for loop is wierd? [solved]

Hello people,

When trying to do some math in my program i came across something i dont understand.

I am reading the value from a sensor A0
what i am trying is getting 1500 results and taking an avg of that.
sounds easy enough right.

but looking at the ouput its puzzeling

69<–> 1420<–> 32684
69<–> 1421<–> 32753
69<–> 1422<–> -32714
68<–> 1423<–> -32646
69<–> 1424<–> -32577

1st field (69-68) is the value from sensor
2nd field is the loop amount
3rd field is the total result (adding all the results together)

WHY is the total amount suddenly going negative at loop number 1422???

PLease can someone explain i tried this on an uno and leonardo same result.

int i;
void setup() {
Serial.begin(115200);  // put your setup code here, to run once:
int tot;
for (i=0;i<1500;i++){ // begin loop
int CO_pin = analogRead(0); 
Serial.print(CO_pin);
tot += CO_pin;
Serial.print("<--> ");
Serial.print(i);
Serial.print("<--> ");
Serial.println(tot);
delay(100);
} //einde loop
tot = tot/1500;
Serial.println(tot);
} // einde setup

void loop() {
}
int tot;

Can hold what range of values?

With readings of around 69 each time, what happens to the value in that variable as you get 1400+ x 69 added to it?

Your output isn't at all puzzling. It's exactly what is expected from the code you have written. :wink: One small change will fix it.

Also look at the standard way to use a for loop. Do the examples show you declaring a Variable to use for the loop counter in the manner you've done?

@track
tot = basicly values 40-100 depending what value come from sensor.

so it starts with 0 and just add up all the (values from sensor) 68+68+69+65 etc
I just noticed as soon as the addition of the numbers come to around 327xx it goes nagative and start adding numbers to the negative value as seen in above output.

can you describe a standard way for a loop? imo the way i did it is pretty standard, but please correct me if im wrong.

without declaring tot i get an error while compiling.

int = basicly values 40-100 depending what value come from sensor

No, specifically int tot;

HInt: It's a sixteen bit "Int"

Also look at the standard way to use a for loop. Do the examples show you declaring a Variable to use for the loop counter in the manner you've done?

There's nothing at all wrong with how the loop control variable is declared.

You need a long int

BareMetal:
You need a long int

Actually, he doesn't in this example.

We're trying to help him learn by working it out as it's a fundamental error.

@OP, as AWOL hinted, what is the range of values a 16 bit int can hold? Next, consider the maximum value you will be assigning to it with your 1500 additions.

Awesum !!!
That long int solved my problem
must do some more reading on 16bit and range value
Thank you guys

UPDATE:
i did some reading :slight_smile:
On the Arduino Uno (and other ATMega based boards) an int stores a 16-bit (2-byte) value. This yields a range of -32,768 to 32,767 (minimum value of -2^15 and a maximum value of (2^15) - 1).

exactly the number i ran into problems:)

Once again ty guys you rock!!

malawi:
Awesum !!!
That long int solved my problem

Thank you guys

But what have you learnt by being given the (wrong) answer?

We were trying to help you learn the fundamentals of variable type appropriate to what you wish to assign to it.

In this case the answer is not a 'long', although it does fix your issue it is actually overkill. You are now using a 32 bit variable where a 16 bit one would be sufficient, and hence using 2 more bytes of that precious 2kB of SRAM that isn't required.

BareMetal has not helped you with that answer really.

That long int solved my problem

But you don't need long int, as has been mentioned.

What range of numbers can an int hold ?
HINT - part of the range is negative. Do you need negative numbers ?
If only you did not need to take negative numbers into account then maybe an int could hold a larger number.
If only that negative sign was not there and all the numbers were positive an int could hold a number twice as big maybe.

@tack,
I get your point
Please enlighten me:)

The values coming from sensors are in the range of 0-1024 so in theorie
the range i want to use is 0- 1536000 ->(1024*1500) ←

so in you opinion what should i use.

But you don't need long int, as has been mentioned

Chaps, yes, you almost certainly do - 1500 readings of any analogRead over 43 is going to require 32 bits.

@UKHeliBob
i see, so i am using only half of int when i dont use negative numbers.
so how would i describe an int with no negatives? or is that not possible?

AWOL:

But you don't need long int, as has been mentioned

Chaps, yes, you almost certainly do - 1500 readings of any analogRead over 43 is going to require 32 bits.

True. I was just looking at his quoted example of the rollover at 1420, but actually it would appear that is his second rollover!

So BareMetal was right on the long type. My apology there. One of the downsides to being on a phone and just using the info presented without checking personally.

It would still help malawi to think about the original question and it's aim to consider the right type for application, and whether he requires negative values.

Understanding each data type and it's value range is useful basic understanding.

16 bit "int" range -32768 to 32767
16 bit "unsigned int" range 0 to 65535.
There are no 24 bit datatypes.

An analogRead cannot return a negative number, so using an unsigned value is pointless, so if your analogRead averages over 43, you're going to need a 32 bit datatype.

There's also nothing wrong with the declaration of your loop counter variable.

Surely keeping variables as local as necessary is good practice to encourage?

for (int i = 0; i < 1500; i++)

It depends whether or not the intention is to re-use the value later, perhaps after a loop break.
Or maybe you want to port your code to C.

Sorry for the overkill, peeps, I was shooting from the hip :slight_smile:

BareMetal:
Sorry for the overkill, peeps, I was shooting from the hip :slight_smile:

Apology is mine. You were right, although unsigned long would be more appropriate. I only looked at the data posted and a rollover at 1420 loops, but he must have had another rollover earlier already as the total should have been about 96000 odd.

My train of thought was unsigned int due to that and only seeming like 80 more loops required.

:blush:

No apology required. Uint would have been better learning…we are all here to help each other out, and/ or amuse! Nice to meet u. The guy got sorted, thats what matters.

Thanks for all the replies,
I got the program working and you guys learned me alot.
much apreciated:)

Malawi