# arduino UNO light LEDs right-to-left and then reversed

I have 8 LEDs lined up on a breadboard, and I am trying to make them run from one end to the other and back, sorta like the lights on KITT (from night rider - hope that clarifies). the LEDs are all set up and correct and work.

I have set up my code like so:

int ledPins[]={2,3,4,5,6,7,8,9};

void setup()
{
for (int i =0; i <8; i++)
{
pinMode(ledPins[i],OUTPUT);
}
}

void loop()
{
for(int i =0; i<8; i++)
{
digitalWrite(ledPins[i],HIGH);
delay(100);
digitalWrite(ledPins[i],LOW);
}

for(int i =7; i<1; i--)
{
digitalWrite(ledPins[i],HIGH);
delay(100);
digitalWrite(ledPins[i],LOW);
}
}


but it doesn't cycle the LEDs from right to left and then back like I was aiming for.

EDIT: What it does do - is light the LEDs from right to left, but doesn't light them the opposite way (left to right). It just keeps going from right to left, right to left

I am using an Arduino UNO, and basing my circuit off the experimentation kit for Arudino CIRC-02

EDIT #2: So I have changed the for-loops to while loops:

int ledPins[]={2,3,4,5,6,7,8,9};

void setup()
{
for (int i =0; i <8; i++)
{
pinMode(ledPins[i],OUTPUT);
}
}

void loop()
{
int i = 0;
while (i<7)
{
{
digitalWrite(ledPins[i],HIGH);
delay(100);
digitalWrite(ledPins[i],LOW);
i++;
}
}

i = 7;
while (i >0)
{
digitalWrite(ledPins[i],HIGH);
delay(100);
digitalWrite(ledPins[i],LOW);
i--;
}
}


and it works as intended! However, I am still interested in where my for-loop went wrong, if anybody has any ideas

• Saying what it DOES do may help. – Russell McMahon Aug 9 '11 at 13:36

In the second 'for' loop, i is immediately not <1, so the loop never executes.

for(int i =7; i<1; i--)


should say

for(int i =7; i>=0; i--)

• looks like I need to revise my for-loops! fantastic, thanks! – snowflakekiller Aug 9 '11 at 15:37
int ledPins[]={2,3,4,5,6,7,8,9,10,11,12};

void setup()
{
for (int i =0; i <11; i++)
{
pinMode(ledPins[i],OUTPUT);
}
}

void loop()
{
for(int i =0; i<11; i++)
{
digitalWrite(ledPins[i],HIGH);
delay(30);
digitalWrite(ledPins[i],LOW);
}

for(int i =10; i>1; i--)
{
digitalWrite(ledPins[i],HIGH);
delay(80);
digitalWrite(ledPins[i],LOW);
}
}

• I cleaned up your code a bit. In addition to that, when the OP asks for an explanation why he was wrong, it's at the very least a bit... odd, to answer with just code, and no explanation whatsoever. – Keelan Aug 31 '14 at 21:13

(a) I'm not 100% familiar with what your language demands but, are your "for" tests of the form (start, while, action) or (start, until, action) ?

If the latter then your "<8" tests should be be ">7"

If tests are start/while then OK as is.

(b) In the second LED lighting loop (the downcount) LOW and HIGH should be swapped - going backwards does not change the way you want the LED to light.

• it's (start,while,action) and yeah the 2nd loop's LOW and HIGH were the wrong way round, I've swapped them (edited the question too) but the for loop still isn't working – snowflakekiller Aug 9 '11 at 13:51
• ahh swapping the for-loops for while loops makes it work. I should have tried this first. Grr! Thanks though – snowflakekiller Aug 9 '11 at 13:56

Count-down loops are more traditionally done with pre-decrement:

for(i=8; i>0; --i)
{
...
}


Would count from 7 to 0. The --i decreases i by 1 as the very first thing that gets done after the comparison.

• for loops in C are entirely equivalent to "expression1; while( expression2 != 0 ){ loop-body-code; expression3 }". I.e., the final expression (whether it's pre- or post- increment or decrement, or something else, is the last thing done after executing the loop-body code. So your expression would execute for values of i of [8,7,...,1]. In this context, --i and i-- are equivalent and you can choose one or the other to aid readability. I usually write standalone increment or decrement expressions as pre-(whichevers) because they read the way I say them, e.g., "decrement i". – JRobert Aug 9 '11 at 16:14
• ... One of many references: cprogramminglanguage.net/c-for-loop-statement.aspx – JRobert Aug 9 '11 at 16:14

Commented example code:

void setup() {
// initialize digital pin 13 to 10 as an output.
pinMode(13, OUTPUT);
pinMode(12, OUTPUT);
pinMode(11, OUTPUT);
pinMode(10, OUTPUT);

}

// the loop function runs over and over again forever
void loop() {
digitalWrite(13, HIGH);   // turn the LED on (HIGH is the voltage level)
delay(50);              // wait for a second
digitalWrite(13, LOW);   // turn the LED off by making the voltage LOW
delay(50);             // wait for a second
digitalWrite(12, HIGH);   // turn the LED on (HIGH is the voltage level)
delay(50);              // wait for a second
digitalWrite(12, LOW);   // turn the LED off by making the voltage LOW
delay(50);
digitalWrite(11, HIGH);   // turn the LED on (HIGH is the voltage level)
delay(50);              // wait for a second
digitalWrite(11, LOW);   // turn the LED off by making the voltage LOW
delay(50);             // wait for a second
digitalWrite(10, HIGH);   // turn the LED on (HIGH is the voltage level)
delay(50);              // wait for a second
digitalWrite(10, LOW);   // turn the LED off by making the voltage LOW
delay(1000);
digitalWrite(10, HIGH);
delay(50);
digitalWrite(10, LOW);   // turn the LED off by making the voltage LOW
delay(50);
digitalWrite(11, HIGH);   // turn the LED on (HIGH is the voltage level)
delay(50);              // wait for a second
digitalWrite(11, LOW);   // turn the LED off by making the voltage LOW
delay(50);
digitalWrite(12, HIGH);   // turn the LED on (HIGH is the voltage level)
delay(50);              // wait for a second
digitalWrite(12, LOW);   // turn the LED off by making the voltage LOW
delay(50);
digitalWrite(13, HIGH);   // turn the LED on (HIGH is the voltage level)
delay(50);              // wait for a second
digitalWrite(13, LOW);   // turn the LED off by making the voltage LOW
delay(1000);

}

• rule of thumb: if you have that many lines in your program that look almost the same, there is most likely room for improvement :) – 0x6d64 Dec 7 '15 at 19:53