[ILUG] gcc optimisation weirdness?

[Burke, Gary : GIG IS&T]

]int input, frequency[9];
]
]//frequency[] initialised to 0.
]
]do {
]                scanf("%d", &input);
]                if (input > 0 && input < 11) {
]			//point 1
]                        (frequency[input-1])++;
]			//point 2
]                        }
]        } while (input);

Hmmm.  int frequency[9]; give you elements 0 -> 8 inclusive.  If input=10,
then you are trying to access frequency[10-1], or frequency[9], which is not
part of your array.  What seems to be happening is that 'input' is being
places in memory where frequency[9] would be, if it existed.  Hence,
'frequency[9]++' is basically being a 'input++', in this case.

Please note that by accessing an array out of bounds, you are in unknown
territory.  Different compilers, and different options, will do things
differently.  There is generally no way to know what you might be accessing.



Gary
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