Go Programming

111 flashcards by MentisLibrary

Go as it is actually written: types and zero values, slices and maps, structs and interfaces, goroutines and channels, error handling, generics, modules and the testing and tooling that ship with the language.

What does := do in Go?

Short variable declaration: it declares a new variable and initializes it, inferring the type from the right-hand side. It only works inside a function.

What is a zero value?

Every declared variable starts as its type's zero value: 0, false, "", and nil for pointers, slices, maps, channels, functions and interfaces.

var vs :=

var works at package and function scope and may omit the initializer; := only works inside functions and always needs a value.

How do you declare a constant?

const name = value for an untyped constant, or const name Type = value for a typed one. Untyped constants take their type from the context they are used in.

What is iota?

A counter inside a const block that starts at 0 and increases by one on each line, used to build enumerations.

Which loop constructs does Go have?

Only for, in three shapes: the C-style three-clause loop, a condition-only loop that acts like while, and for range.

What does range yield over a slice, a map, a string and a channel?

Slice: index and element. Map: key and value. String: byte offset and rune. Channel: each received value.

How does switch differ from C?

Cases do not fall through, so no break is needed; a case may list several values, and a tagless switch takes boolean expressions.

defer

Schedules a function call to run when the surrounding function returns; multiple deferred calls run in last-in, first-out order.

When are deferred arguments evaluated?

At the moment the defer statement executes, even though the call itself runs later.

How do multiple return values work?

A function lists several result types in parentheses and returns them together; the caller receives them into several variables.

What are named return values?

Result parameters with names, which declares them as zero-valued variables in the function and lets a bare return send back their current values.

How do you write a variadic function?

Make the final parameter ...T; callers pass zero or more values and the function sees them as a []T.

What is a closure in Go?

A function literal that captures variables from the enclosing scope by reference, so it can read and modify them after that scope has returned.

What changed about loop variables in Go 1.22?

Each iteration of a for loop now gets its own copy of the loop variable, so closures created in different iterations capture different values.

Are arguments passed by value or by reference?

Always by value: the callee gets a copy. Slices, maps, channels, functions and pointers copy a small header or address, so the underlying data is shared.

How do pointers work in Go?

&x takes the address of x, *p dereferences the pointer p, and *T is the pointer-to-T type.

new vs make

new(T) allocates a zeroed T and returns a *T. make initializes a slice, map or channel and returns the value itself, ready to use.

Arrays vs slices

An array has a fixed length that is part of its type and is copied on assignment. A slice is a window onto an array, with a length and a capacity, and assignment shares the backing array.

What are a slice's length and capacity?

len is how many elements the slice currently holds; cap is how many the backing array can hold counting from the slice's first element.

How does append work?

It adds elements to a slice, reusing the backing array when capacity allows and otherwise allocating a larger array and copying. It returns the new slice header.

What does s[lo:hi:max] mean?

The full slice expression: elements lo up to but excluding hi, with the capacity capped at max - lo.

What does copy do?

copy(dst, src) copies min(len(dst), len(src)) elements from src into dst and returns how many it copied.

How do maps behave?

map[K]V is a hash table with comparable keys. Reading a missing key returns the zero value of V rather than an error.

What is the comma ok idiom?

The two-value map lookup v, ok := m[k] returns the value and a boolean saying whether the key exists.

How do you remove a map entry?

delete(m, key) removes it; deleting a key that is not there is a harmless no-op.

Why can a slice not be a map key?

Map keys must support ==, and slices, maps and functions are not comparable.

How do you sort a slice?

slices.Sort for ordered element types, or slices.SortFunc with a comparison function returning negative, zero or positive.

How do you define and create a struct?

type Name struct { fields } defines it; a composite literal with keyed fields creates a value.

What is struct embedding?

Listing a type inside a struct without a field name embeds it and promotes its fields and methods to the outer type.