How to check for empty array (array of struct)

Question

A mySQL Query function returns an array of rows witch is defined as

type Row []interface{}

I would like to check if the returned array is empty, but I get a runtime panic:

s := fmt.Sprintf("select id, secret, shortname from beehives where shortname = '%s'", beehive)

rows, res, err := database.Query(s)
if err == nil {
    if len(rows) != 1 {

The expression len(rows) seems to cause the runtime panic if rows is empty.

How can I check for an empty array? I tried also rows == nil, which also panics.

Which library is this? Because golang.org/pkg/database/sql/#DB.Query returns two variables. You could check if rows.Next() returns false on the first check I suppose? — Taco de Wolff
– Taco de Wolff, Commented Jan 19, 2015 at 22:40

dave · Accepted Answer · 2018-02-01 18:11:12Z

7

It looks like you could use QueryRow, since that query is expected to return only one row.

QueryRow executes a query that is expected to return at most one row. QueryRow always return a non-nil value. Errors are deferred until Row's Scan method is called.

In this case, if there is no row, the ErrNoRows occurs, but is deferred until a .Scan occurs.

ErrNoRows is returned by Scan when QueryRow doesn't return a row. In such a case, QueryRow returns a placeholder *Row value that defers this error until a Scan.

So what you want to do is something like:

var id int
var secret string
var shortname string
err := db.QueryRow("SELECT ...").Scan(&id, &secret, &shortname)
switch {
    case err == sql.ErrNoRows:
        log.Printf("Not found.")
    case err != nil:
        log.Fatal(err)
    default:
        //do stuff
}

Otherwise, since you need to be looping over rows.Next anyways, you can easily set a flag:

defer rows.Close()
has_results := false
for rows.Next() {
    has_results = true
    //do stuff
}
if (!has_results) {
    //error handling
}

edited Feb 1, 2018 at 18:11

answered Jan 19, 2015 at 22:51

dave

65.1k6 gold badges81 silver badges98 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Michael Over a year ago

Thanks. It works now. I changed from github.com/ziutek/mymysql package to github.com/go-sql-driver/mysql. I much happier with it.

Collectives™ on Stack Overflow

How to check for empty array (array of struct)

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related