7. The Garden with a Condition <<< Contents >>> 9. Peter in a Maze

8. Peter Walks on Marks

In another program, we will learn how to use conditional performing of commands. We will let Peter walk around the sheet, onto which we will put various marks. According to the marks, Peter will choose his direction.

First, we will create a new program. As you sure remember from chapter 4, a new program is created by clicking the New button in the program window. The new program will be called Marks.

In the program, we will need two items that will be used as marks. In the Global Variables and Functions window, copy (with the right mouse button) the empty square element into two new elements. Name them Left and Right. To remind you: You can assign a name by, for example, selecting the element by clicking its icon, and pressing Alt+Enter to edit the name.

Put the new items into editing mode by double-clicking their icons. Into the Left element, draw an arrow turned to the left; into the Right element, draw an arrow turned to the right, e.g. like this: and .

We will prepare the sheet. Double-click the sheet element. In the editing window, the program sheet appears. We will create a path for Peter. Put the Left and Right elements on the sheet in such a way that Peter can follow them in a closed track. Remember that Peter starts from the bottom left corner to the right. To put the items on the sheet, drag both of them from the Global Variables and Functions window to a place on the sheet first, and drop them there. Then you can move the items with the left mouse button, or copy them with the right mouse button. An item that is not necessary can be deleted by moving it out of the sheet. The sheet may look like this (Peter’s path is indicated by the dashed line with arrows):

When the program sheet is prepared, we can start to assemble our program. This time we will begin with the main cycle. Peter has to keep moving along the marks, which means that the program will be based on a never-ending cycle. For this reason, we will prepare the conditional repeating of commands element. Actually, the cycle will not be never-ending; it will be possible to end it by pressing the Esc key. However, we will learn a few facts about the keyboard first.

When a key on the keyboard is pressed, it sends its numeric code to the computer, which is something like a serial number of the key on the keyboard. The computer converts this code into a character, which can be passed to a program, e.g. as a letter or a number. In our program, not only the characters, but also the numeric codes of the keys are available. The function for character input is used for typing texts from the keyboard, as we know it from common writing of texts (e.g. holding Shift generates capitals). The function for key input serves for controlling programs and games (even control keys, like Ctrl, generate key codes, although they do not generate characters).

When a key is pressed, its character or code is stored in a stack. The reason is that when the key is pressed, the program might not be ready to accept it. When the program is ready, it accepts the key from the stack. The key is deleted from the stack then. When you use keyboard input functions, you have to remember that when you load the character or the code from the keyboard, it is discarded, and the next loading returns something else.

Characters and keys have separate stacks, which are independent of each other. They are as separate data flows. One channel is used for characters, the other for key codes.

Now we can prepare the condition for quitting the main program cycle with the Esc key. We will use a function for the input of the key code from the keyboard. We know that it returns a numeric code, and so we will need a function for comparing numbers. Such functions are located in the calculations group under comparisons . We will use the is not equal to function. Drag it into the while is valid cycle condition.

The first element to be compared is key input (does not wait for press) . We will drag it to the comparative function from the controls group, keyboard sub-group. We have already used a similar element (waiting for a key to be pressed) in the previous chapters. The difference between these two elements is that this element does not wait for a key to be pressed. If no key code is ready, it returns the code of a situation where no key is pressed (it is the number 0, but we do not have to know the value, as we have a no key pressed symbolic code).

The second element to compare is the Esc key. It is into the same group as the key input function, but it is nested deeper, under keys and control keys . This element is a numeric constant with the value of the Esc key code, which means that we do not have to know the code. Put this element under the key input function element in the comparative function.

What does the cycle do now? Read the notes besides the elements: "While it is valid that key input is not equal to Esc key, repeat commands (something)." This sounds rather complicated, but the meaning is clear, perhaps. The cycle will be repeated, until the Esc key is pressed on the keyboard.

Let us fill in the repeat commands cycle body. During his walk, Peter will decide his direction accordingly to the square in front of him. The decision will be made using a new element, conditional executing of commands . It is in the program control group. Drag it into the repeat commands cycle body.

When you insert the element for conditional execution of commands, you may notice that it contains another three elements. The first one is if valid . It is a test of a condition. We already used a similar element in the conditional cycle. The element tests a condition, and if it is true, the commands in the then do elements will be executed. If the condition is not true, the commands in the else do element will be performed.

In the condition’s test, we will detect if there is an item for turning left in front of Peter. We know a similar test from the previous chapter, and so it is clear that we will use the following elements: item identity , item in front of Peter and Left .

Into the first branch (then do ), we will put two commands, step and left . The conditional command construction has this meaning now: "If there is an item for turning to the left in front of Peter, Peter will make a step and turn left, in other cases, he will do something else."

Now we will create that "something else". Using the right button, copy the whole construction of the conditional command to a lower place. Drag the newly created conditional command with the left mouse button, and drop it in the second branch of the first conditional command (the else do branch). This is a quick way to prepare the part of the program for the second case, turning to the right.

In the new conditional command, replace Left with Right . In a similar way, replace left with right . Now you probably know what Peter will do if there is not an item for turning left in front of him. He will detect, whether there is an item telling him to turn right. If there is, he will make a step and turn to the right. If not... What should he in fact do otherwise? We do not want anything more; it is enough if he makes a step . Copy the element for a step with the right mouse button from the second branch of the condition.

Did it look complicated? Don’t worry — the program is ready (its picture follows). Run the program. If everything is all right, Peter will run along the marks and turn to the left or right on them. Do you want to make Peter go faster? Double-click the Peter element to edit Peter’s sprite. Click the Properties button. A window for setting the sprite properties appears. Into the Phases per Step field, type 4, and then press Enter. Run the program again. Peter will run like crazy now, but he will not gasp for his breath.

Try to prepare a more complicated track for Peter. Here is one example:

7. The Garden with a Condition <<< Contents >>> 9. Peter in a Maze