Math expression error using parenthesis

Projects Programming
1

I have some expressions in my code to control motor response settings. For some odd reason I'm getting a weird result when I use parenthesis. I've tried using different variable types in case the problem is with roll-over. The problem only seem to go away when I rewrite the expression without the parenthesis. Any help with this issue would be greatly appreciated as there are quite a few expressions in my motor control code I'd have to reformat! The following is an example of one of the expressions I'm being troubled by.

void loop() {
static unsigned long tSerial = 0;
int ansInt = 0;
long ansLong = 0;
double ansDouble = 0;
int ans2Int = 0;
int a = 48;
int b = 99;
int c = 100;

ansInt = 60E6/(abc);
ansLong = 60E6/(abc);
ansDouble = 60E6/(abc);
ans2Int = 60E6/a/b/c;

if(millis() - tSerial > 2000){
tSerial = millis();
Serial.print("Expression 1 stored as int: ");
Serial.println(ansInt);
Serial.print("Expression 1 stored as long: ");
Serial.println(ansLong);
Serial.print("Expression 1 stored as double: ");
Serial.println(ansDouble);
Serial.print("Expression 2 stored as int: ");
Serial.println(ans2Int);
Serial.println("\n");
} //end if
} //end function

Capture.JPG

2

try using 'float' since division will yield you decimals.

3

I tired float and even 60. instead of 60 with the same result as before.

Note: E6 has been defined as 1000000 using #define in the header.

4

For some reason, I can't see the JPEG, nor can I see what the value of E6 is.

What is the value of

(a*b*c)

?

5 
int a = 48;
int b = 99;
int c = 100;

abc is 475200, which won't fit into an "int" (as required by your use of parentheses).

6

The JPEG was a screenshot of the serial output:

Expression 1 stored as int: 3647
Expression 1 stored as long: 3647
Expression 1 stored as double: 3647.00
Expression 2 stored as int: 126

E6 has been defined as 1000000

jremington - your observation is correct. I changed the three variables a,b,c to long and got the correct solution. Any reason these variables are driving the variable type for the parenthesis and not ans?

7

Cast them to long if they're ints.

8

The problem is integer overflow. This (abc) is a product of "int" whose result is defined to be "int".
4899100 = 475,200. The result will be truncated to 16 bits (the size of an int on the 386-based Arduinos). 475,200 = 74040(hex). Truncate to 16-bits is 4040(hex) and convert back to decimal is 16448.
60000000/16448 = 3647.86 which is where the 3647 result comes from.

With /a/b/c, the divisions are done one at a time and involve a long integer being divided by an int. The division will produce the correct result because the intermediate results are long integers.

Pete

9

The reason I asked the question about the product of a, b and c in reply #3 was to prompt you to find out by printing it, then you would have seen the problem immediately.

10

@AWOL - sorry, I didn't see your question otherwise I wouldn't have spoiled it by providing the answer.

Pete

11

No worries - got there in the end.

12

Don't worry AWOL. As soon as jremington suggested the int issue I casted 'a' to long and re-ran. I get it now. Thanks for the help - I thought I was going crazy! lol