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Python loop through range list

Photo by bantersnaps on UnsplashThe for loop. It is a cornerstone of programming -- a technique you learn as a novice and one that you'll carry through the rest of your programming journey.If you're coming from other popular languages such as PHP or JavaScript, you're familiar with using a variable to keep track of your current index.It's critical to understand that these for loops do not actually iterate over the array; they manually iterate via an expression that serves as a proxy for referencing each array value.In the example above, i has no explicit relation to scores, it simply happens to coincide with each necessary index value.The traditional for loop as shown above does not exist in Python. However, if you're like me, your first instinct is to find a way to recreate what you're comfortable with.As a result, you may have discovered the range() function and come up with something like this.The problem with this for loop is that it isn't very "Pythonic". We're not actually iterating over the list itself, but rather we're using i as a proxy index.In fact, even in JavaScript there are methods of directly iterating over arrays (forEach() and for...of).If you want to properly keep track of the "index value" in a Python for loop, the answer is to make use of the enumerate() function, which will "count over" an iterable--yes, you can use it for other data types like strings, tuples, and dictionaries.The function takes two arguments: the iterable and an optional starting count.If a starting count is not passed, then it will default to 0. Then, the function will return tuples with each current count and respective value in the iterable.This code is so much cleaner. We avoid dealing with list indices, iterate over the actual values, and explicitly see each value in the for loop's definition.Here's a bonus, have you ever wanted to print a numbered list but had to print i + 1 since the first index is 0? Simply pass the value 1 to enumerate() and watch the magic!I hope this tutorial was helpful. What other uses of the enumerate() function have you found? Do you find the syntax easier to read than range(len())? Share your thoughts and experiences below! Get Python Cookbook now with O'Reilly online learning. O'Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers. Credit: Andy McKayYou need to loop through every item of multiple lists. There are basically three approaches. Say you have:a = ['a1', 'a2', 'a3'] b = ['b1', 'b2']Using the built-in function map, with a first argument of None, you can iterate on both lists in parallel: print "Map:" for x, y in map(None, a, b): print x, yThe loop runs three times. On the last iteration, y will be None. Using the built-in function zip also lets you iterate in parallel: print "Zip:" for x, y in zip(a, b): print x, yThe loop runs two times; the third iteration simply is not done.A list comprehension affords a very different iteration:print "List comprehension:" for x, y in [(x,y) for x in a for y in b]: print x, yThe loop runs six times, over each item of b for each item of a. Using map with None as the first argument is a subtle variation of the standard map call, which typically takes a function as the first argument. As the documentation indicates, if the first argument is None, the identity function is used as the function through which the arguments are mapped. If there are multiple list arguments, map returns a list consisting of tuples that contain the corresponding items from all lists (in other words, it's a kind of transpose operation). The list arguments may be any kind of sequence, and the result is always a list. Note that the first technique returns None for sequences in which there are no more elements. Therefore, the output of the first loop is: Map: a1 b1 a2 b2 a3 Nonezip lets you iterate over the lists in a similar way, but only up to the number of elements of the smallest list. Therefore, the output of the second technique is: Zip: a1 b1 a2 b2Python 2.0 introduced list comprehensions, with a syntax that some found a bit strange: [(x,y) for x in a for y in b]This iterates over list b for every element in a. These elements are put into a tuple (x, y). We then iterate through the resulting list of tuples in the outermost for loop. The output of the third technique, therefore, is quite different: List comprehension: a1 b1 a1 b2 a2 b1 a2 b2 a3 b1 a3 b2The Library Reference section on sequence types; documentation for the zip and map built-ins in the Library Reference. Get Python Cookbook now with O'Reilly online learning. O'Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers. The for loop in Python is used to iterate over a sequence (list, tuple, string) or other iterable objects. Iterating over a sequence is called traversal. Syntax of for Loop for val in sequence: Body of for Here, val is the variable that takes the value of the item inside the sequence on each iteration. Loop continues until we reach the last item in the sequence. The body of for loop is separated from the rest of the code using indentation. Flowchart of for Loop Flowchart of for Loop in Python Example: Python for Loop # Program to find the sum of all numbers stored in a list # List of numbers numbers = [6, 5, 3, 8, 4, 2, 5, 4, 11] # variable to store the sum sum = 0 # iterate over the list for val in numbers: sum = sum+val print("The sum is", sum) When you run the program, the output will be: The sum is 48 The range() function We can generate a sequence of numbers using range() function. range(10) will generate numbers from 0 to 9 (10 numbers). We can also define the start, stop and step size as range(start, stop,step_size). step_size defaults to 1 if not provided. The range object is "lazy" in a sense because it doesn't generate every number that it "contains" when we create it. However, it is not an iterator since it supports in, len and __getitem__ operations. This function does not store all the values in memory; it would be inefficient. So it remembers the start, stop, step size and generates the next number on the go. To force this function to output all the items, we can use the function list(). The following example will clarify this. print(range(10)) print(list(range(10))) print(list(range(2, 8))) print(list(range(2, 20, 3))) Output range(0, 10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] [2, 3, 4, 5, 6, 7] [2, 5, 8, 11, 14, 17] We can use the range() function in for loops to iterate through a sequence of numbers. It can be combined with the len() function to iterate through a sequence using indexing. Here is an example. # Program to iterate through a list using indexing genre = ['pop', 'rock', 'jazz'] # iterate over the list using index for i in range(len(genre)): print("I like", genre[i]) Output I like pop I like rock I like jazz for loop with else A for loop can have an optional else block as well. The else part is executed if the items in the sequence used in for loop exhausts. The break keyword can be used to stop a for loop. In such cases, the else part is ignored. Hence, a for loop's else part runs if no break occurs. Here is an example to illustrate this. digits = [0, 1, 5] for i in digits: print(i) else: print("No items left.") When you run the program, the output will be: 0 1 5 No items left. Here, the for loop prints items of the list until the loop exhausts. When the for loop exhausts, it executes the block of code in the else and prints No items left. This for...else statement can be used with the break keyword to run the else block only when the break keyword was not executed. Let's take an example: # program to display student's marks from record student_name = 'Soyuj' marks = {'James': 90, 'Jules': 55, 'Arthur': 77} for student in marks: if student == student_name: print(marks[student]) break else: print('No entry with that name found.') Output No entry with that name found. There are two types of loops in Python, for and while. The "for" loop For loops iterate over a given sequence. Here is an example: primes = [2, 3, 5, 7] for prime in primes: print(prime) For loops can iterate over a sequence of numbers using the "range" and "xrange" functions. The difference between range and xrange is that the range function returns a new list with numbers of that specified range, whereas xrange returns an iterator, which is more efficient. (Python 3 uses the range function, which acts like xrange). Note that the range function is zero based. # Prints out the numbers 0,1,2,3,4 for x in range(5): print(x) # Prints out 3,4,5 for x in range(3, 6): print(x) # Prints out 3,5,7 for x in range(3, 8, 2): print(x) "while" loops While loops repeat as long as a certain boolean condition is met. For example: # Prints out 0,1,2,3,4 count = 0 while count < 5: print(count) count += 1 # This is the same as count = count + 1 "break" and "continue" statements break is used to exit a for loop or a while loop, whereas continue is used to skip the current block, and return to the "for" or "while" statement. A few examples: # Prints out 0,1,2,3,4 count = 0 while True: print(count) count += 1 if count >= 5: break # Prints out only odd numbers - 1,3,5,7,9 for x in range(10): # Check if x is even if x % 2 == 0: continue print(x) Can we use "else" clause for loops? Unlike languages like C,CPP.. we can use else for loops. When the loop condition of "for" or "while" statement fails then code part in "else" is executed. If a break statement is executed inside the for loop then the "else" part is skipped. Note that the "else" part is executed even if there is a continue statement. Here are a few examples: # Prints out 0,1,2,3,4 and then it prints "count value reached 5" count=0 while(count

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