为什么这个计算不起作用？

Question

I was throwing together a quickie program to take mainframe output blocked in 133 byte lengths, all ending with a CRLF, and it was working except for my calculated number of lines in the output. Because the output size was X pages of 133 bytes with 2 bytes (CRLF) at the end, I was calculating the line count as:

lineCount = fileLength - 2 / 133;

For a file length of 3194, that works out to 24 lines. Take 3194, subtract 2 for the CRLF and you get 3192, and that is divided by 133 to come up with 24. Simple! The crazy thing is, I was getting the lineCount equal to the fileLength !

What could I be doing wrong?

Answer 1

After examining this several times, I finally hit on it! It's a matter of the infamous Order of Operations !

lineCount = fileLength - 2 / 133;

If I evaluate this from left to right, according to my description above, it works fine, but I happen to be a human, not a CPU. Different rule! The computer processor has to use a different rule: MiDAS: multiplications, divisions, additions and subtractions.

My code was calculating 2 / 133, which for integers equals 0. It was then subtracting that 0 from fileLength, and of course set lineCount to that value. I am ancient of days, sort of, and should have known better from the start, but I guess I was in a hurry. The correct code?

int lineCount = ((fileLength - 2) / 133);

So, remember MiDAS and you will be Golden!

NOTE: it's more complicated than this, actually. The full rule encompasses parentheses and exponentiation. For an expanded look at this check Wikipedia for Order of Operations .

In the US the mnemonic is more like: PEMDAS - Please Excuse My Dear Aunt Sally - and refers to Parentheses, Exponents, Multiplications, Divisions, Additions and Subtractions.