Lesson 10 Using and Sorting Lists

Transcription

1 Lesson What you will learn: how to create variables how to create lists how to add values to lists how to create ordered lists how to use IF ELSE how to create a list in a random order with no duplicates how to sort a random list back into order using a bubble sort Contents Exercise 1: Setting all the Variables and Lists to Zero... 3 Exercise 2: Creating a Random List... 4 Exercise 3: Bubble Sort... 5 Exercise 4: Using the List to Set Coordinates... 6 Introduction to Scratch Programming (2 nd Edition) Page 1 of 9 ZigZag Education, 2013

2 Sorting data into lists is a very useful process in computing. For example, in this exercise we will use a list of numbers to set the position of sprites on a screen using coordinates. We will create an ordered list, then reorder the values in the list into a random order and then use a technique called bubble sort to reorder the list. 1. Start by creating the variables as shown: End will be the highest value in the sorted list. Length will be the total number of values used in the list. Position will be the position of a value in the list. Start will be the lowest value in the sorted list. Temp will be used to store a temporary value in the bubble sort process. Value will hold the range of numbers in between the start and end values. Randomlist will hold the random values without repeats. Sortedlist will be used to create the sorted values in a given range between start and end. - Introduction to Scratch Programming (2 nd Edition) Page 2 of 9 ZigZag Education, 2013

3 Exercise 1: Setting all the Variables and Lists to Zero 2. Working on the stage, create a script as shown. 3. Create a script that will create the ordered list (push the first script out of the way). -3 has been used here because it will determine a coordinate on the screen later in another example any number would work. 3 has been used here because it will determine a coordinate on the screen later in another example any number would work. This section sets value to start then adds it to the list. Then value goes up by one and that is added to the list, and so on. Introduction to Scratch Programming (2 nd Edition) Page 3 of 9 ZigZag Education, 2013

4 Exercise 2: Creating a Random List It is easy to create random numbers the trick here is to produce random numbers that never repeat. We can achieve this by pulling the numbers from the sorted list in random order then deleting that number so that it cannot be used again. 4. Still working on the stage, create the script as shown: The purpose of the r is so that each section of the exercise can be run separately. The purpose of the wait is so that you can see each stage happening. The purpose of the broadcast command at the end of this script is to move some sprites around the screen using coordinates. We will set this up later in the exercise. Just select new from the drop-down list and then type randomise. Test the scripts that you have created so far you should see the ordered list populate then the numbers move across to the random list. Introduction to Scratch Programming (2 nd Edition) Page 4 of 9 ZigZag Education, 2013

5 Exercise 3: Bubble Sort Now we will create the script that sorts the numbers back into the correct order. This is done by a process known as bubble sort. In a bubble sort, the first two numbers are compared; if the second is smaller, then they are swapped over. Then the next two numbers are compared and so on. Eventually all the numbers are in order. It is an effective method for short lists and is one of many methods for sorting lists of numbers. 5. Still working on the stage, create the script as shown: At the beginning, length has to be reset because the last process reset it to 1. The If Else section compares the first two values if the first value is LARGER, it is copied into temp and the second value is used to replace it. The value in temp is then copied into the second value place. Then the position is changed by one. If the first value ISN T LARGER, the position value just moves to the next position. The wait is just so that you can see what is going on and the broadcast reorder is to trigger another process that we will come to next. Introduction to Scratch Programming (2 nd Edition) Page 5 of 9 ZigZag Education, 2013

6 Exercise 4: Using the List to Set Coordinates 6. Introduce some new sprites onto the screen which are the numbers Set the stage out as shown. We have left the cat to give instructions to the end user. Introduction to Scratch Programming (2 nd Edition) Page 6 of 9 ZigZag Education, 2013

7 8. For the cat sprite, create the script as shown: 9. For the number 1 sprite, create the script as shown: Introduction to Scratch Programming (2 nd Edition) Page 7 of 9 ZigZag Education, 2013

8 Each number sprite has a different starting position shown in the first script. The item number corresponds to the actual number of the sprite. Sprite number Start position (x) Start position (y) When you run the script, the numbers should go into the correct order, change order randomly when you type r and then go back into order again when you type s. Introduction to Scratch Programming (2 nd Edition) Page 8 of 9 ZigZag Education, 2013

9 In this example, the number sprites have been replaced by playing cards. The effect is that the cards shuffle, then automatically rearrange back into order again. Introduction to Scratch Programming (2 nd Edition) Page 9 of 9 ZigZag Education, 2013