CODEWITHSUNDEEP
javamathrounding
MikroByte
MikroByte

Reputation: 65

How can I round manually?

I'd like to round manually without the round()-Method. So I can tell my program that's my number, on this point i want you to round. Let me give you some examples:
Input number: 144
Input rounding: 2
Output rounded number: 140

Input number: 123456
Input rounding: 3
Output rounded number: 123500

And as a litte addon maybe to round behind the comma: Input number: 123.456
Input rounding: -1
Output rounded number: 123.460

I don't know how to start programming that... Has anyone a clue how I can get started with that problem?

Thanks for helping me :)

I'd like to learn better programming, so i don't want to use the round and make my own one, so i can understand it a better way :)

Upvotes: 5

Views: 5839

Answers (5)

Geeky Guy
Geeky Guy

Reputation: 9399

public static int round (int input, int places) {
  int factor = (int)java.lang.Math.pow(10, places);
  return (input / factor) * factor;
}

Basically, what this does is dividing the input by your factor, then multiplying again. When dividing integers in languages like Java, the remainder of the division is dropped from the results.

edit: the code was faulty, fixed it. Also, the java.lang.Math.pow is so that you get 10 to the n-th power, where n is the value of places. In the OP's example, the number of places to consider is upped by one.

Re-edit: as pointed out in the comments, the above will give you the floor, that is, the result of rounding down. If you don't want to always round down, you must also keep the modulus in another variable. Like this:

int mod = input % factor;

If you want to always get the ceiling, that is, rounding up, check whether mod is zero. If it is, leave it at that. Otherwise, add factor to the result.

int ceil = input + (mod == 0 ? 0 : factor);

If you want to round to nearest, then get the floor if mod is smaller than factor / 2, or the ceiling otherwise.

Upvotes: 1

Barranka
Barranka

Reputation: 21047

A simple way to do it is:

  1. Divide the number by a power of ten
  2. Round it by any desired method
  3. Multiply the result by the same power of ten in step 1

Let me show you an example:

You want to round the number 1234.567 to two decimal positions (the desired result is 1234.57).

x = 1234.567;
p = 2;
x = x * pow(10, p);  // x = 123456.7
x = floor(x + 0.5);  // x = floor(123456.7 + 0.5) = floor(123457.2) = 123457
x = x / pow(10,p);   // x = 1234.57
return x;

Of course you can compact all these steps in one. I made it step-by-step to show you how it works. In a compact java form it would be something like:

public double roundItTheHardWay(double x, int p) {
    return ((double) Math.floor(x * pow(10,p) + 0.5)) / pow(10,p);
}

As for the integer positions, you can easily check that this also works (with p < 0).

Hope this helps

Upvotes: 2

jfrank
jfrank

Reputation: 733

For integers, one way would be to use a combination of the mod operator, which is the percent symbol %, and the divide operator. In your first example, you would compute 144 % 10, resulting in 4. And compute 144 / 10, which gives 14 (as an integer). You can compare the result of the mod operation to half of the denominator, to find out if you should round the 14 up to 15 or not (in this case not), and then multiply back by the denominator to get your answer.

In psuedo code, assuming n is the number to round, p is the power of 10 representing the position of the significant digits:

denom = power(10, p)
remainder = n % denom
dividend = n / denom
if (remainder < denom/2)
  return dividend * denom
else
  return (dividend + 1) * denom

Upvotes: 0

user1209809
user1209809

Reputation: 98

Divide (positive)/Multiply (negative) by the "input rounding" times 10 - 1 (144 / (10 * (2 - 1)). This will give you the same in this instance. Get the remainder of the last digit (4). Determine if it is greater than or equal to 5 (less than). Make it equal to 0 or add 10, depending on the previous answer. Multiply/Divide it back by the "input rounding" times 10 - 1. This should give you your value.

If this is for homework. The purpose is to teach you to think for yourself. I may have given you the answer, but you still need to write the code by yourself.

Next time, you should write your own code and ask what is wrong

Upvotes: 0

user902383
user902383

Reputation: 8640

if you need some advice how to start,

  • step by step write down calculations what you need to do to get from 144,2 --> 140
  • replace your math with java commands, that should be easy, but if you have problem, just look here and here

Upvotes: 1

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