Format Specifiers

Inside {} interpolation you can add :spec to control value formatting. The supported specifiers are:

Spec	Effect	Example
`:.Nf`	N decimal places	`{pi:.2f}` → `3.14`
`:0Nd`	integer with N digits zero-padded	`{n:04d}` → `0042`
`:#x`	integer in hexadecimal with `0x` prefix	`{255:#x}` → `0xff`

Float precision, zero-padding, and hex in interpolation.

04-format-specs.zolo

Playground

// Feature: Format specs in interpolation

// Syntax: `"{expr:spec}"` — spec after `:`

// When to use: float precision, zero-padding, numeric bases.


// -- Float precision (`.Nf`) ------------------------------------

let pi = 3.14159265
print("{pi:.2f}")  // 3.14

print("{pi:.4f}")  // 3.1416

print("{pi:.0f}")  // 3


// -- Numeric zero-padding (`0Nd`) -------------------------------

let n = 42
print("{n:04d}")  // 0042

print("{n:08d}")  // 00000042


// -- Hex with prefix (`#x`) ------------------------------------

print("{255:#x}")  // 0xff

print("{16:#x}")  // 0x10


// -- Practical combinations ------------------------------------

let price = 9.5
print("$ {price:.2f}")  // $ 9.50


let id = 7
print("ID: {id:04d}")  // ID: 0007


// Coordinates with 3 decimal places.

let x = 1.234567
let y = -0.5
print("({x:.3f}, {y:.3f})")

// expected: (1.235, -0.500)


// Sequential numeric IDs with zero-pad.

var i = 1
while i <= 3 {
  print("user_{i:03d}")
  i = i + 1
}
// expected:

// user_001

// user_002

// user_003


// NOTE: alignment (`>` and `<`) and binary/octal bases are not yet

// supported in the current interpolator. Use precision and zero-pad

// for the most common cases.

Note: alignment (>, <) and octal/binary bases are not available in the current interpolator. Use precision and zero-padding for the most common cases.