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Oracle Tips by Burleson |
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
Notice that the variable v_hold is given an
initial value of 1. This is important because if it were not
initialized it would start with a NULL value and the code would return
NULL due to NULL math (refer to the Null Math section in Chapter 1,
“Introduction to PL/SQL”). Likewise if a value is initialized to 0,
the code would be multiplying by zero (0) always resulting in zero
(0).
SQL> declare
2 v_seed number := &numb;
3 v_hold number;
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: 4
!4 = --(NULL)
SQL> declare
2 v_seed number := &numb;
3 v_hold number := 0;
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: 4
!4 = 0 -- (multiplication by 0)
The above book excerpt is from:
Easy Oracle PL/SQL Programming
Get Started
Fast with Working PL/SQL Code Examples
ISBN 0-9759135-7-3
John Garmany
http://www.rampant-books.com/book_2005_1_easy_plsql.htm
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