How to declare a constant array in Go?

Question

I have tried:
const ascii = "abcdefghijklmnopqrstuvwxyz"
const letter_goodness []float32  = { .0817,.0149,.0278,.0425,.1270,.0223,.0202, .0609,.0697,.0015,.0077,.0402,.0241,.0675, .0751,.0193,.0009,.0599,.0633,.0906,.0276, .0098,.0236,.0015,.0197,.0007 }
const letter_goodness  = { .0817,.0149,.0278,.0425,.1270,.0223,.0202, .0609,.0697,.0015,.0077,.0402,.0241,.0675, .0751,.0193,.0009,.0599,.0633,.0906,.0276, .0098,.0236,.0015,.0197,.0007 }
const letter_goodness = []float32 { .0817,.0149,.0278,.0425,.1270,.0223,.0202, .0609,.0697,.0015,.0077,.0402,.0241,.0675, .0751,.0193,.0009,.0599,.0633,.0906,.0276, .0098,.0236,.0015,.0197,.0007 }

First, declaration and initialization works fine, but the second, third and fourth don't work. How can I declare and initialize const array of floats?

Chris Gregori · Accepted Answer

An array isn't immutable by nature; you can't make it constant. The nearest you can get is: var letter_goodness = [...]float32 {.0817, .0149, .0278, .0425, .1270, .0223, .0202, .0609, .0697, .0015, .0077, .0402, .0241, .0675, .0751, .0193, .0009, .0599, .0633, .0906, .0276, .0098, .0236, .0015, .0197, .0007 } Note the [...] instead of []: it ensures you get a (fixed size) array instead of a slice. So the values aren't fixed but the size is.

Gopher · Answer

From Effective Go: Constants in Go are just that—constant. They are created at compile time, even when defined as locals in functions, and can only be numbers, strings or booleans. Because of the compile-time restriction, the expressions that define them must be constant expressions, evaluatable by the compiler. For instance, 1<<3 is a constant expression, while math.Sin(math.Pi/4) is not because the function call to math.Sin needs to happen at run time. Slices and arrays are always evaluated during runtime: var TestSlice = []float32 {.03, .02} var TestArray = [2]float32 {.03, .02} var TestArray2 = [...]float32 {.03, .02} [...] tells the compiler to figure out the length of the array itself. Slices wrap arrays and are easier to work with in most cases. Instead of using constants just make the variables unaccessible to other packages by using a lower case first letter: var ThisIsPublic = [2]float32 {.03, .02} var thisIsPrivate = [2]float32 {.03, .02} thisIsPrivate is available only in the package it is defined. If you need read access from outside you can then write a simple getter function (see Getters in golang).

Go Language

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