|
 |
|
PL/SQL FOR Loop tips
Oracle Tips by Burleson Consulting |
The PL/SQL FOR Loop
The FOR loop executes for a specified number of
times, defined in the loop definition. Because the number of loops
is specified, the overhead of checking a condition to exit is
eliminated. The number of executions is defined in the loop
definition as a range from a start value to an end value (inclusive).
The integer index in the FOR loop starts at the start value and
increments by one (1) for each loop until it reaches the end value.
SQL> begin
2 for idx in 2..5 loop
3
dbms_output.put_line (idx);
4 end loop;
5 end;
6 /
2
3
4
5
PL/SQL
procedure successfully completed.
In the example below a variable idx is
defined, assigning it the value 100. When the FOR loop executes,
the variable idx is also defined as the index for the FOR loop.
The original variable idx goes out of scope when the FOR loop
defines its index variable. Inside the FOR loop, the idx
variable is the loop index. Once the FOR loop terminates, the loop
index goes out of scope and the original idx variable is again in
scope.
SQL> declare
2 idx number := 100;
3 begin
4 dbms_output.put_line (idx);
5
for idx in 2..5 loop
6
dbms_output.put_line (idx);
7 end loop;
8 dbms_output.put_line (idx);
9 end;
10 /
100
2
3
4
5
100
PL/SQL
procedure successfully completed.
You can use the loop index inside the loop, but you
can not change it. If you want to loop by an increment other than
one, you will have to do so programmatically as the FOR loop will only
increment the index by one.
SQL> begin
2 for i in 4 .. 200 loop
3
i := i + 4;
4 end
loop;
5 end;
6 /
i
:= i + 4;
*
ERROR at line 3:
ORA-06550: line 3, column 5:
PLS-00363: expression 'I' cannot be used
as an assignment target
ORA-06550: line 3, column 5:
PL/SQL:
Statement ignored
The loop index start and stop values can be
expressions or variables. They are evaluated once at the start of
the loop to determine the number of loop iterations. If their
values change during the loop processing, it does not impact the number
of iterations.
SQL> declare
2 n_start number := 3;
3
n_stop number := 6;
4 begin
5
for xyz in n_start .. n_stop loop
6
n_stop := 100;
7
dbms_output.put_line (xyz);
8 end loop;
9 end;
10 /
3
4
5
6
PL/SQL
procedure successfully completed.
Line 6 changes the stop value, setting it to 100.
But the loop still terminates at the value of 6. The loop index
start and stop values are always defined from lowest to highest.
If you want the index to count down use the REVERSE key word.
SQL> begin
2 for num in 4 .. 7 loop
3
dbms_output.put_line (num);
4 end loop;
5
6 for num in reverse 4 .. 7 loop
7 dbms_output.put_line (num);
8
end loop;
9
10 for num in 7 .. 4 loop
11 dbms_output.put_line (num);
12
end loop;
13 end;
14 /
4
5
6
7
7
6
5
4
PL/SQL
procedure successfully completed.
Notice that the third FOR loop COMPILED BUT DID NOT
EXECUTE! The FOR loop calculated the number of loop iterations and
got a negative number, therefore the loop count was zero.
In the next example a FOR loop is used to calculate
the factorial of a number. A factorial value is commonly used to
determine all possible values for a number and is defined as
x*(x-1)*(x-2)….(0) = !x.
!8 =
8*7*6*5*4*3*2*1 = 40320
SQL> declare
2 v_seed number := &numb;
3
v_hold number := 1;
4 begin
5
for i in reverse 1 .. v_seed loop
6
v_hold := v_hold * i;
7 end loop;
8 dbms_output.put_line ('!'||v_seed||' = '||v_hold);
9 end;
10 /
Enter value for
numb: 8
!8 = 40320
SQL> /
Enter
value for numb: 4
!4 = 24
Related PL/SQL FOR Loop Articles:
|
