awk problems remembering previous line - awk

Question

I have the following awk file;
BEGIN { FS=":" };
{if (NR%2==1) { host=$1 }};
{if (NR%2==0) { print $host ":" $0 }};
I want to do the following;
If the line # is odd, store the 1st field.
If the line # is even, print the previously stored field, a colon, plus the current line.
currently this outputs the even numbered line twice "evenline:evenline"
not sure what i'm doing wrong.

Answer 1

It should be:
BEGIN { FS=":" };
{if (NR%2==1) { host=$1 }};
{if (NR%2==0) { print host ":" $0 }};
$host -> host.
Why?
Field names in awk start with a dollar $ in front. You can access fields in awk statically like $1, $2 etc. or dynamically like $variable. Variables will get casted to integers when used for field names because field names in awk are numbers. The variable host contains a string which will get casted to 0. This makes awk print $0 twice.
Note that you can simplify this:
BEGIN { FS=":" }
NR%2==1 { host=$1 };
NR%2==0 { print host ":" $0 }