randon number generation from a given set of numbers

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hello guys,
i'm a beginner, and wanted to make a program to generate random number's from a given set of numbers, ten times. i came up with this so far. i want it to generate a number from the array "numbers" but it generates numbers which r not in the declared array. i don't understand why?
please help me.

CSS

int numbers[] = { 3, 13, 22, 57, 417 };
int length = sizeof(numbers);
for (int aa=1;aa<=20;aa++)
    {cout <<numbers[rand()%length]<< endl;}

i you want to compile it for yourself and see, then here is the link to my entire code
link to my whole code

Posted 18-Aug-13 4:56am

A.U.K

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pasztorpisti · Accepted Answer · 2013-08-18T05:08:00

C++

int length = sizeof(numbers) / sizeof(numbers[0]);

sizeof returns the size of the array in bytes and not the number of items in the array. Since this array contains integers that are 4 bytes one-by-one sizeof probably returns 5*4 so sometimes you read garbage from memory by overindexing the array.

In my codebase I follow a convention that is pretty practical. I refer to size of something in bytes with "size" while I refer to the number of items in a sequence (array or string or whatever) with "length". For example if I had an array class then I would name the method that returns the number of items as GetLength().