awk: transpose column header to first field of row - awk

Question

My input looks like this:
A|B|C
1|2|3
4|5|6
Using awk, I am trying to get:
A|1
B|2
C|3
A|4
B|5
C|6
My code:
gawk '
BEGIN{FS=OFS="|"}
NR==1{
for(i=1; i<=NF; i++){
x_i=$i
}
}
NR>1{
for(i=1; i<=NF; i++){
print x_i FS $i
}
}' input
But it keeps only the last iteration of the NR==1 block even if I use the same loop in NR>1 bock:
C|1
C|2
C|3
C|4
C|5
C|6
Any trick?
EDIT
Thanks to Jose, I needed to change x_i by x[i].
What about if using the same input, I would need to output:
A;B;C|1|2|3
A;B;C|4|5|6

Answer 1

$ awk 'BEGIN{FS=OFS="|"} NR==1{split($0,h);next} {for (i=1;i<=NF;i++) print h[i], $i}' file
A|1
B|2
C|3
A|4
B|5
C|6
$ awk 'BEGIN{FS=OFS="|"} NR==1{gsub(/\|/,";");h=$0;next} {print h, $0}' file
A;B;C|1|2|3
A;B;C|4|5|6
Read Effective Awk Programming, 4th Edition, by Arnold Robbins.

Answer 2

you can try,
awk 'BEGIN{FS=OFS="|"}
NR==1{for(i=1; i<=NF; ++i) d[i]=$i; next}
{for(i=1; i<=NF; ++i) print d[i], $i}
' input
you get
A|1
B|2
C|3
A|4
B|5
C|6
Important Note
your logic is correct, only x[i] instead of x_i
gawk '
BEGIN{FS=OFS="|"}
NR==1{
for(i=1; i<=NF; i++){
x[i]=$i
}
}
NR>1{
for(i=1; i<=NF; i++){
print x[i] FS $i
}
}' input

Answer 3

Here is another using split and for:
$ awk 'NR==1 { split($0,a,"|") }
NR>1 { n=split($0,b,"|");
for(i=1;i<=n;i++)
print a[i] "|" b[i] }' file
A|1
B|2
C|3
A|4
B|5
C|6