Java rounding up to an int using Math.ceil

问题:

int total = (int) Math.ceil(157/32);

Why does it still return 4? 157/32 = 4.90625, I need to round up, I've looked around and this seems to be the right method.

I tried total as double type, but get 4.0.

What am I doing wrong?

回答1:

You are doing 157/32 which is dividing two integers with each other, which always result in a rounded down integer. Therefore the (int) Math.ceil(...) isn't doing anything. There are three possible solutions to achieve what you want. I recommend using either option 1 or option 2. Please do NOT use option 0.

Option 0

Convert a and b to a double, and you can use the division and Math.ceil as you wanted it to work. However I strongly discourage the use of this approach, because double division can be imprecise. To read more about imprecision of doubles see this question.

int n = (int) Math.ceil((double) a / b));

Option 1

int n = a / b + ((a % b == 0) ? 0 : 1); 

You do a / b with always floor if a and b are both integers. Then you have an inline if-statement witch checks whether or not you should ceil instead of floor. So +1 or +0, if there is a remainder with the division you need +1. a % b == 0 checks for the remainder.

Option 2

This option is very short, but maybe for some less intuitive. I think this less intuitive approach would be faster than the double division and comparison approach:

int n = (a + b - 1) / b;

To reduce the chance of overflow you could use the following. However please note that it doesn't work for a = 0 and b < 1.

int n = (a - 1) / b + 1;

Explanation behind the "less intuitive approach"

Since dividing two integer in Java (and most other programming languages) will always floor the result. So:

int a, b;
int result = a/b (is the same as floor(a/b) )

But we don't want floor(a/b), but ceil(a/b), and using the definitions and plots from Wikipedia:

With these plots of the floor and ceil function you can see the relationship.

You can see that floor(x) <= ceil(x). We need floor(x + s) = ceil(x). So we need to find s. If we take 1/2 <= s < 1 it will be just right (try some numbers and you will see it does, I find it hard myself to prove this). And 1/2 <= (b-1) / b < 1, so

ceil(a/b) = floor(a/b + s)
          = floor(a/b + (b-1)/b)
          = floor( (a+b-1)/b) )

This is not a real proof, but I hope your are satisfied with it. If someone can explain it better I would appreciate it too. Maybe ask it on MathOverflow.

回答2:

157/32 is int/int, which results in an int.

Try using the double literal - 157/32d, which is int/double, which results in a double.

回答3:

157/32 is an integer division because all numerical literals are integers unless otherwise specified with a suffix (d for double l for long)

the division is rounded down (to 4) before it is converted to a double (4.0) which is then rounded up (to 4.0)

if you use a variables you can avoid that

double a1=157;
double a2=32;
int total = (int) Math.ceil(a1/a2);

回答4:

int total = (int) Math.ceil((double)157/32);

回答5:

In Java adding a .0 will make it a double...

int total = (int) Math.ceil(157.0 / 32.0);

回答6:

When dividing two integers, e.g.,

int c = (int) a / (int) b;

the result is an int, the value of which is a divided by b, rounded toward zero. Because the result is already rounded, ceil() doesn't do anything. Note that this rounding is not the same as floor(), which rounds towards negative infinity. So, 3/2 equals 1 (and floor(1.5) equals 1.0, but (-3)/2 equals -1 (but floor(-1.5) equals -2.0).

This is significant because if a/b were always the same as floor(a / (double) b), then you could just implement ceil() of a/b as -( (-a) / b).

The suggestion of getting ceil(a/b) from

int n = (a + b - 1) / b;, which is equivalent to a / b + (b - 1) / b, or (a - 1) / b + 1

works because ceil(a/b) is always one greater than floor(a/b), except when a/b is a whole number. So, you want to bump it to (or past) the next whole number, unless a/b is a whole number. Adding 1 - 1 / b will do this. For whole numbers, it won't quite push them up to the next whole number. For everything else, it will.

Yikes. Hopefully that makes sense. I'm sure there's a more mathematically elegant way to explain it.

回答7:

int total = (int) Math.ceil( (double)157/ (double) 32);

回答8:

Also to convert a number from integer to real number you can add a dot:

int total = (int) Math.ceil(157/32.);

And the result of (157/32.) will be real too. ;)

回答9:

Check the solution below for your question:

int total = (int) Math.ceil(157/32);

Here you should multiply Numerator with 1.0, then it will give your answer.

int total = (int) Math.ceil(157*1.0/32);

回答10:

Use double to cast like

Math.ceil((double)value) or like

Math.ceil((double)value1/(double)value2);

回答11:

Java provides only floor division / by default. But we can write ceiling in terms of floor. Let's see:

Any integer y can be written with the form y == q*k+r. According to the definition of floor division (here floor) which rounds off r,

floor(q*k+r, k) == q  , where 0 ≤ r ≤ k-1

and of ceiling division (here ceil) which rounds up r₁,

ceil(q*k+r₁, k) == q+1  , where 1 ≤ r₁ ≤ k

where we can substitute r+1 for r₁:

ceil(q*k+r+1, k) == q+1  , where 0 ≤ r ≤ k-1

Then we substitute the first equation into the third for q getting

ceil(q*k+r+1, k) == floor(q*k+r, k) + 1  , where 0 ≤ r ≤ k-1

Finally, given any integer y where y = q*k+r+1 for some q,k,r, we have

ceil(y, k) == floor(y-1, k) + 1

And we are done. Hope this helps.

回答12:

Nobody has mentioned the most intuitive:

int x = Math.round(Math.ceil((double) 157 / 32));

This solution fixes the double division imprecision.

回答13:

There are two methods by which you can round up your double value.

1. Math.ceil
2. Math.floor

If you want your answer 4.90625 as 4 then you should use Math.floor and if you want your answer 4.90625 as 5 then you can use Math.ceil

You can refer following code for that.

public class TestClass {

    public static void main(String[] args) {
        int floorValue = (int) Math.floor((double)157 / 32);
        int ceilValue = (int) Math.ceil((double)157 / 32);
        System.out.println("Floor: "+floorValue);
        System.out.println("Ceil: "+ceilValue);

    }

}

回答14:

int total = (157-1)/32 + 1

or more general

(a-1)/b +1