How do I round floating point numbers to the nearest integer? For example, if I want 2.7 to become 3?

Question

Ryan McCombe · Accepted Answer

There are several ways to round floating-point numbers in C++, each with its own specific behavior. Let's explore the main approaches:

Using Standard Library Functions

The <cmath> library provides several useful rounding functions:

1#include <cmath>
2#include <iostream>
3using namespace std;
4
5int main() {
6  float damage{24.7f};
7
8  // Round to nearest integer
9  int roundedDamage{
10    static_cast<int>(std::round(damage))};
11
12  // Round down (floor)
13  int floorDamage{
14    static_cast<int>(std::floor(damage))};
15
16  // Round up (ceiling)
17  int ceilingDamage{
18    static_cast<int>(std::ceil(damage))};
19
20  cout << "Original: " << damage << "\n";
21  cout << "Rounded: " << roundedDamage << "\n";
22  cout << "Floor: " << floorDamage << "\n";
23  cout << "Ceiling: " << ceilingDamage;
24}

1Original: 24.7
2Rounded: 25
3Floor: 24
4Ceiling: 25

Rounding to Decimal Places

Sometimes you want to round to a specific number of decimal places:

1#include <cmath>
2#include <iostream>
3using namespace std;
4
5float roundToDecimalPlaces(
6  float value, int decimalPlaces
7) {
8  double multiplier{pow(10.0f, decimalPlaces)};
9  return round(
10    value * multiplier) / multiplier;
11}
12
13int main() {
14  float health{95.6789f};
15
16  // Round to various decimal places
17  float oneDecimal{
18    roundToDecimalPlaces(health, 1)};
19  float twoDecimals{
20    roundToDecimalPlaces(health, 2)};
21
22  cout << "Original: " << health << "\n";
23  cout << "One decimal place: "
24    << oneDecimal << "\n";
25  cout << "Two decimal places: "
26    << twoDecimals;
27}

1Original: 95.6789
2One decimal place: 95.7
3Two decimal places: 95.68

Practical Example: Damage Calculations

Here's a more practical example showing how rounding can be used in game mechanics:

1#include <cmath>
2#include <iostream>
3using namespace std;
4
5float calculateDamage(
6  float baseDamage, float multiplier
7) {
8  float rawDamage{baseDamage * multiplier};
9
10  // Round to nearest whole number
11  return round(rawDamage);
12}
13
14int main() {
15  float weaponDamage{10.0f};
16  float criticalMultiplier{1.5f};
17
18  float damage{
19    calculateDamage(
20      weaponDamage, criticalMultiplier)};
21  cout << "Critical hit damage: "
22    << damage << "\n";
23
24  // Multiple calculations
25  float multipliers[]{
26    0.8f, 1.0f, 1.2f, 1.5f
27  };
28  cout <<
29    "\nDamage with different multipliers:";
30  for (float mult : multipliers) {
31    cout << "\nMultiplier " << mult << ": "
32      << calculateDamage(
33        weaponDamage, mult);
34  }
35}

1Critical hit damage: 15
2
3Damage with different multipliers:
4Multiplier 0.8: 8
5Multiplier 1: 10
6Multiplier 1.2: 12
7Multiplier 1.5: 15

Numbers

Rounding Floating Point Numbers

Using Standard Library Functions

Rounding to Decimal Places

Practical Example: Damage Calculations

Numbers

Intro to C++ Programming

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