在C ++中对布尔进行NOT运算符

Question

I'm trying to use this Arduino code in my program, the LED will stay on for 5 sec then turns itself off

#include <elapsedMillis.h>

int led = 13 ;
elapsedMillis timer0 ;
#define interval 5000
boolean timer0Fired ;

// the setup routine runs once when you press reset:
void setup() 
{
    pinMode(led, OUTPUT);
    digitalWrite(led, HIGH);
    timer0Fired = false;
    timer0 = 0; // clear the timer at the end of startup
}

void loop()
{
    if ((!timer0Fired) && (timer0 > interval))
    {
        timer0Fired = true;      // don't execute this again
        digitalWrite(led, LOW);  // turn led off after 5 sec
    }
}

I don't understand how the if statement works inside the loop, !timer0Fired should evaluates to 1 but when I print it out it's 0, so when timer0 exceeds the interval the if statement should evaluates to be false, I tried it on a UNO and it works.

Answer 1

!timer0Fired should evaluates to 1 but when I print it out it's 0

When you print it, do you print "!timer0Fired" or "timer0Fired"? It would explain the output.

so when timer0 exceeds the interval the if statement should evaluates to be false

Currently, "(timer0 > interval)" evaluates to true when timer0 exceeds the interval, not the contrary. Isn't it correct?

Answer 2

This is just a basic programming logic.

Imagine that you want to count to 5 and call some function, eg DoSomething , in and endless loop.

int count = 0;
bool called = false; // DoSomething not called yet

while (true) // main loop
{
    ++count;

    if (count == 5)
    {
        // the count is 5
        if (called == false)
        {
            // function not called yet, call it!
            called = true;
            DoSomething();
        }
    }
}

This might seem as a non-sense in a way. Just wait for it...

However, the problem you're facing is that you don't have a simple counter like mine count . Instead, you're using a timer that could be a few milliseconds off and you still want the code to execute even if it's late.

For example:

int count = 0;
bool called = false; // DoSomething not called yet

while (true) // main loop
{
    count += 2; // emulate counter being late

    if (count >= 5) // note that count == 5 will be always false here...
    {
        // the count is 5 or more
        if (called == false)
        {
            // function not called yet, call it!
            called = true;
            DoSomething();
        }
    }
}

Now that completely inverses the value of the if s. This way, the application almost always passes the first if but only once the second. In order to optimize this, you can just swap the if s:

if (called == false)
{
    if (count >= 5)
    {
        called = true;
        DoSomething();
    }
}

And now, as you probably know, these nested if statements can be easily grouped into one using && operator. Also, called == false can be made less verbose by using !called . This in the end results in

while (true) // main loop
{
    count += 2; // emulate counter being late

    if (!called && count >= 5)
    {
        // function not called yet and count is already 5 or more
        called = true;
        DoSomething();
    }
}

So what is going on is:
a) you want to execute a piece of code only once
- represented by bool called = false; if (!called) called = true; bool called = false; if (!called) called = true; logic or timer0fired in your code for the matter

b) you want to delay the call until some timer reaches a certain amount
- represented by count >= 5 or timer >= interval in your example

tl;dr
Your code is working correctly.