Cannot use word (type interface {}) as type string in assignment: need type assertion

Question

I'm new to Go and what i'm doing for some reason doesn't seem very straight-forward to me.

Here is my code:

for _, column := range resp.Values {
  for _, word := range column {

    s := make([]string, 1)
    s[0] = word
    fmt.Print(s, "\n")
  }
}

and I get the error:

Cannot use word (type interface {}) as type string in assignment: need type assertion

resp.Values is an array of arrays, all of which are populated with strings.

reflect.TypeOf(resp.Values) returns [][]interface {},

reflect.TypeOf(resp.Values[0]) (which is column) returns []interface {},

reflect.TypeOf(resp.Values[0][0]) (which is word) returns string.

My end goal here is to make each word its own array, so instead of having:

[[Hello, Stack], [Overflow, Team]], I would have: [[[Hello], [Stack]], [[Overflow], [Team]]]

You need to first type assert that the word is actually a string. One, prone-to-panic, way would be s[0] = word.(string). — mkopriva
– mkopriva, Commented Jan 19, 2018 at 17:11
Have a look if this helps you: stackoverflow.com/a/71930507/18579038 — Mohammad Reza Karimi
– Mohammad Reza Karimi, Commented Apr 19, 2022 at 19:56

maerics · Accepted Answer · 2018-01-19 17:58:21Z

6

The prescribed way to ensure that a value has some type is to use a type assertion, which has two flavors:

s := x.(string) // panics if "x" is not really a string.
s, ok := x.(string) // the "ok" boolean will flag success.

Your code should probably do something like this:

str, ok := word.(string)
if !ok {
  fmt.Printf("ERROR: not a string -> %#v\n", word)
  continue
}
s[0] = str

answered Jan 19, 2018 at 17:58

maerics

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