Loops ===== While loop videos ----------------- - `While Loop | Python | Tutorial 20 `_ - Counter variable - Loop condition - `Beginner Python Tutorial 69 - do while Loop in Python `_ - Post-test loop (Do while) - ``break`` - `Beginner Python Tutorial 70 - Indefinite Loop `_ - Post-test loop with user input - `How to Use "break" and "continue" in Python "while" Loops `_ - ``break`` - ``continue`` For loop videos --------------- - `Beginner Python Tutorial 51 - for Loop `_ - `Beginner Python Tutorial 53 - range() Function `_ - `Beginner Python Tutorial 54 - Range Starting Position `_ - `Beginner Python Tutorial 55 - Step in Range Explained `_ Loop with a counter variable ---------------------------- Three steps to looping with a counter variable: - what is the first value I want to output? - Set 'i' to that value. - what is the final value I want to output? - Include that in the while condition. - what do I want it to go up by? - Increase 'i' by that amount. .. code-block:: python i = 0 while i < 5: print("hello") i += 1 Practice Question Generator ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Try the `Loop Practice Question Generator `_. Press the "play" button to run the program. Loop with an accumulator variable --------------------------------- .. code-block:: python total = 0 # accumulator i = 1 # counter while i <= 10: total += i # add to the accumulator i += 1 # increase the counter break ----- .. code-block:: python i = 0 while i <= 10: print(i) if i == 5: break i += 1 continue -------- .. code-block:: python i = 0 while i <= 10: if i == 5 or i == 7: i += 1 continue print(i) i += 1 Get user input in a loop ------------------------ This is broken down in three patterns: 1. Create a loop that iterates five times. 2. Take user input in the loop. 3. Use an accumulator variable to store the input. .. code-block:: python i = 0 # counter total = 0 # accumulator while i < 5: # set to loop 5 times num = int(input("Enter number: ")) total += num i += 1 print(total) Quit loop with sentinel value ----------------------------- The sentinel value in the code below is ``-1``. The loop will continue forever until the user enters that vaue. We ``break`` when the sentinel value is encountered. .. code-block:: python total = 0 while True: num = int(input("Enter number, -1 to stop: ")) if num == -1: break total += num print(total) Loop through a string (while) ----------------------------- .. code-block:: python name = "Mr. Gallo" i = 0 while i < len(name): print(i, name[i]) i += 1 :: 0 M 1 r 2 . 3 4 G 5 a 6 l 7 l 8 o Loop through a list (while) --------------------------- .. code-block:: python friends = ["Frank", "Sally", "Jimbo"] i = 0 while i < len(friends): print(i, friends[i]) i += 1 :: 0 Frank 1 Sally 2 Jimbo Loop through a string (for) --------------------------- For loops can be a short cut, but, can sometimes be limiting because as they are looping, they don't have access to the index values of the elements like we see in :ref:`loops:Loop through a string (while)` and :ref:`loops:Loop through a list (while)`. For a work-around, see :ref:`loops:Loop using enumerate`. .. code-block:: python name = "Mr. Gallo" for character in name: print(character) :: M r . G a l l o Loop through a list (for) ------------------------- .. code-block:: python friend_list = ["Frank", "Sally", "Jimbo"] # For every friend in my friend list # print the friend for friend in friend_list: print(friend) :: Frank Sally Jimbo Loop using range ---------------- .. code-block:: python # while loop print 50-100 i = 50 while i <= 100: print(i) i += 1 # range(start, end)... Goes up to, but, doesn't include the end for num in range(50, 101): print(num) # while loop print 1-25 by 5 i = 0 while i <= 25: print(i) i += 5 # range(start, end, step) for num in range(0, 26, 5): print(num) Loop using enumerate -------------------- To have access to the index number while looping through a collection, we can use the ``enumerate`` function. This allows for the simplicity of a for loop, while still having access to the index value. .. code-block:: python name = "Mr. Gallo" for i, character in enumerate(name): print(i, character) :: 0 M 1 r 2 . 3 4 G 5 a 6 l 7 l 8 o String building and filtering ----------------------------- To build a string with a loop: 1. Create an empty string (*before* the loop). 2. Loop over whatever you need to loop over. 3. Add to the string inside the loop. .. code-block:: python :linenos: my_string = "" for n in range(65, 70): my_string += str(n) + " " print(my_string) # "65 66 67 68 69 " The same concept applies to building and filtering lists. .. code-block:: python :linenos: nums = [1, 6, -4, 1, -5, 1] only_ones = [] for n in nums: if n == 1: only_ones.append(n) print(only_ones) # [1, 1, 1]