Write PL/SQL code to generate Armstrong number from 1 to 500 - oracle

Question

I am getting output as a = 1. I have taken a for loop from 1 to 500 and while loops inside the outer for loop.
declare
n number;
s number:=0;
r number;
len number;
m number;
begin
for a in 1..500 loop
m:=a;
n:=a;
len:=length(to_char(n));
while(n>0) loop
r:=mod(n,10);
s:=s+power(r,len);
n:=trunc(n/10);
end loop;
if m=s then
dbms_output.put_line('a='||to_char(a)');
end if;
end loop;
end;

Answer 1

How about this?
substr splits i to 3 separate digits
nvl is here to avoid adding null value if those digits don't exist (yet)
power function calculates i's cube
display i if it is equal to r (as "result")
SQL> declare
2 r number;
3 begin
4 for i in 1 .. 500 loop
5 r := power(to_number(substr(to_char(i), 1, 1)), 3) +
6 nvl(power(to_number(substr(to_char(i), 2, 1)), 3), 0) +
7 nvl(power(to_number(substr(to_char(i), 3, 1)), 3), 0);
8
9 if r = i then
10 dbms_output.put_line(i);
11 end if;
12 end loop;
13 end;
14 /
1
153
370
371
407
PL/SQL procedure successfully completed.
SQL>

Answer 2

You never reset s and you do not need the final IF statement (unless you are only interested in the Armstrong numbers which equal the original number):
declare
n number;
s number:=0;
r number;
len number;
m number;
begin
for a in 1..500 loop
m:=a;
n:=a;
s:=0; -- Reset s for each loop
len:=length(to_char(n));
while(n>0) loop
r:=mod(n,10);
s:=s+power(r,len);
n:=trunc(n/10);
end loop;
dbms_output.put_line(a || '=' || s); -- Output values for every loop.
end loop;
end;
/
db<>fiddle here