Mathematical operation to keep number not less than zero

Question

In programming, modulus is useful to keep numbers in range not exceeding an upper bound limit.

For example:

int value = 0;
for (int x=0; x<100; x++)
    cout << value++%8 << " "; //Keeps number in range 0-7

Output: 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7...

---

Now consider this situation:

int value = 5;
for (int x=0; x<100; x++)
    cout << value-- << " ";

Output: 5 4 3 2 1 0 -1 -2 -3 -4 -5 -6 -7...

My question is: How to set the lower bound limit to 0 WITHOUT using any conditional statement like if or switch case?

Output I want: 5 4 3 2 1 0 0 0 0 0 0 0 ...

Birkle

Is it even possible? But I just feel that it is possible.. – Birkle

@TheForestAndTheTrees Please don't do that. – Platonic

If I use unsigned int, I thought it will loop back to the max value like...65535 when it hits -1? :P – Birkle

@Birkle If you like the negatives to be turned into positive integers you might be interested in std::abs(int) (coliru.stacked-crooked.com/a/0f0e6aa6243f114b) - And yes it might overflow with unsigned int. – Platonic

Answer 1

How about std::max?

int value = 5;
for (int x=0; x<100; x++) {
    cout << value << " ";
    value = std::max(0, --value);
}

Bracing

That's one possibility..hmm any other suggestion guys? – Birkle

@Birkle How about accepting an answer or refining your requirements? I don't see why this isn't sufficient answer. – Gerdy

@Gerdy Yes, his answer is fine. But I was looking for possibilites of using mathematical operations to get the solution. – Birkle

max is a mathematical operation just as much as addition is. You give it two elements from an ordered set as input, and it returns an element from the same set. It's closed, and it even distributes over min. – Bracing

Answer 2

Use std::max(value,0) or the ternary operator. Both will compile to code without conditional jumps.

Congregate

Answer 3

Perhaps it can be done with

typedef unsigned u32b;

u32b sat_subu32b(u32b x, u32b y)
{
    u32b res = x - y;
    res &= -(res <= x); 
    return res;
}

which is taken from here. More specifically it seems to implement saturated subtraction; the saturated decrement would be

u32b sat_dec(u32bx)
{
    u32b res = x-1;
    res &= -(res <= x); 
    return res;
}

and seems to be branch-free.

Boudreaux

Answer 4

The two situations are different. As you've shown, the % will make the numbers loop around from 0 to 7, whereas in the negative case you seem to be asking for the max.

Depending on whether you count the ternary operator as a conditional, you could do:

int value = 5;
for (int x=0; x<100; x++)
    cout << value > 0 ? value-- : 0 << " ";

This has the side effect that value will no longer be decremented once it has reached 0.

Alternatively, you could use a std::max:

int value = 5;
for (int x=0; x<100; x++)
    cout << std::max(0, value--) << " ";

Blabbermouth

Answer 5

using max function in std.
max function detail:

template <class T> const T& max (const T& a, const T& b) {
  return (a<b)?b:a;     
}

Narcotism

Now where have I seen that before? You should include your source and remove the comment as it makes no sense out of context. – Blabbermouth

@Tom Fenech New user to this forum. I dont know i am required to attach the link. here we go, cplusplus.com/reference/algorithm/max – Narcotism