Weird math errors

Projects Programming
1

In the following code why does it not multiply correctly?
I should have a result of 39600 but instead I get -25936.
Weird or what?

void setup() {
  Serial.begin(9600);
}

void loop() {
  Serial.println((3600*11));
  delay(2000);   
}
2

I should have a result of 39600 but instead I get -25936.

Why would you expect to fit 39600 in an int?

Serial.println((3600UL*11UL));
will print 39600.

3

Hmmm, I guess I don't know all the rules yet.

So because the arduino has some computing limitations I assume I will not be able to do the following:

int monthDays[12] = {31,28,31,30,31,30,31,31,30,31,30,31};
void setup() {
  Serial.begin(9600);
  Serial.println("setup");
}

void loop() {
  Serial.println("START LOOP");
   unsigned long years = 2012;
   unsigned long months = 1;
   unsigned long days  = 27;
   unsigned long hours = 11;
   unsigned long mins = 27;
   unsigned long secs = 30;
   unsigned long monthsInDays = 0;
   unsigned long monthsInSecs;
   unsigned long i;
   unsigned long leapYearsInSec = ((years-1972)/4)*31622400UL;
   unsigned long yearsInSec = ((years - 1970)-((years-1972)/4))*31536000UL;
   Serial.println("VAR");
   if (((years-1972)%4)==0){
     if (months > 2){
       monthsInDays = 1;
       for (i=0;i<=(months-2);i++){
         monthsInDays = (monthsInDays + monthDays[i]);
       }
     }else{
       monthsInDays = 0;
       for (i=0;i<=(months-2);i++){
         monthsInDays = (monthsInDays + monthDays[i]);
       }
     }
     monthsInSecs = monthsInDays*86400UL;
   }else{
     monthsInDays = 0;
       for (i=0;i<=(months-2);i++){
         monthsInDays = (monthsInDays + monthDays[i]);
       }
       monthsInSecs = monthsInDays*86400UL;
   }
   Serial.println("AFTER IF");
   unsigned long daysInSecs = (days*86400UL);
   Serial.println(hours);
   unsigned long hoursInSecs = (hours*3600UL);
   unsigned long minsInSecs = (mins*60UL);
   
   unsigned long unix = (secs+minsInSecs+hoursInSecs+daysInSecs+monthsInSecs+yearsInSec+leapYearsInSec);
   Serial.println("START");
   Serial.println(secs);
   Serial.println(minsInSecs);
   Serial.println(hoursInSecs);
   Serial.println(daysInSecs);
   Serial.println(monthsInSecs);
   Serial.println(yearsInSec);
   Serial.println(leapYearsInSec);
   Serial.print("UNIX IS: ");
   Serial.println(unix);
   Serial.println();

  
}
4

Sparked:
I will not be able to do the following:

The code you wrote here is using unsigned long, which does have the range needed to do this. I do not believe you'll have any problem.

Your first example was using unadorned integers, which have a range of only -32768...32767. This is not sufficient to evaluate the calculations correctly.

5

With this number in a multiplication 31622400UL, you are however getting close to the Unsigned long limit, 4294967295UL

6

Thirty one and a half million is a good long way from four billion, where years since 1972 are involved.