Программирование LEGO NXT роботов на языке NXC - Управляющие структуры
Автор: Daniele Benedettelli
Перевод: © Ботов Антон aka =DeaD=, 2009
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Управляющие структуры
In the previous chapters we saw the repeat and while statements. These statements control the way the other statements in the program are executed. They are called “control structures”. In this chapter we will see some other control structures.
Оператор "if"
Sometimes you want that a particular part of your program is only executed in certain situations. In this case the if statement is used. Let me give an example. We will again change the program we have been working with so far, but with a new twist. We want the robot to drive along a straight line and then either make a left or a right turn. To do this we need random numbers again. We pick a random number that is either positive or negative. If the number is less than 0 we make a right turn; otherwise we make a left turn. Here is the program:
- define MOVE_TIME 500
- define TURN_TIME 360
task main() { while(true) { OnFwd(OUT_AC, 75); Wait(MOVE_TIME); if (Random() >= 0) { OnRev(OUT_C, 75); } else { OnRev(OUT_A, 75); } Wait(TURN_TIME); } } The if statement looks a bit like the while statement. If the condition between the parentheses is true the part between the brackets is executed. Otherwise, the part between the brackets after the word else is executed. Let us look a bit better at the condition we use. It reads Random() >= 0. This means that Random() must be greater-than or equal to 0 to make the condition true. You can compare values in different ways. Here are the most important ones: == equal to < smaller than <= smaller than or equal to > larger than >= larger than or equal to != not equal to You can combine conditions use &&, which means “and”, or ||, which means “or”. Here are some examples of conditions: true always true false never true ttt != 3 true when ttt is not equal to 3 (ttt >= 5) && (ttt <= 10) true when ttt lies between 5 and 10 (aaa == 10) || (bbb == 10)true if either aaa or bbb (or both) are equal to 10 - 17 - Note that the if statement has two parts. The part immediately after the condition, which is executed when the condition is true, and the part after the else, which is executed when the condition is false. The keyword else and the part after it are optional. So you can omit them if there is nothing to do when the condition is false.
Оператор "do"
There is another control structure, the do statement. It has the following form: do { statements; } while (condition); The statements between the brackets after the do part are executed as long as the condition is true. The condition has the same form as in the if statement described above. Here is an example of a program. The robot runs around randomly for 20 seconds and then stops. int move_time, turn_time, total_time; task main() { total_time = 0; do { move_time = Random(1000); turn_time = Random(1000); OnFwd(OUT_AC, 75); Wait(move_time); OnRev(OUT_C, 75); Wait(turn_time); total_time += move_time; total_time += turn_time; } while (total_time < 20000); Off(OUT_AC); } Note also that the do statement behaves almost the same as the while statement. But in the while statement the condition is tested before executing the statements, while in the do statement the condition is tested at the end. For the while statement, the statements might never be executed, but for the do statement they are executed at least once. Summary In this chapter we have seen two new control structures: the if statement and the do statement. Together with the repeat statement and the while statement they are the statements that control the way in which the program is executed. It is very important that you understand what they do. So better try some more examples yourself before continuing.
