PL/SQL Program to Reverse a String

Writing a program to reverse a string (for example, turning "hello" into "olleh") is a classic programming challenge. It's a great way to learn how to use loops and string functions to manipulate data one character at a time.

This simple guide will show you the logic and a complete PL/SQL program to solve this problem.

What You Need to Know

To write this program, you will use a few basic PL/SQL concepts:

1. Enabling Output: You must run this command once in your SQL tool (like SQL*Plus or SQL Developer) to see the printed results:SET SERVEROUTPUT ON;
2. Anonymous Block: We will write our code in a DECLARE...BEGIN...END; block.
3. Variables: We'll need a variable for the original string (v_string), a variable to hold the new reversed string (v_reversed_string), and a variable for the string's length (v_length).
4. LENGTH(string): This built-in function tells us how many characters are in the string.
5. SUBSTR(string, position, length): This function extracts a part of a string. We will use SUBSTR(v_string, i, 1) to get the single character at position i.
6. FOR Loop with REVERSE: We will use a FOR loop to iterate through the string. The REVERSE keyword makes the loop count backward (e.g., from 10 down to 1), which is perfect for this task.
7. Concatenation (||): We use the || operator to build the new string by appending one character at a time.

PL/SQL Program: Reverse a String

This program will take the string stored in v_string, reverse it, and print the result.

PL/SQL Program

SET SERVEROUTPUT ON;

DECLARE
  -- The string we want to reverse
  v_string VARCHAR2(100) := 'Hello World';
  
  -- A variable to hold the new string as we build it
  v_reversed_string VARCHAR2(100);
  
  -- A variable to store the length
  v_length NUMBER;

BEGIN
  
  -- 1. Get the length of the string
  v_length := LENGTH(v_string);
  
  -- 2. Loop from the end of the string down to the beginning
  FOR i IN REVERSE 1..v_length LOOP
    
    -- 3. Get the character at the current position 'i'
    --    and append it to our new string
    v_reversed_string := v_reversed_string || SUBSTR(v_string, i, 1);
    
  END LOOP;
  
  -- 4. Print the final results
  DBMS_OUTPUT.PUT_LINE('Original string: ' || v_string);
  DBMS_OUTPUT.PUT_LINE('Reversed string: ' || v_reversed_string);

END;
/

Result (for v_string := 'Hello World')

Original string: Hello World
Reversed string: dlroW olleH

Program Explanation

1. DECLARE section: We create v_string ('Hello World'), v_reversed_string (which is NULL at the start), and v_length.
2. BEGIN section: The logic starts.
3. v_length := LENGTH(v_string);: This calculates the length of 'Hello World' and stores 11 in the v_length variable.
4. FOR i IN REVERSE 1..v_length LOOP: This is our loop. Because we used REVERSE and v_length is 11, the loop will run with i taking the values: 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, and finally 1.
5. v_reversed_string := v_reversed_string || SUBSTR(v_string, i, 1);: This is where the reversed string is built.
   • Loop 1 (i=11): v_reversed_string = NULL || SUBSTR('Hello World', 11, 1) = 'd'
   • Loop 2 (i=10): v_reversed_string = 'd' || SUBSTR('Hello World', 10, 1) = 'dl'
   • Loop 3 (i=9): v_reversed_string = 'dl' || SUBSTR('Hello World', 9, 1) = 'dlr'
   • ...This continues, appending each character from the end of the original string to the new string.
   • Loop 11 (i=1): v_reversed_string = 'dlroW olle' || SUBSTR('Hello World', 1, 1) = 'dlroW olleH'
6. END LOOP;: The loop finishes.
7. DBMS_OUTPUT.PUT_LINE(...): The program prints the original string and the final value of v_reversed_string.