Go: What is the difference between := and =?

:= — short variable declaration: creates a new variable and assigns. = — assignment: updates an existing variable.

Short declarations are the standard way to introduce variables in Go functions. The type is inferred. At least one variable on the left must be new.

1func main() {
2    x := 42       // new: x
3    y, z := 1, 2  // new: y, z
4    x, w := 3, 4  // OK: x reassigned, w is new
5}

Assignment updates existing variables. Use for reassignment, swapping, or receiving values.

1var count int
2for i := 0; i < 10; i++ {
3    count++  // assignment
4}
5
6a, b := 1, 2  // short declaration
7a, b = b, a   // swap

When to use:

• := — introducing new variables.
• = — updating existing variables.

Common mistakes:

• Shadowing: := in nested blocks hides outer variables.
• Using := when all variables exist — error.
• := at package level — not allowed.
• Shadowing in for loops.