Format Double up to Max Non-Zero Significant Digits

Question

Is there a format specifier to format a double to only include up to a max number of non-zero significant digits after the decimal place?

For example, if I want a max of 4 significant digits and the number is:

3.14159265359, I want to display 3.1459

5.7350, I want 5.735

2.680, I want 2.68

9.200, I want 9.2

7.000, I want 7 (no decimal point)

What have you tried? Does the NumberFormatter documentation mention anything about this? — Alexander, Sep 02 '20 at 21:07
What you described as *significant digits after the decimal place* is called *fraction digits* in the documentation. — pawello2222, Sep 02 '20 at 21:19

score 3 · Accepted Answer · answered Sep 02 '20 at 21:34

You can use NumberFormatter. You just need to set the maximumFractionDigits to 4 and the minimumFractionDigits to 0:

let nf = NumberFormatter()
nf.numberStyle = .decimal
nf.maximumFractionDigits = 4
nf.minimumFractionDigits = 0

nf.string(for: 3.14159265359)  // "3.1416"
nf.string(for: 5.7350)         // "5.735"
nf.string(for: 2.680)          // "2.68"
nf.string(for: 9.200)          // "9.2"
nf.string(for: 7.000)          // "7"

If you would like to round down the result i.e. 3.1415 you just need to set the NumberFormatter roundingMode property to .floor:

let nf = NumberFormatter()
nf.numberStyle = .decimal
nf.maximumFractionDigits = 4
nf.minimumFractionDigits = 0
nf.roundingMode = .floor
nf.string(for: 3.14159265359)  // "3.1415"

score 0 · Answer 2 · answered Sep 02 '20 at 21:23

This should work for you:

`let number: Float = 3.14159

let str1 = String(format: "%.2f", number) // 3.14

let str2 = String(format: "%.4f", number) // 3.1416 (rounded)

There is a really good tutorial here

Let me know if that works! This is formatting the number once it is presented as a string value.

If you are trying to actually change the number value, this was already answered on stackoverflow.

Format Double up to Max Non-Zero Significant Digits

2 Answers2