How do I round to 1 decimal on the bbc:microbit with javascript w/o toFixed(), the usual multiplication + divide gives trailing 9999 digits?

Question

temperatureReading = Math.round(temperatureReading * 10) / 10

gives me 26.29999999999999999999 instead of 26.3

And 26.00000000001 instead of 26.0

I get alternating 2 values from the temperature sensor: 26.33 and 26.3200000

After the conversion I have: 26.2999999999999

The number of the repeating digits above is just an example. My display on the micro bit is not wide enough to see them all.

I use toString() to display the values.

Unfortunately, toFixed() and toPrecision() is not available on the micro:bit

Can the rounding be achieved by some other operations?

Is `temperatureReading` `26.29999999999999999999` to begin with? — Jack Bashford, Jul 27 '19 at 10:03
Also should be worth noting that pasting `temperatureReading = 26.29999999999999999999` into the Safari console yields `26.3`. — Jack Bashford, Jul 27 '19 at 10:04
Javascript is not the language you want here. For precision calculations, where its crucial your number has no rounding errors, you should not use JS. FORTRAN is made for that kind of thing, if I recall correctly! — somethinghere, Jul 27 '19 at 10:44

score 1 · Accepted Answer · answered Jul 27 '19 at 12:33

With the following code I can now get numbers with 1 decimal as a string:

let temperatureStr = Math.round(temperatureReading * 10).toString()
let temperature = temperatureStr.slice(0, temperatureStr.length - 1) + "." + temperatureStr.slice(temperatureStr.length - 1);

I first multiply the number by 10 and convert the result to a string. Then, I insert the decimal point before the last digit. This gives me the string I want to print.

How do I round to 1 decimal on the bbc:microbit with javascript w/o toFixed(), the usual multiplication + divide gives trailing 9999 digits?

1 Answers1