This is simple, but interesting programming puzzle. Given three integers, low, high and x such that high >= low. How to check if x lies in range [low, high] or not using single comparison. For example, if range is [10, 100] and number is 30, then output is true and if the number is 5, then output is false for same range.
A simple solution is compare x with low and high
using namespace std;
// Returns true if x is in range [low..high], else false
bool inRange(unsigned low, unsigned high, unsigned x)
return (low <= x && x <= high);
inRange(10, 100, 30)? cout << "Yes\n": cout <<"No\n";
inRange(10, 100, 5)? cout << "Yes\n": cout <<"No\n";
The above solution does two comparisons, Can we do the same task using one comparison?
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The idea is to compare “x-low” with “high-x”. x is in range [low, high] if and only if x is greater than or equal to low and smaller than or equal to high.
How does this work for [10, 100] and x = 5?
When we subtract 10 from 5, we get -5 which is considered as UNIT_MAX-4 in unsigned int form. UNIT_MAX is maximum possible unsigned int value. The assumption here is, numbers are stored in 2’s complement form. In 2’s complement form, -1 represents UINT_MAX, -2 represets UINT_MAX-1,..etc.
Thanks to Utkarsh for suggesting this solution.
A Solution that works for negative numbers also
The idea is to multiply (x-low) and (x-high). If x is in range, then it must be greater than or equal to low, i.e., (x-low) >= 0. And must be smaller than or equal to high i.e., (high – x) <= 0. So if result of the multiplication is less than or equal to 0, then x is in range. Else not. Thanks to eva for suggesting this method.
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