* jlforr...@berkeley.edu <jlforr...@berkeley.edu> [190329 11:16]:
> I get the error
>
> assignment mismatch: 2 variable but 1 values
>
> for the assignment statement in Part 2 but the assignment statement in
> Part 1 compiles. Both assignment statements have the same number of
> values, but it appears that referencing an integer in a structure
> field in the 2nd assignment statement, rather than a simple integer,
> confuses the compiler.
> ---------------------------
>
> var a = map[int]int{
> 1: 2,
> }
>
> var b = map[int]Word{
> 1: {2, 3},
> }
>
> // Part 1
> num, ok := a[1]
>
> // Part 2
> num, ok = b[1].code // assignment mismatch: 2 variable but 1 values
To add to what others have said, look at the Go Language Specification
under Index expressions at https://golang.org/ref/spec#Index_expressions
just above the heading "Slice expressions".
It says that if a is a map, an assignment or initialization of the
special form «v, ok = a[x]» yields an additional untyped boolean. In
Part 2 above, you are not assigning b[1] (which would work), but
b[1].code which does not match the special form whose purpose is to
allow the programmer to determine at runtime if the map contains the
given key.
...Marvin
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