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

:= — short variable declaration: creates a new variable and assigns a value. = — assignment: sets the value of an existing variable.

Short declarations are the idiomatic way to introduce variables in Go functions. The type is inferred from the right-hand side, making the code concise and readable.

1func main() {
2    x := 42         // declares x as int
3    y, z := 1, "a"  // declares y as int, z as string
4    x = 100         // assignment — x already exists
5}

Assignment (=) updates existing variables. It's used for reassignment, swapping, and receiving values from channels or functions.

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 via assignment

When to use:

• := — introducing new variables, capturing returns, if/for init.
• = — updating, reassigning, swapping.

Common mistakes:

• Shadowing outer variables with := in nested blocks.
• Using := when all variables are already declared — error.
• := at package level — not allowed; use var.
• Shadowing in for loops: new variable per iteration instead of accumulating.