might be confused by Python's for loops. Python doesn't actually have for loops... at least not the same kind of for loop that C-based languages have. Python's for loops are actually foreach loops. In this article I'll compare Python's for

loops to those of other languages and discuss the usual ways we solve common problems with for loops in Python. For loops in other languages Before we look at Python's loops, let's take a look at a for loop in JavaScript: 1 2 3 4 var colors = ["red", "green", "blue", "purple"]; for (var i = 0; i < colors.length; i++) { console.log(colors[i]); } This

JavaScript loop looks nearly identical in C/C++ and Java. In this loop we: Set a counter variable i to 0 Check if the counter is less than the array length Execute the code in the loop or exit the loop if the counter is too high Increment the counter variable by 1 Looping in Python Now let's talk about loops in Python. First we'll look at two slightly more

familiar looping methods and then we'll look at the idiomatic way to loop in Python. while If we wanted to mimic the behavior of our traditional C-style for loop in Python, we could use a while loop: 1 2 3 4 5 colors = ["red", "green", "blue", "purple"] i = 0 while i < len(colors): print(colors[i]) i += 1 This involves the same 4 steps as the for loops in other

languages (note that we're setting, checking, and incrementing i) but it's not quite as compact. This method of looping in Python is very uncommon. range of length I often see new Python programmers attempt to recreate traditional for loops in a slightly more creative fashion in Python: 1 2 3 colors = ["red", "green", "blue", "purple"] for i in

range(len(colors)): print(colors[i]) This first creates a range corresponding to the indexes in our list (0 to len(colors) - 1). We can loop over this range using Python's for-in loop (really a foreach). This provides us with the index of each item in our colors list, which is the same way that C-style for loops work. To get the actual color, we use colors[i]. for-

in: the usual way Both the while loop and range-of-len methods rely on looping over indexes. But we don't actually care about the indexes: we're only using these indexes for the purpose of retrieving elements from our list. Because we don't actually care about the indexes in our loop, there is a much simpler method of looping we can use: 1 2 3 colors

= ["red", "green", "blue", "purple"] for color in colors: print(color) So instead of retrieving the item indexes and looking up each element, we can just loop over our list using a plain for-in loop. The other two methods we discussed are sometimes referred to as anti-patterns because they are programming patterns which are widely considered

unidiomatic. What if we need indexes? What if we actually need the indexes? For example, let's say we're printing out president names along with their numbers (based on list indexes). range of length We could use range(len(our_list)) and then lookup the index like before: 1 2 3 presidents = ["Washington", "Adams", "Jefferson", "Madison", "Monroe",

"Adams", "Jackson"] for i in range(len(presidents)): print("President {}: {}".format(i + 1, presidents[i])) But there's a more idiomatic way to accomplish this task: use the enumerate function. enumerate Python's built-in enumerate function allows us to loop over a list and retrieve both the index and the value of each item in the list: 1 2 3 presidents =

["Washington", "Adams", "Jefferson", "Madison", "Monroe", "Adams", "Jackson"] for num, name in enumerate(presidents, start=1): print("President {}: {}".format(num, name)) The enumerate function gives us an iterable where each element is a tuple that contains the index of the item and the original item value. This function is meant for solving the

task of: Accessing each item in a list (or another iterable) Also getting the index of each item accessed So whenever we need item indexes while looping, we should think of enumerate. Note: the start=1 option to enumerate here is optional. If we didn't specify this, we'd start counting at 0 by default. What if we need to loop over multiple things? Often

when we use list indexes, it's to look something up in another list. enumerate For example, here we're looping over two lists at the same time using indexes to look up corresponding elements: 1 2 3 4 5 colors = ["red", "green", "blue", "purple"] ratios = [0.2, 0.3, 0.1, 0.4] for i, color in enumerate(colors): ratio = ratios[i] print("{}% {}".format(ratio *

100, color)) Note that we only need the index in this scenario because we're using it to lookup elements at the same index in our second list. What we really want is to loop over two lists simultaneously: the indexes just provide a means to do that. zip We don't actually care about the index when looping here. Our real goal is to loop over two lists at

once. This need is common enough that there's a special built-in function just for this. Python's zip function allows us to loop over multiple lists at the same time: 1 2 3 4 colors = ["red", "green", "blue", "purple"] ratios = [0.2, 0.3, 0.1, 0.4] for color, ratio in zip(colors, ratios): print("{}% {}".format(ratio * 100, color)) The zip function takes multiple lists

and returns an iterable that provides a tuple of the corresponding elements of each list as we loop over it. Note that zip with different size lists will stop after the shortest list runs out of items. You may want to look into itertools.zip_longest if you need different behavior. Also note that zip in Python 2 returns a list but zip in Python 3 returns a lazy

iterable. In Python 2, itertools.izip is equivalent to the newer Python 3 zip function. Looping cheat sheet Here's a very short looping cheat sheet that might help you remember the preferred construct for each of these three looping scenarios. Loop over a single list with a regular for-in: 1 2 for n in numbers: print(n) Loop over multiple lists at the same

time with zip: 1 2 for header, rows in zip(headers, columns): print("{}: {}".format(header, ", ".join(rows))) Loop over a list while keeping track of indexes with enumerate: 1 2 for num, line in enumerate(lines): print("{0:03d}: {}".format(num, line)) In Summary If you find yourself tempted to use range(len(my_list)) or a loop counter, think about

whether you can reframe your problem to allow usage of zip or enumerate (or a combination of the two). In fact, if you find yourself reaching for enumerate, think about whether you actually need indexes at all. It's quite rare to need indexes in Python. If you need to loop over multiple lists at the same time, use zip If you only need to loop over a single

list just use a for-in loop If you need to loop over a list and you need item indexes, use enumerate If you find yourself struggling to figure out the best way to loop, try using the cheat sheet above. Practice makes perfect You don't learn by putting information in your head, you learn by attempting to retrieve information from your head. So you've just

read an article on something new, but you haven't learned yet. Write some code that uses enumerate and zip later today and then quiz yourself tomorrow on the different ways of looping in Python. You have to practice these skills if you want to actually remember them. If you'd like to get hands-on experience practicing Python every week, I have a

Python skill-building service you should consider joining. If you sign up for Python Morsels I'll give you a Python looping exercise that right now and then I'll send you one new Python exercise every week after that. I won't share you info with others (see the Python Morsels Privacy Policy for details). This form is reCAPTCHA protected (Google Privacy

Policy & TOS) Fill out the form above to sign up for Python Morsels, get some practice with the zip function, and start leveling-up your Python skills every week.

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