3. Arithmetic order errors

A subset of logic errors are arithmetic order errors. In maths, you should be familiar with the order of operations when working out a calculation (BODMAS). Or

1. Brackets
2. Orders
3. Division
4. Multiplication
5. Addition
6. Subtraction

Having a standard order means that users can expect to get the same results when they run a calculation, regardless of the program carrying it out.

Arithmetic errors creep in when the programmer forgets the order of operations, and ends up with a result they don't expect:

	Line 30:	SET startMiles TO 20
	Line 31:	SET endMiles TO 26
	Line 32:	SET petrolUsed TO 2
	Line 33:	SET milesPerGallon TO EndMiles - StartMiles/petrolUsed

Here, the programmer tries calculating miles per gallon incorrectly. If we slot in the numbers, the calculation looks like this

          miles per gallon = 26 - 20/2

With BODMAS the answer is going to be 16 which is incorrect. To get the correct answer, brackets must be added, like this

          miles per gallon = (26 - 20)/2

which works out as 3 miles per gallon which is now correct. The pseudocode is now:

                 
  Line 33: SET milesPerGallon TO (EndMiles - StartMiles)/petrolUsed