Thread: Array shifted when initializing variable

I'm working on Project Euler 22. I think I got the logic and my code is a step away from the answer. But something creepy is going on.

Basically I have four functions: "read" to read the input of names, "swap" to swap two character strings, "sort" to sort an array of character strings in alphabetical order, "worth" to calculate the worth of a name.

I ran into two troubles:

(1) If you notice, my "sort" function actually sort the array in reversed alphabetical order. When I tried "if(strcmp(names[j-1],names[j])>0)", it gives "Illegal instruction: 4". Why is that so?

I can still continue with the reversed sorting. But the second problem is weirder.

(2) At the very last step, I need to define an integer "sum" to record the total name scores. But when I defined "int sum=0", the array names[] is shifted! names[1] becomes names[0], names[2] becomes names[1], and so on. How did this happen?

I expect the shift downward was caused by your sort function. j=0 is OK, but then you start comparing and swapping j with j-1. j-1 is array[-1], so start your loop with j=1, instead of j=0. Your sort function doesn't "bubble up" it "settles downward".

The strcmp line of code worked fine for me, but that j-1 problem may cause some sort of error or warning to appear.

Ah, stupid mistake. It does work now, thanks!

I don't completely get the mechanism though. Why exactly the array is shifted when I initialized "int sum=0"?

Short answer: When you're messing with undefined behavior, like accessing arrays at negative indexes, all bets are off and anything can happen.

As Salem's signature says:

If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.