awk sum addition math on fields & colums $ - strange results - awk

Question

Please run this script. I wonder why I can not assing $1, $2 and $3 in BEGIN in order to calculate and print them:
BEGIN {
OFS=FS=";" ;
# #include getopt.awk
geb = 4;
dis = 3;
$1 = 10;
$2 = 0.19;
$3 = 20;
summe = geb+dis+$1;
colsum = $1+$2+$3
}
{
print $1 FS $2 FS $3 FS "Fee" " "summe FS $1+$3 FS 3+4+$1 FS colsum}
For example I hoped that
print $1+$3
gives me 30 ?!
Can't I assign new values to fields?

Answer 1

The BEGIN block happens before awk begins processing the file, so it doesn't make sense to assign to the individual fields, as they will be overwritten once the first record is read.
If you wish to perform calculations on the records that awk reads, this should be done in a normal block, like the one you are using to print.

Answer 2

BEGIN blcoks are executed before any input is read (unless use getline), so consequently none of the variables that refer to input, like NR,FNR,NF, fields like $0, $1, $2 ...$10 will be defined in any of the BEGIN blocks.
In fact BEGIN block is for actions to execute before you read the first line
(unless use getline) - Not Recommended
akshay#db-3325:/tmp$ seq 1 5 >test
akshay#db-3325:/tmp$ cat test
1
2
3
4
5
# Default action by awk
akshay#db-3325:/tmp$ awk 'BEGIN{print $1}' test
# if you use getline
akshay#db-3325:/tmp$ awk 'BEGIN{getline;print $1}' test
1