Figure 1: Food list with its items and corresponding indexes.<\/figcaption><\/figure>\nHere’s how we access each index number.<\/p>\n
## List example #1\r\n\r\n['burger', 'pizza', 'pasta', 'bacon omelette']\r\nfood = ['burger', 'pizza', 'pasta', 'bacon omelette']\r\nprint(food[0])\r\n\r\n#Return value\r\nburger\r\n<\/pre>\nThe integer contained in the square brackets is the index. Accessing food[0]<\/code> will return the first item in the food<\/code> list, which is burger<\/code>. Now that we’ve covered the fundamentals behind loops and lists let’s see how we can make them work together in a Python for loop!<\/p>\nFor Loop in Python with List<\/h2>\n
Using a for loop in combination with the range() function is the most common (and easiest) way to loop through a list in Python. I’ve gone ahead and added all of us to a new list called coders<\/code>. I’ll modify the original for loop that I showed you earlier so that it interacts with this new list.<\/p>\ncoders = ['Zack', 'Jesus', 'Dan', 'You']\r\nfor i in range(len(coders)):\r\n print('Index ' + str(i) + ' in the coder list is: ' + coders[i])\r\n print(coders[i] + ' ' + 'will get better at coding!')<\/pre>\nFor this next example, I’ll use range(len(coders))<\/code>, since my variable i<\/code> (short for item) will access the index and the item at that index as coders[i]<\/code>. I’ve also asked our program to give us some much-needed encouragement throughout our journey! This is what our terminal shows:<\/p>\nIndex 0 in the coder list is: Zack\r\nZack will get better at coding!\r\nIndex 1 in the coder list is: Jesus\r\nJesus will get better at coding!\r\nIndex 2 in the coder list is: Dan\r\nDan will get better at coding!\r\nIndex 3 in the coder list is: You\r\nYou will get better at coding!<\/pre>\nThis line of code will also iterate through all the indexes of the list coders. By default, the range<\/code> function starts at 0<\/code> and then increments its value by 1<\/code> until the number that we’ve specified in our clause is reached. Note: The final value is\u00a0not<\/em> included in the output. This is referred to as an “exclusive” bound in that the upper bound is\u00a0excluded<\/em>.<\/p>\n‘Break’ in Python Loop<\/h2>\n
An infinite loop that never exits is a fairly common programming bug. Thankfully, there’s a shortcut that gets the program to break out of a loop. What’s best is that break<\/code> keyword in Python loop works in for<\/code> loops and<\/em> while<\/code> loops.<\/p>\nFor our exiting for loop in Python example, we’ll modify our initial for<\/code> loop demonstration, where we asked Python to print all the items in the food<\/code> list. Now, we will ask our program to break out of the loop once it finds and prints pasta<\/code>.<\/p>\nfood = ['burger', 'pizza', 'pasta', 'bacon omelette']\r\n\r\nfor dish in food:\r\n print(dish)\r\n if dish == 'pasta':\r\n break\r\n\r\nburger\r\npizza\r\npasta<\/pre>\nNotice how bacon omelette<\/code> is no longer printed on our terminal window. This means we’ve successfully broken out of a loop that would’ve otherwise printed every item on our list!<\/p>\nThe Continue Statement<\/h2>\n
The continue<\/code> statement, just like the break<\/code> statement, can only be used inside loops. Contrary to break<\/code>, the continue<\/code> statement forces the program to immediately<\/em> jump back to the start of the loop and re-evaluate the loop’s condition.<\/p>\nIn for<\/code> loops, you can use the continue<\/code> keyword to force the program to proceed to the next value in the counter \u2014 effectively skipping values at your convenience. An equally common and basic use of the continue<\/code> statement is for creating a login and password that grants users access to your program’s functions. Here’s an example:<\/p>\nwhile True:\r\n print(\"Please enter your login ID\")\r\n login_id = input()\r\n if login_id != \"Dan\":\r\n continue\r\n print(\"Welcome back, Dan. Please enter your password\")\r\n password = input()\r\n if password == \"psw123\":\r\n break\r\nprint(\"Thank you. Access Granted.\")<\/pre>\nPython Remove Item From List in For Loop<\/h2>\n
Generally speaking, you do not want to remove items from a list while iterating over it. An alternative approach is realized by building a new list that filters undesired items. However, as shown below, you can attempt to remove the undesired elements coupled with a ValueError<\/code> exception.<\/p>\nnewfood3 = ['sushi', 'kebab', 'capacollo', 'lasagna', 'grilled cheese', 'jelly', 'popcorn', 'ham']\r\ntry:\r\n newfood3.remove('lasagna')\r\nexcept ValueError:\r\n pass\r\nprint(newfood3)\r\n\r\n['sushi', 'kebab', 'capacollo', 'grilled cheese', 'jelly', 'popcorn', 'ham']<\/pre>\nYou can also use a List Comprehension that removes all the elements that match a specific condition.<\/p>\n
newfood = ['burger', 'bacon omellete', 'tacos', 'salad', 'lasagna', 'lasagna', 'lasagna', 'lasagna']\r\nnewfood = [x for x in newfood if x != 'lasagna']\r\nprint(newfood)\r\n\r\n['burger', 'bacon omellete', 'tacos', 'salad']<\/pre>\nPython Create Lists in For Loop<\/h2>\n
Python allows you to append items from one list to another or create entirely new lists with a for loop. We’ll go over a couple of ways to do this using the append()<\/code> method.
\nYou can use the append()<\/code> method to add arguments to the end of the list as follows:<\/p>\ndesserts = ['chocolate cake', 'vanilla cake', 'ice cream']\r\ndesserts.append('apple pie')\r\nprint(desserts)\r\n\r\n['chocolate cake', 'vanilla cake', 'ice cream', 'apple pie']<\/pre>\nIf you’d like your for<\/code> loop to merge two lists, we can ask Python to append all the items in list 1 to list 2 with a for<\/code> loop. Here’s an example:<\/p>\nlist1 = ['lasagna','manicotti','cannolis']\r\nlist2 = ['riccota pie','escarole']\r\nfor item in list2:\r\n list1.append(item)\r\nprint (list1)\r\n\r\n['lasagna', 'manicotti', 'cannolis', 'riccota pie', 'escarole']<\/pre>\nIf we want to Python create lists in for loop, we can also define an entirely new list and append all previous items to it. Here’s how I did it:<\/p>\n
list1 = ['lasagna','manicotti','cannolis']\r\nlist2 = ['riccota pie','escarole']\r\nlist3 = []\r\nfor item in list1:\r\n list3.append(item)\r\nfor item in list2:\r\n list3.append(item)\r\nprint (list3)\r\n\r\n['lasagna', 'manicotti', 'cannolis', 'riccota pie', 'escarole']<\/pre>\nYou can also modify this code to make all sorts of calculations and append all the results to a new list. Here’s another way for Python to create lists in for<\/code> loop:<\/p>\ncalculuslist1 = [3,6,9,12,15]\r\ncalculuslist2 = []\r\nfor number in calculuslist1:\r\n calc = number * 10\r\n calculuslist2.append(calc)\r\nprint(calculuslist2)\r\n\r\n[30, 60, 90, 120, 150]\r\n<\/pre>\nWith these Python create lists for<\/code> loop techniques, you can create entirely new lists and append items or results to them.<\/p>\nLoop in Python List with Enumerate<\/h2>\n
We used the range function for our first for loop example and added a string to our code that identified each item with its corresponding index. As it turns out, there’s a loop in Python list that can automatically do this for us.<\/p>\n
When we combine the enumerate()<\/code> method with a for loop, we can iterate each item in our list by index. This is particularly useful when we need to track both the index and value of each element. Let’s use our original coders<\/code> list and compare their outputs.<\/p>\nfor i, val in enumerate(coders):\r\n print (i, \":\",val)\r\n\r\n0 : Zack\r\n1 : Jesus\r\n2 : Dan\r\n3 : You<\/pre>\nHere\u2019s the output for our original loop:<\/p>\n
Index 0 in the coder list is: Zack\r\nZack will get better at coding!\r\nIndex 1 in the coder list is: Jesus\r\nJesus will get better at coding!\r\nIndex 2 in the coder list is: Dan\r\nDan will get better at coding!\r\nIndex 3 in the coder list is: You\r\nYou will get better at coding!<\/pre>\nLoop With List Python<\/h2>\n
Another popular loop with list Python comes via list comprehension. We briefly used this handy resource earlier to filter out items in a list with a loop.<\/p>\n
A list comprehension can iterate through lists to access (or outright change) items and list values. This saves you the trouble of using the append method to new lists, saving you considerable time. Let’s take a closer look at how this loop with list Python works:<\/p>\n
listComp = [22, 44, 66, 88, 110]\r\nprint([item * 2.0 for item in listComp])\r\n\r\n[44.0, 88.0, 132.0, 176.0, 220.0]<\/pre>\nWhen used correctly, list comprehensions can make your code leaner and easier to understand.<\/p>\n
Looping Through a List in Python With While Loops<\/h2>\n
You can also loop through your lists with a traditional while<\/code> loop. A while<\/code>loop will execute a block of code over and over as long as the while statement condition is True<\/code>. A while<\/code> loop has:<\/p>\n\n- The
while<\/code> keyword<\/li>\n- A condition that evaluates to
True<\/code> or False<\/code><\/li>\n- A clause<\/li>\n<\/ul>\n
Here’s what a while<\/code> loop looks like:<\/p>\ngreetings = 0\r\nwhile greetings < 5:\r\n print('Hello there!')\r\n greetings = greetings + 1\r\n\r\nHello there!\r\nHello there!\r\nHello there!\r\nHello there!\r\nHello there!<\/pre>\nThis while<\/code> loop stops after five prints because the integer in greetings<\/code> is incremented by one at the end of each loop iteration. Once the next iteration begins, the condition is checked, and if it evaluates to True<\/code>, then the clause is executed once more. The cycle continues until the condition is found to be False<\/code>.<\/p>\nWe can use a while<\/code> loop to create lists that work with our user’s input. Since most of our lists have featured popular foods and dishes, let’s create an automated service that takes your lunch order!<\/p>\nlunch_order = []\r\nwhile True:\r\n print('Enter dish #' + str(len(lunchOrder) + 1) + ' (Or enter nothing to stop.):')\r\n dishes = input()\r\n if dishes == ' ':\r\n break\r\n lunchOrder = lunchOrder + [dishes]\r\nprint('Here is your order:')\r\nfor dishes in lunchOrder:\r\n print(' ' + dishes)<\/pre>\nThis while<\/code> loop will take your customer’s order – one dish at a time – and concatenate the dishes into a lunch_order<\/code> list. Customers can input an empty space to break the loop. Here’s what it would look like:<\/p>\nEnter dish #1 (Or enter nothing to stop.):\r\nmanicotti\r\nEnter dish #2 (Or enter nothing to stop.):\r\ncannolis\r\nEnter dish #3 (Or enter nothing to stop.):\r\napple pie\r\nEnter dish #4 (Or enter nothing to stop.):\r\nchocolate cake\r\nEnter dish #5 (Or enter nothing to stop.):\r\n \r\nHere is your order:\r\n manicotti\r\n cannolis\r\n apple pie\r\n chocolate cake<\/pre>\nNote<\/strong>: Using a while True<\/code> loop (a.k.a. infinite loop) is considered a code smell<\/a> and bad practice by many. Unless one has a specific reason for which a loop should use the while True<\/code> condition it is recommended to always have an explicitly-stated condition that could conceivably evaluate False.<\/p>\nFinal Words<\/h2>\n
Lists are, by far, one of the most powerful data structures in Python. Combined with loops, they can automate a tremendous amount of work in just a few lines of code. A well-coded loop through a list in Python will allow you to create complex calculators and input-dependent programs that save you time and effort while dramatically improving its users’ experience.<\/p>\n","protected":false},"excerpt":{"rendered":"
Learn how to loop through a list in Python in record time with our short, step-by-step guide! Learning how to loop through a list in Python is a basic yet incredibly powerful building block that you’ll use throughout your career as a programmer. It’s an excellent starting point for coders who wish to understand how […]<\/p>\n","protected":false},"author":4,"featured_media":2909,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"inline_featured_image":false,"footnotes":""},"categories":[186,655],"tags":[538,143],"acf":[],"_links":{"self":[{"href":"https:\/\/www.alpharithms.com\/wp-json\/wp\/v2\/posts\/2807"}],"collection":[{"href":"https:\/\/www.alpharithms.com\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.alpharithms.com\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.alpharithms.com\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/www.alpharithms.com\/wp-json\/wp\/v2\/comments?post=2807"}],"version-history":[{"count":9,"href":"https:\/\/www.alpharithms.com\/wp-json\/wp\/v2\/posts\/2807\/revisions"}],"predecessor-version":[{"id":2932,"href":"https:\/\/www.alpharithms.com\/wp-json\/wp\/v2\/posts\/2807\/revisions\/2932"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.alpharithms.com\/wp-json\/wp\/v2\/media\/2909"}],"wp:attachment":[{"href":"https:\/\/www.alpharithms.com\/wp-json\/wp\/v2\/media?parent=2807"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.alpharithms.com\/wp-json\/wp\/v2\/categories?post=2807"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.alpharithms.com\/wp-json\/wp\/v2\/tags?post=2807"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
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