Using variables in printf format

Question

Suppose I have a file like this:

$ cat a
hello this is a sentence
and this is another one

And I want to print the first two columns with so

Answer 1

does this count?

idea is building the "dynamic" fmt, used for printf.

kent$   awk '{n=7;fmt="%"n"-s%s\n"; printf fmt, $1, $2}' f 
hello  this
and    this

Answer 2

Using simple string concatenation.

Here "%", n and "-s%s\n" concatenates as a single string for the format. Based on the example below, the format string produced is %7-s%s\n.

awk -v n=7 '{ printf "%" n "-s%s\n", $1, $2}' file
awk '{ n = 7; printf "%" n "-s%s\n", $1, $2}' file

Output:

hello  this
and    this

Answer 3

If you use * in your format string, it gets a number from the arguments

awk '{printf "%*-s%s\n", 17, $1, $2}' file
hello            this
and              this

awk '{printf "%*-s%s\n", 7, $1, $2}' file
hello  this
and    this

---

As read in The GNU Awk User’s Guide #5.5.3 Modifiers for printf Formats:

The C library printf’s dynamic width and prec capability (for example, "%*.*s") is supported. Instead of supplying explicit width and/or prec values in the format string, they are passed in the argument list. For example:

w = 5
p = 3
s = "abcdefg"
printf "%*.*s\n", w, p, s

is exactly equivalent to:

s = "abcdefg"
printf "%5.3s\n", s

Answer 4

you can use eval (maybe not the most beautiful with all the escape characters, but it works)

i=15
eval "awk '{printf \"%$i-s%s\\n\", \$1, \$2}' a"

output:

hello          this
and            this