ListsAndLoops - Again

In this tutorial we will provide additional examples of loop algorithms that process lists of data.

Using a While Loop to Create a List

Recall the MakeList procedure, which uses a while loop to create a list of random numbers:

To MakeList do:
   Set global theList to make-an-empty-list
   While length(theList) < listSize do:
      add a random number between 0 and 9 inclusive to theList

Each time the 'do' slot is executed, a random integer between 0 and 9 is added to thelist. Each time through the loop, the list will grow in size until it reaches a length of size at which point the test will be false and the loop will terminate.

While Statement Vs. While Structure

As shown in this figure a correct while-loop statement must be combined with initialization steps and must contain update steps.

Initialize, Test, Update

Initialization Step: Before we enter a while loop we need to perform some initializations. In this case we initialize theList to the empty list. And the listSize argument is also initialized to the value given when the procedure is called.

Loop Entry Condition: Before the while loop performs the do slot, it must test that the loop-entry-condition is true.

Update Step: At least one operation in the do slot must make progress toward the termination of the loop -- e.g., increasing the size of the list.

Question: What would happen if we didn't make progress toward termination? Question: What would happen if the test never became false? Watson: What is an infinite loop?

Using a For-each loop to Process a List

When dealing with lists, App Inventor provides a for-each loop that will perform some operation for each element on the list.

There are three slots that need to be filled in to define a for-each loop:

1. The variable slot -- Create your own named variable using Definition > name
2. The in list slot -- This should a global list variable or a list argument
3. The do -- This is where the operations go.

Sum the Elements in a List of Numbers

For example, suppose we want to compute the sum of the numbers on our random list. Let's write a Pseudocode function to do that, using the idea of for-each-item-in-a-list:

# Function using a for-each loop to compute the sum of a list of numbers.
# In this case the variable is named item
# In this case the in list is named aList

To computeListSum(aList) :
   Set global Sum to 0
   For each item is aList do:
      Set Sum to Sum + item
   Return Sum

And here is the App Inventor definition.

Let's use App Inventor's Watch and DoIt features to test our definition:

Abstraction

Question: Would you say that the for-each loop structure is an abstraction? Why or why not?

Exercises

Working in pairs, perform each of the following exercises. If you do not finish in class, do the exercises for homework. HINT: Some of the operations required for these exercises use built-in functions that can be found in the App Inventor Lists and Text menus.

1. Rewrite the ComputeListSum function to use a while loop instead of a for-each loop.

2. Write and test a function named filterOutZeroes that uses a for-each loop to remove all the 0s from a list of numbers. For example, if the original list is (0 1 0 0 2 0 2), then your function should return the list (1 2 2). Your function should have 1 parameter -- i.e., the list.

3. Write and test a function named countZeroes that will count the number of zeroes in a list. For example, if the list is (0 1 0 0 2 0 2), the function should return 4.

4. Write and test a function named convertListToText that uses a for-each loop to convert a list of numbers into a string where the numbers are separated by commas. For example, if the list was (1 2 4), the function would return '1,2,4'. (HINT: Join each item to a string.)