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PL/SQL FOR Loop tips
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
Related PL/SQL FOR Loop Articles:
The FOR loop runs one or more executable
statements placed with in its loop structure while the loop index value
is between the lower bound and the upper bound.
The below prototype defines the basic structure of the
FOR loop.
For <loop_index> in [Reverse] <lower_bound> .. <upper_bound>
loop
<Executable statements>;
End loop <loop_index>;
%
Note: Reverse keyword is optional
in the FOR loop’s structure. The <loop_index> placed after the
END LOOP syntax is optional and it is meant for identifying a
particular loop’s end with ease.
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The <lower_bound> value cannot be greater than the <upper_bound>
value, else the loop will not run even once.
The <upper_bound> value cannot be lower the <lower_bound>
value, else the loop will not run even once.
%
Note: If the <lower_bound> and
the <upper_bound> values are equal, the FOR loop executes only
once irrespective of its bound value.
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While the reverse keyword is placed, the <loop_index>
value starts at the <lower_bound> and increments itself by 1 for each iteration
of the loop until it reaches the <upper_bound>.
While the reverse keyword is not placed, the <loop_index>
value starts at the <upper_bound> and decrements itself by 1 for each iteration
of the loop until it reaches the <lower_bound>.
PL/SQL FOR loop tips The below script runs the loop for 5 times starting from the lower bound value 1 incrementing itself by 1 until it reaches the upper bound value 5. 1. BEGIN 2. FOR loop_index IN 1..5 3. LOOP 4. dbms_output.put_line(loop_index); 5. END LOOP loop_index; 6. END; 7. / Result: 1 2 3 4 5 Line no. | Description | 1 | Start of the execution section of the block | 2,3 | Start of the FOR loop statement with lower bound value as 1 and upper bound value as 5 | 4 | The loop index value is printed using the DBMS_OUTPUT.PUT_LINE procedure | 5 | End of the FOR loop | 6,7 | End of the execution section of the block | For loop with REVERSE keyword The below script runs the loop for 5 times starting from the upper bound value 5 decrementing itself by 1 until it reaches the lower bound value 1. 1. BEGIN 2. FOR loop_index IN reverse 1 .. 5 3. LOOP 4. dbms_output.put_line(loop_index); 5. END LOOP loop_index; 6. END; 7. / Result: 5 4 3 2 1 Line no. | Description | 1 | Start of the execution section of the block | 2,3 | Start of the FOR loop statement with lower bound value as 1, upper bound value as 5 and reverse keyword placed. | 4 | The loop index value is printed using the DBMS_OUTPUT.PUT_LINE procedure | 5 | End of the FOR loop | 6,7 | End of the execution section of the block | FOR loop with decimal loop index value The below FOR loop has decimal values for its lower bound and upper bound values. These loop index values are rounded off to its nearest value and then the loop starts its execution. In the below example, the lower bound value of 1.1 is rounded off to 1 and the upper bound value of 5.5 is rounded off to 6, thus the loop runs for 6 times starting from the rounded lower bound value of 1 through the rounded upper bound value of 6. 1. BEGIN 2. FOR loop_index IN 1.1 .. 5.5 3. LOOP 4. dbms_output.put_line(loop_index); 5. END LOOP loop_index; 6. END; 7. / Result: 1 2 3 4 5 6 Line no. | Description | 1 | Start of the execution section of the block | 2,3 | Start of the FOR loop statement with lower bound value as 1.1 and upper bound value as 5.5 | 4 | The loop index value is printed using the DBMS_OUTPUT.PUT_LINE procedure | 5 | End of the FOR loop | 6,7 | End of the execution section of the block | FOR loop with non-numeric loop index value The loop index is implicitly created with PLS_INTEGER data type and when the lower bound and the upper bound values of a FOR loop are of non-numeric literals, the loop’s execution fails resulting in an exception. The below example immediately raises the predefined exception VALUE_ERROR when executed. 1. BEGIN 2. FOR loop_index IN 'A' .. 'D' 3. LOOP 4. dbms_output.put_line(loop_index); 5. END LOOP loop_index; 6. END; 7. / Error report: ORA-06502: PL/SQL: numeric or value error: character to number conversion error ORA-06512: at line 2 06502. 00000 - "PL/SQL: numeric or value error%s" Line no. | Description | 1 | Start of the execution section of the block | 2,3 | Start of the FOR loop statement with lower bound value as ‘A’ and upper bound value as ‘D’ | 4 | The loop index value is printed using the DBMS_OUTPUT.PUT_LINE procedure | 5 | End of the FOR loop | 6,7 | End of the execution section of the block | FOR loop index The loop index value has its scope inside the FOR loop’s structure only and when it is accessed outside its structure, the loop fails with the below compilation error. 1. BEGIN 2. FOR loop_index IN 1 .. 5 3. LOOP 4. dbms_output.put_line(loop_index); 5. END LOOP loop_index; 6. dbms_output.put_line(loop_index); 7. END; 8. / Error report: ORA-06550: line 6, column 24: PLS-00201: identifier 'LOOP_INDEX' must be declared ORA-06550: line 6, column 3: PL/SQL: Statement ignored 06550. 00000 - "line %s, column %s:\n%s" *Cause: Usually a PL/SQL compilation error. Line no. | Description | 1 | Start of the execution section of the block | 2,3 | Start of the FOR loop statement with lower bound value as 1 and upper bound value as 5 | 4 | The loop index value is printed using the DBMS_OUTPUT.PUT_LINE procedure | 5 | End of the FOR loop | 6 | The loop index value is printed using the DBMS_OUTPUT.PUT_LINE procedure | 7,8 | End of the execution section of the block | The statements inside the loop can access the loop index’s value but it can never be assigned with any value. Once the loop’s execution is completely done, the loop index’s value is reset to Null. The below example fails with an error when its loop index is used as an assignment target. 1. BEGIN 2. FOR loop_index IN 1 .. 5 3. LOOP 4. loop_index:=10; 5. dbms_output.put_line(loop_index); 6. END LOOP loop_index; 7. END; 8. / Error report: ORA-06550: line 4, column 5: PLS-00363: expression 'LOOP_INDEX' cannot be used as an assignment target ORA-06550: line 4, column 5: PL/SQL: Statement ignored 06550. 00000 - "line %s, column %s:\n%s" *Cause: Usually a PL/SQL compilation error. Line no. | Description | 1 | Start of the execution section of the block | 2,3 | Start of the FOR loop statement with lower bound value as 1 and upper bound value as 5 | 4 | The loop_index is assigned to the value 10. | 5 | The loop index value is printed using the DBMS_OUTPUT.PUT_LINE procedure | 6 | End of the FOR loop | 7,8 | End of the execution section of the block |
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