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 ...

If I use unsigned int, I thought it will loop back to the max value like...65535 when it hits -1? :P — user3437460, Apr 03 '14 at 15:05
@user3437460 If you like the negatives to be turned into positive integers you might be interested in std::abs(int) (http://coliru.stacked-crooked.com/a/0f0e6aa6243f114b) - And yes it might overflow with unsigned int. — Floris Velleman, Apr 03 '14 at 15:09

score 6 · Accepted Answer · answered Apr 03 '14 at 15:03

6

How about std::max?

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

answered Apr 03 '14 at 15:03

bstamour

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That's one possibility..hmm any other suggestion guys? – user3437460 Apr 03 '14 at 15:11
1

@user3437460 How about accepting an answer or refining your requirements? I don't see why this isn't sufficient answer. – tgmath Apr 03 '14 at 15:19
@tgmath Yes, his answer is fine. But I was looking for possibilites of using mathematical operations to get the solution. – user3437460 Apr 03 '14 at 15:44
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`. – bstamour Apr 03 '14 at 16:00

score 2 · Answer 2 · answered Apr 03 '14 at 15:04

2

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

answered Apr 03 '14 at 15:04

anderas

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Codor · Answer 3 · 2014-04-03T15:57:20.683

1

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.

edited Apr 03 '14 at 15:57

answered Apr 03 '14 at 15:09

Codor

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score 0 · Answer 4 · answered Apr 03 '14 at 15:09

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--) << " ";

score 0 · Answer 5 · edited Apr 03 '14 at 18:55

0

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;     
}

edited Apr 03 '14 at 18:55

Tom Fenech

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answered Apr 03 '14 at 16:40

Now [where have I seen that before](http://www.cplusplus.com/reference/algorithm/max/)? You should include your source and remove the comment as it makes no sense out of context. – Tom Fenech Apr 03 '14 at 17:41
@Tom Fenech New user to this forum. I dont know i am required to attach the link. here we go, http://www.cplusplus.com/reference/algorithm/max/ – Apr 03 '14 at 18:52

Mathematical operation to keep number not less than zero

5 Answers5