Awk variable and summation - awk

Question

I would like to understand the difference in result for the following awk commands.
I have read that when awk introduces numerical variables they are set to zero by default, so would assume that sum=0 would be implicitly assumed.
However 1) gives an incorrect result, while 2) is correct.
Aim: Find the total number of lines in a file without using NR
financial.txt
14D 20190503 0.31 0.31 0.295 0.295 117949
14DO 20190503 0.00 0.00 0.00 0.07 0
1AD 20190503 0.18 0.19 0.18 0.19 54370
1AG 20190503 0.041 0.042 0.041 0.042 284890
1AL 20190503 0.00 0.00 0.00 0.88 0
1ST 20190503 0.05 0.05 0.049 0.049 223215
3DP 20190503 0.049 0.054 0.048 0.048 2056379
3PL 20190503 1.055 1.06 1.02 1.05 120685
4CE 20190503 0.00 0.00 0.00 0.009 0
4DS 20190503 0.072 0.076 0.072 0.075 2375896
$ awk 'BEGIN {sum+=1} END {print sum}' financial.txt
1
$ awk 'BEGIN {sum=0}{sum+=1} END {print sum}' financial.txt
5527
Thanks
After reviewing comments, I found the solution I was looking for without using BEGIN.
$ awk '{sum+=1}END{print sum}' financial.txt
5527

Answer 1

All awk variables are initialized to zero-or-null. If first used in a numeric context they become 0 at that point while if first used in a string context they become null at that point. Wrt your code samples, this:
BEGIN {sum+=1} END {print sum}
means:
BEGIN {sum+=1}
END {print sum}
while this:
BEGIN {sum=0}{sum+=1} END {print sum}
means:
BEGIN {sum=0}
<true> {sum+=1}
END {print sum}
See the difference? Add ;print sum before every } to trace how sum is being populated if it's not obvious what's happening.

Answer 2

From GNU AWK Manual :
A BEGIN rule is executed once only, before the first input record is read. Likewise, an END rule is executed once only, after all the input is read.
Thus, the following will execute "{sum+=1}" statement only once.
awk 'BEGIN {sum+=1} END {print sum}' financial.txt
But, in 2nd case "{sum+=1}" is executed for every line read from the file.
awk 'BEGIN {sum=0}{sum+=1} END {print sum}' financial.txt