PL/SQL FOR Loop tips

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

Source: http://www.dba-oracle.com/concepts/pl_sql_for_loop.htm