Explicit Arguments Indexes With Fmt Package

Golang has a very handy package called fmt for formating data values.

fmt.Printf("Hello %s", "world") // Hello World

But by default, the fmt family of functions work by successively applying args passed after the format string to matching format verbs by argument order.

tablePrefix := "mydb"

query := fmt.Sprintf("SELECT %s_table1.*, %s_table2.id from %s_table1 left join %s_table2 on %s_table2.id = %s_table1.id;",tablePrefix, tablePrefix, tablePrefix, tablePrefix, tablePrefix)

fmt.Print(query)

// SELECT mydb_table1.*, mydb_table2.id from mydb_table1 left join mydb_table2 on mydb_table2.id = mydb_table1.id;

while the above does achieves our desired outcome, it does so by duplicating code heavily. it would be better if tablePrefix didn't have to be passed several times to fmt function if the same data value is used by many format specifiers.

Thankfully, fmt package designers saw a need for that and provided a way to avoid such a duplication of code.

from the fmt documentation.

import "fmt"

Explicit argument indexes:

In Printf, Sprintf, and Fprintf, the default behavior is for each formatting verb to format successive arguments passed in the call. However, the notation [n] immediately before the verb indicates that the nth one-indexed argument is to be formatted instead.

This means we can simplify the function call to look like below.

tablePrefix := "mydb"

query := fmt.Sprintf("SELECT %[1]s_table1.*, %[1]s_table2.id from %[1]s_table1 left join %[1]s_table2 on %[1]s_table2.id = %[1]s_table1.id;",tablePrefix) // notice only one data argument is needed if you use a position based trick

fmt.Print(query)

// SELECT mydb_table1.*, mydb_table2.id from mydb_table1 left join mydb_table2 on mydb_table2.id = mydb_table1.id;

The position starts from index 1 instead of 0 probably because the format string is the zeroth argument to fmt functions.

Such a simple tricks is easy to forget and I am puting it here as a reminder to my self when I need it and I also hope someone in future may find it helpful too.

Happy coding!