Why doesn't this work int/long?

Projects Programming
1

this one works but yr has to be a long. if int you get wrong answers.
char buf[11];
int dy = 16;
int mo = 6;
long yr = 21;
long xx = 0;
xx = dy;
xx += mo * 100;
xx += yr * 10000;
ltoa(xx, buf, 10);
Serial.println("date "); Serial.println(buf);

but this one gives wrong answers even tho 21 easily fits into int.
char buf[11];
int dy = 16;
int mo = 6;
int yr = 21;
long xx = 0;
xx = dy;
xx += mo * 100;
xx += yr * 10000;
ltoa(xx, buf, 10);
Serial.println("date "); Serial.println(buf);

2

yr is an int. So is 10,000. When you multiply them together you get integer overflow before trying to put the result in a long.

3

Do the calculations by hand. See how big the numbers get. Learn how large a number can be and fit into an int and a long variable.

a7

4 
void setup()
{
   Serial.begin(115200);
   char buf[11];
   int dy = 16;
   int mo = 6;
   int yr = 21;
   long xx = 0;
   xx = dy;
   xx += mo * 100L;  // add L to force long calculation
   xx += yr * 10000L;  // add L
   ltoa(xx, buf, 10);
   Serial.println("date "); Serial.println(buf);
}

void loop()
{

}

The L in the calculations force the calculation to be done with long data type. See here.

1 Like
5

depends if it is an African or European int.

6

I had surmised the compiler would use the target for the answer as the workplace or define a long to multiply the two ints. But I guessed wrongly I guess.

7

one thing about the old mainframes, overflow would abend your program.

8

C doesn't care: "You want overflow? I got your stinkin' overflow right here!"

The compiler looks at the expression first and decides it's all ints, so ok, integer math. Oh look, he wants to put it in a long comes later, after the overflow.

9

You guessed right about guessing wrong. And I am not guessing.

Pro tip: guess work and computer programming do not mix well.

Documentation can be consulted for the "truth".

Or get empirical - a few minutes of hacking with various changes can reveal a lifetime of knowledge.

The more you know already, the more focussed and meaningful your experments.

a7

10

you have to guess. often the answer is not obvious.

11

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