I’m working on a c++ round down where I need to round down floating-point numbers to the nearest integer. I know about standard rounding functions, but I specifically need to ensure that numbers always round downward (i.e., towards negative infinity).
For example:
4.7 should become 4
-2.3 should become -3
After some research, I found that C++ provides several ways to achieve this, and I’d love to hear your thoughts on the best approach.
- Using floor() from <cmath>
One of the simplest ways is using the floor() function from the <cmath> library.
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#include <iostream>
#include <cmath>
int main() {
double num1 = 4.7;
double num2 = -2.3;
std::cout << "Floor of " << num1 << " is " << std::floor(num1) << std::endl;
std::cout << "Floor of " << num2 << " is " << std::floor(num2) << std::endl;
return 0;
}
Output:
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Floor of 4.7 is 4
Floor of -2.3 is -3
This function is efficient and works perfectly for my case.
- Using Type Casting (Only for Positive Numbers)
Another approach (which only works for positive values) is using an implicit conversion to int.
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#include <iostream>
int main() {
double num = 4.7;
std::cout << "Implicit cast: " << (int)num << std::endl;
return 0;
}
However, this truncates towards zero rather than always rounding down, so it won’t work correctly for negative numbers.
- Using Integer Division (For Specific Cases)
In some cases, if you're dealing with division and want to round down, you can use integer division.
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int roundDown(int a, int b) {
return a / b; // Integer division naturally discards decimal places
}
Again, this method is useful in some situations but doesn’t apply to general floating-point rounding.
- Using std::floor() in Modern C++ with std::vector (Example for Multiple Values)
If you have a list of numbers, you can apply std::floor() to all of them using a loop or a lambda function with std::transform.
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#include <iostream>
#include <vector>
#include <cmath>
#include <algorithm>
int main() {
std::vector<double> numbers = {3.9, 5.2, -1.7, -4.3};
std::vector<int> flooredNumbers;
std::transform(numbers.begin(), numbers.end(), std::back_inserter(flooredNumbers),
[](double num) { return std::floor(num); });
for (int n : flooredNumbers) {
std::cout << n << " ";
}
return 0;
}
Conclusion
From what I’ve tested, std::floor() is the best and most reliable method to always round numbers downward. Type casting and integer division can work in some cases, but they don’t guarantee correct behavior for negative numbers.
