String method crashes program - c++

Question

Alright so i have two identical string methods...
string CreateCust() {
string nameArray[] ={"Tom","Timo","Sally","Kelly","Bob","Thomas","Samantha","Maria"};
int d = rand() % (8 - 1 + 1) + 1;
string e = nameArray[d];
return e;
}
string CreateFood() {
string nameArray[] = {"spagetti", "ChickenSoup", "Menudo"};
int d = rand() % (3 - 1 + 1) + 1;
string f = nameArray[d];
return f;
}
however no matter what i do it the guts of CreateFood it will always crash. i created a test chassis for it and it always fails at the cMeal = CreateFood();
Customer Cnow;
cout << "test1" << endl;
cMeal = Cnow.CreateFood();
cout << "test1" << endl;
cCustomer = Cnow.CreateCust();
cout << "test1" << endl;
i even switched CreateCust with CreateFood and it still fails at the CreateFood Function...
NOTE: if i make createFood a int method it does work...
Also guys even if i changed CreateFood to just COUT a message and nothing more it still crashed...

Answer 1

Take out the + 1 on both of them, you access arrays starting from 0:
int d = rand() % (8 - 1 + 1); // 0-7, not 1-8
int d = rand() % (3 - 1 + 1); // 0-2, not 1-3
Otherwise you're accessing a non-existent element, and this is undefined behavior. (That means it could appear to work, like in CreateCust, crash like in CreateFood, do nothing, or do anything.)
I'm not sure what the purpose of subtracting 1 then adding 1 is. In any case, now is the perfect time to learn: Don't Repeat Yourself. Even if you do something just twice, make a function out of it, it'll be less cryptic and more concise:
int random(int min, int max)
{
return rand() % ((b - a) + 1) + a;
}
This is a simple function that returns a random number between a and b, inclusive. (Means it can include both a and b in the results.) Now your code reads:
// I'll leave CreateCust up to you
string CreateFood(void)
{
string nameArray[] = {"spagetti", "ChickenSoup", "Menudo"};
int d = random(0, 2); // either 0, 1, or 2, randomly
string f = nameArray[d];
return f;
}
And you'll see even just one function makes it much easier to read; your goal is to make your code easy to read by humans. Also, this is much more concise:
string CreateFood(void)
{
string nameArray[] = {"spagetti", "ChickenSoup", "Menudo"};
return nameArray[random(0, 2)];
}
Another bad thing to do is hardcode magic numbers into your program. For example, why 3 or 8? It can be deduced those are array sizes, but that doesn't stand on its own. What you might want is something like:
string CreateFood(void)
{
const size_t ArraySize = 3; // 3 elements, 0-2
string nameArray[ArraySize] = {"spagetti", "ChickenSoup", "Menudo"};
// ^ Ensure it matches
return nameArray[random(0, ArraySize - 1)];
}
Now the range for the number number makes sense up front.
The rest may be a bit advanced (which you won't understand until you get to templates), but shows how we might go on:
template <typename T, size_t N>
char (&countof_detail(T (&)[N]))[N];
#define countof(pX) sizeof(countof_detail(pX))
This nifty tool will give you the number of elements in an array. The code might turn into this
string CreateFood(void)
{
string nameArray[] = {"spagetti", "ChickenSoup", "Menudo"};
// ^ no explicit size
return nameArray[random(0, countof(nameArray) - 1)];
}
We got rid of any numbers altogether, you can just manipulate the array as you please. Lastly, we're repeating ourselves again: getting a random element from an array. We should make a function for that:
template <typename T, size_t N>
T& random_element(T (&pArray)[N])
{
return pArray[random(0, N - 1)];
}
This returns a random element from any array. Your function would then simply be:
string CreateFood(void)
{
string nameArray[] = {"spagetti", "ChickenSoup", "Menudo"};
return random_element(nameArray);
}
Note in this refactoring (refactoring is taking code and factoring it into new, simpler parts) it reads much better: To get a food, we have an array of foods and we pick one at random.
Keep this kind of stuff in mind while you work, and as you learn C++ you can make better code. Anytime you repeat a task that isn't trivial, make it a function. Suddenly that task is trivial, because you don't care about how the function works (that's in the function), just what the function does (that's the function name).

Answer 2

The crash is happening because you are accessing an invalid index. This is because array indexes start from 0 and not 1, so you don't want to add a 1 to the rvalue of the modulus operator.
Here is a neat trick that you can use to make your code a little more maintainable:
template <class T>
T getRandElem( const T[] arr )
{
return arr[ rand() % ( sizeof(arr) / sizeof((arr)[0]) ) ];
}
string CreateCust(){
static string nameArray[] = {"Tom","Timo","Sally","Kelly","Bob","Thomas","Samantha","Maria"};
return getRandElem<string>( nameArray );
}
string CreateFood(){
static string nameArray[] = {"spagetti", "ChickenSoup", "Menudo"};
return getRandElem<string>( nameArray );
}

Answer 3

Look here:
int d = rand() % (8 - 1 + 1) + 1;
This will return a number between 1 and 8 inclusive. What you need is this:
int d = rand() % 8;

Answer 4

You're going outside the bounds of the array. The array object begins at 0.

Answer 5

I don't understand why you have
int d = rand() % (8 - 1 + 1) + 1;
Why not just use
int d = rand() % 8;

Answer 6

I think modern c++ compilers will still let you use an old C trick for static arrays:
string CreateFood()
{
char* nameArray = {"spagetti", "ChickenSoup", "Menudo"};
// note the trick to get the compiler to count the array elements for you:
int d = rand() % (sizeof(nameArray) / sizeof(nameArray[0]));
return std::string( nameArray[d] );
}