Why are my test for checking floats keeps returning false?

Question

I created a javascript method that checks if a number is a float. Each time I test the method for a float it keeps returning FALSE. I am not sure what I am doing wrong here. Any help would be really appreciated!

Below is my code.. Thanks!

function isFloat(n){

    var patt =  new RegExp('[+-]([0-9]*[.])?[0-9]+');
    return patt.test(n)
}


console.log(isFloat(12.40));

Why do you use `new Regexp(...)` When you can just use `/.../g`? — Bálint, Jul 08 '16 at 20:41
Why don't you try this instead: http://stackoverflow.com/a/3886106/693275 — Rob M., Jul 08 '16 at 20:44
See, you can make a regex by inserting it inside 2 `/`s like I wrote above. Then you can add flags at the end of it — Bálint, Jul 08 '16 at 20:50
Your regex looks like you are trying to check if a string is numeric, but your description is like you want to check if a number is not an integer. I'm confused. — Oriol, Jul 08 '16 at 20:50

Mike Cluck · Accepted Answer · 2016-07-08T20:47:38.267

3

There's a much easier way to check if a number is a float. Just compare the floored value against the original value. If they're the same, it's an integer.

function isFloat(n) {
  return Math.floor(n) !== n;
}

console.log(isFloat(1));
console.log(isFloat(1.2));
console.log(isFloat(12));
console.log(isFloat(12.4));

This will work if, and only if, you only use it on numbers. You can perform an additional check if you're worried about someone passing in non-numbers.

function isFloat(n) {
  return typeof n === 'number' && Math.floor(n) !== n;
}

Or you can simplify this even further by using Number.isInteger, provided you're running an environment that supports it.

function isFloat(n) {
  return typeof n === 'number' && !Number.isInteger(n);
}

console.log(isFloat(1));
console.log(isFloat(1.2));
console.log(isFloat(12));
console.log(isFloat(12.4));
console.log(isFloat('a'));

edited Jul 08 '16 at 20:47

answered Jul 08 '16 at 20:43

Mike Cluck

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FYI `isFloat('a') === true` (don't you just love `NaN`?! :P) – Rob M. Jul 08 '16 at 20:44
@PhilVarg `12.0 === 12` so it's effectively an integer, as long as you care about actual values and not string representations. – Mike Cluck Jul 08 '16 at 20:45
Thanks a lot Mike C! – json2021 Jul 08 '16 at 22:58

score 0 · Answer 2 · answered Jul 08 '16 at 20:45

0

Your RegEx is wrong, it should be

/[+-]?\d+\.\d+/g

Also, as others noted, this is not the best solution for this. Use !Number.isInteger(num)

answered Jul 08 '16 at 20:45

Bálint

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score 0 · Answer 3 · edited May 23 '17 at 12:32

0

The problem is that toString does not insert '+' before a number:

12.40.toString(); // "12.4", not "+12.4"

So you could use a regular expression like

new RegExp('[+-]?([0-9]*[.])?[0-9]+');

However, I recommend Validate decimal numbers in JavaScript - IsNumeric()

edited May 23 '17 at 12:32

Community

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answered Jul 08 '16 at 20:45

Oriol

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Why are my test for checking floats keeps returning false?

3 Answers3