# Why does the LED turn on after the while loop becomes false?

**URL:** <https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393>\
**Category:** Programming\
**Created:** [August 23, 2021, 2:36pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393 "2021-08-23T14:36:03Z")\
**Posts on this page:** 20\
**Page:** 1

<div class="post-metadata">

**Author:** ![adrianoles](https://avatars.discourse-cdn.com/v4/letter/a/9fc348/32.png) [@adrianoles](https://forum.arduino.cc/u/adrianoles)\
**Post date:** [August 23, 2021, 2:36pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/1 "2021-08-23T14:36:03Z")

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I don't understand why the LED won't blink until I let go of the button, (making the while loop false.) Would it not continue to blink the LED until the while loop becomes false?

```cpp
void setup(){
  pinMode(13,OUTPUT);
  pinMode(3, INPUT_PULLUP); //Not required
}

void loop(){

  if(digitalRead(3)== 0)
  {
    while(digitalRead(3) == 0);
    digitalWrite(13, HIGH);
    delay(1000);
    digitalWrite(13, LOW);
    delay (1000);

    digitalWrite(13, HIGH);
    delay(1000);
    digitalWrite(13, LOW);
    delay (1000);
  
  }
}

```

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<div class="post-metadata">

**Author:** ![gcjr](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/gcjr/32/314015_2.png) [@gcjr](https://forum.arduino.cc/u/gcjr)\
**Post date:** [August 23, 2021, 2:53pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/2 "2021-08-23T14:53:47Z")

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> [@adrianoles](#):
>
> ` while(digitalRead(3) == 0);`

the ";" at the end of the line is the while loop body which will be executed while the pin is LOW.

i don't think you need the while loop

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<div class="post-metadata">

**Author:** ![adrianoles](https://avatars.discourse-cdn.com/v4/letter/a/9fc348/32.png) [@adrianoles](https://forum.arduino.cc/u/adrianoles)\
**Post date:** [August 23, 2021, 3:20pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/3 "2021-08-23T15:20:22Z")

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I see. I'm just trying to understand other people's code so I can implement their pieces into mine.

I think I understand it now though...  
The while loop will continue to check the button's value once it is low. But when you let go to make it high, the while loop stops and it executes the blinks.

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<div class="post-metadata">

**Author:** ![adrianoles](https://avatars.discourse-cdn.com/v4/letter/a/9fc348/32.png) [@adrianoles](https://forum.arduino.cc/u/adrianoles)\
**Post date:** [August 23, 2021, 3:28pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/4 "2021-08-23T15:28:49Z")

</div>

This question is off topic, but to avoid making another thread I thought asking you would be much easier...

When you use the increment operator, is there a way to make it continuously increment until a certain value (ex. 8), then reset to 0 and repeat?

As this code only increments every time the button is pressed.

```cpp
void loop(){

  if(digitalRead(3)== 0)
  {
    while(digitalRead(3) == 0);

    ledMode++; 
  
  }

```

---

<div class="post-metadata">

**Author:** ![gcjr](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/gcjr/32/314015_2.png) [@gcjr](https://forum.arduino.cc/u/gcjr)\
**Post date:** [August 23, 2021, 3:35pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/5 "2021-08-23T15:35:24Z")

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> [@adrianoles](#):
>
> is there a way to make it continuously increment until a certain value (ex. 8), then reset to 0 and repeat?

no. you'll need to test for that value and reset it or use the mod operator

```arduino
ledMode = ++ledMode % 8;

```

it's odd the way this code is conditional on the button being pressed (== 0) and then waits for it to go HIGH.

doing things in conventional ways makes it easier to recognize what the code is doing and recognizing bugs

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<div class="post-metadata">

**Author:** ![adrianoles](https://avatars.discourse-cdn.com/v4/letter/a/9fc348/32.png) [@adrianoles](https://forum.arduino.cc/u/adrianoles)\
**Post date:** [August 23, 2021, 3:43pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/6 "2021-08-23T15:43:36Z")

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Ah, okay. Thank you, your help is much appreciated!

Agreed, I thought this was an interesting workaround for the debounce and button press issue I had earlier. But, I suppose it's best for the long term to stay conventional.

---

<div class="post-metadata">

**Author:** ![TomGeorge](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/tomgeorge/32/215261_2.png) [@TomGeorge](https://forum.arduino.cc/u/TomGeorge)\
**Post date:** [August 23, 2021, 3:46pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/7 "2021-08-23T15:46:05Z")

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Hi,  
How have you got your button wired?  
1.Between digital input and gnd.  
**OR**  
2. Between digital input and 5V

If 1. Have you got a 10K pullup resistor between the digital input and 5V?  
If 2. Have you got a 10K pulldown resistor between the digital input and gnd?

What model Arduino are you using?

Thanks... Tom... 😃 👍 ☕ 🇦🇺

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<div class="post-metadata">

**Author:** ![gcjr](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/gcjr/32/314015_2.png) [@gcjr](https://forum.arduino.cc/u/gcjr)\
**Post date:** [August 23, 2021, 3:46pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/8 "2021-08-23T15:46:59Z")

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> [@adrianoles](#):
>
> I thought this was an interesting workaround for the debounce

because the code being executed has delays, there's no need to debounce.

in other cases recognizing a change and delaying for 20 msec is sufficient

---

<div class="post-metadata">

**Author:** ![adrianoles](https://avatars.discourse-cdn.com/v4/letter/a/9fc348/32.png) [@adrianoles](https://forum.arduino.cc/u/adrianoles)\
**Post date:** [August 23, 2021, 3:48pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/9 "2021-08-23T15:48:33Z")

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The button is just wired via GND and pin 3. To workaround using an external resistor I just implemented `pinMode(3, INPUT_PULLUP);`

- UNO r3 (Third party - ELEGOO)

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<div class="post-metadata">

**Author:** ![alto777](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/alto777/32/317385_2.png) [@alto777](https://forum.arduino.cc/u/alto777)\
**Post date:** [August 23, 2021, 3:56pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/10 "2021-08-23T15:56:45Z")

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> [@gcjr](#):
>
> it's odd the way this code is conditional on the button being pressed (== 0) and then waits for it to go HIGH.

I do this alla time, it is a lazy man’s deboincing. Sprinkle a few 10 ms delays around in there and get on with the bigger problem.

Although here it does seem wrongly done even as a hack.

I am so lazy. Usually I just use an externally denounced pushbutton… so catch the press and hang until it goes unpressed.  
.  
a7

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<div class="post-metadata">

**Author:** ![blh64](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/blh64/32/605986_2.png) [@blh64](https://forum.arduino.cc/u/blh64)\
**Post date:** [August 23, 2021, 4:26pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/11 "2021-08-23T16:26:29Z")

</div>

> [@gcjr](#):
>
> `ledMode = ++ledMode % 8;`

Isn't that undefined behavior according to the standard? I am no expert but

```arduino
ledMode = (ledMode+1) % 8;

```

will always work

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<div class="post-metadata">

**Author:** ![gcjr](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/gcjr/32/314015_2.png) [@gcjr](https://forum.arduino.cc/u/gcjr)\
**Post date:** [August 23, 2021, 4:32pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/12 "2021-08-23T16:32:03Z")

</div>

> [@blh64](#):
>
> Isn't that undefined behavior according to the standard?

why?

++ledMode (vs ledMode++) mean increment before the value is used

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<div class="post-metadata">

**Author:** ![blh64](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/blh64/32/605986_2.png) [@blh64](https://forum.arduino.cc/u/blh64)\
**Post date:** [August 23, 2021, 4:35pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/13 "2021-08-23T16:35:01Z")

</div>

> [@gcjr](#):
>
> ledMode = ++ledMode % 8;

But I don't believe there is a sequence point in that expression that guarantees the preincrement completes before the assignment. But again, I am NOT a standards expert guy.

---

<div class="post-metadata">

**Author:** ![gcjr](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/gcjr/32/314015_2.png) [@gcjr](https://forum.arduino.cc/u/gcjr)\
**Post date:** [August 23, 2021, 4:35pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/14 "2021-08-23T16:35:44Z")

</div>

what do you think preincrement means?

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<div class="post-metadata">

**Author:** ![gcjr](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/gcjr/32/314015_2.png) [@gcjr](https://forum.arduino.cc/u/gcjr)\
**Post date:** [August 23, 2021, 4:40pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/15 "2021-08-23T16:40:02Z")

</div>

the following output and code

```cpp
      1
      2
      3
      4
      5
      6
      7
      0
      1
      2

```

```cpp
#include <stdio.h>

int
main ()
{
    int ledMode = 0;
    for (int i = 10; i > 0; i--) {
#if 1
        ledMode = ++ledMode % 8;
#else
        ledMode = ledMode++ % 8;
#endif
        printf (" %6d\n", ledMode);
    }
}

```

and this is the result doing a post increment

```cpp
      0
      0
      0
      0
      0
      0
      0
      0
      0
      0

```

```cpp
#include <stdio.h>

int
main ()
{
    int ledMode = 0;
    for (int i = 10; i > 0; i--) {
#if 1
        ledMode = ++ledMode % 8;
#else
        ledMode = ledMode++ % 8;
#endif
        printf (" %6d\n", ledMode);
    }
}

```

---

<div class="post-metadata">

**Author:** ![PerryBebbington](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/perrybebbington/32/238766_2.png) [@PerryBebbington](https://forum.arduino.cc/u/PerryBebbington)\
**Post date:** [August 23, 2021, 5:39pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/16 "2021-08-23T17:39:15Z")

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@adrianoles  
I think you will find the following tutorials helpful:

My take on reading and responding to buttons:

> [@Buttons and other electro-mechanical inputs (introduction)](https://forum.arduino.cc/t/buttons-and-other-electro-mechanical-inputs-introduction/688576):
>
> Intended audience This tutorial is intended for people who have mastered the basics of electronics and C/C++ programming. If you are a beginner and find the concepts difficult, please review the basic examples in the Arduino IDE. In this tutorial BUTTON will be a generic term for any kind of switch or other input device with mechanical contacts. Introduction A common requirement in many projects is to have push buttons, switches or other electro-mechanical devices connected as inputs to an A…

> [@Using millis() for timing. A beginners guide](https://forum.arduino.cc/t/using-millis-for-timing-a-beginners-guide/483573):
>
> Part 1 It is not usually long before new Arduino users discover that although the delay() function is easy to use it has side effects, the main one of which is that its stops all activity on the Arduino until the delay is finished (not quite true, I know, but that is usually how the problem presents itself). When this occurs the new user is usually directed to the BlinkWithoutDelay example sketch (BWoD) and/or the excellent thread entitled Several things at the same time. [Several things at the…](http://forum.arduino.cc/index.php?topic=223286.0)

There is also blink without delay in the Arduino IDE.

I hope those help you.

---

<div class="post-metadata">

**Author:** ![blh64](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/blh64/32/605986_2.png) [@blh64](https://forum.arduino.cc/u/blh64)\
**Post date:** [August 23, 2021, 7:13pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/17 "2021-08-23T19:13:50Z")

</div>

preincrement means exactly what you think, you add 1 to the variable before using said value. It also stores this incremented value back into the original location. The assignment operator also stores a value back into the same location. You now have two writes to the same memory location and no guarantee about which one will happen first. Just because the arduino code does the right thing, does not mean it doesn't fall into the undefined behavior category.

EDIT: This article confirms it: [c++ - Undefined behavior and sequence points - Stack Overflow](https://stackoverflow.com/questions/4176328/undefined-behavior-and-sequence-points)

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<div class="post-metadata">

**Author:** ![alto777](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/alto777/32/317385_2.png) [@alto777](https://forum.arduino.cc/u/alto777)\
**Post date:** [August 23, 2021, 7:35pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/18 "2021-08-23T19:35:15Z")

</div>

> [@blh64](#):
>
> But again, I am NOT a standards expert guy.

So let’s don’t learn from you all about this confusing matter you bring up.

google out of curiosity, I found this

```
 i = ++i + 2; // undefined behavior until C++11

```

I tend to have trouble learning about these things, so I’m just as happy to write things like that out as a few statements. Clearer for every body, no disagreements.

```
 number++; number &= 0x7; // ;-)

```

a7

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<div class="post-metadata">

**Author:** ![gcjr](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/gcjr/32/314015_2.png) [@gcjr](https://forum.arduino.cc/u/gcjr)\
**Post date:** [August 23, 2021, 7:52pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/19 "2021-08-23T19:52:00Z")

</div>

> [@blh64](#):
>
> You now have two writes to the same memory location and no guarantee about which one will happen first.

the pre-increment is performed first before the value is used for other operations.

arduino uses the industry standard GNU CC compiler

i don't understand what is underfined. the compiler needs to access the value of the variable. the value is affected by the increment -- first.

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<div class="post-metadata">

**Author:** ![alto777](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/alto777/32/317385_2.png) [@alto777](https://forum.arduino.cc/u/alto777)\
**Post date:** [August 23, 2021, 7:58pm UTC](https://forum.arduino.cc/t/why-does-the-led-turn-on-after-the-while-loop-becomes-false/898393/20 "2021-08-23T19:58:53Z")

</div>

There is nothing to order the two times when a value of the variable is put into the memory for that variable.

The sequence points idea is exploited by compiler optimisations, this is exactly why the statement as it was written can produce unintended effects.

Until C++11.

But I’m not standards and experts “guy”.

a7

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