• Frequently:
  • We need to prompt a user to provide input, validate the input and re-prompt if the input is invalid
• Variants:
  • The initial prompt and the subsequent prompts are different
  • All prompts are the same

• The Most Obvious Approach:
  • Use an if statement to validate and re-prompt
• An Example in Pseudocode:

  • Prompt the user to enter their age;
    Read the user's response;
    Assign the response to the variable named age;
    
    if (age is negative) {
        Prompt the user to enter their age;
        Read the user's response;
        Assign the response to the variable named age;
    }
    
    Use the variable named age;
    	  

• A Shortcoming of this Approach:
  • The user may need to be re-prompted multiple times for the same input

• What's Needed:
  • A way to repeat a block of code an indefinite number of times (i.e., until the user enters a valid response)
• What Should be Brought to Mind:
  • while and do loops

1. Enter a do loop. In the body of the loop:
   1. Prompt the user.
   2. Retrieve the user's response.
2. Repeat when the response is invalid.

1. Prompt the user with the normal message.
2. Retrieve the user's response.
3. Enter a while loop when the response is invalid. In the body of the loop:
   1. Prompt the user with the alternative message.
   2. Retrieve the user's response.
4. Repeat.

javaexamples/programmingpatterns/reprompt/RepromptJMUConsole.java (Fragment: identical)

        do {
            JMUConsole.printf("Enter a non-negative age: ");
            age = JMUConsole.readInt();
        } while (age < 0);
        

javaexamples/programmingpatterns/reprompt/RepromptJMUConsole.java (Fragment: different)

        JMUConsole.printf("Enter your age: ");
        age = JMUConsole.readInt();
        while (age < 0) {
            JMUConsole.printf("  Enter a non-negative value: ");
            age = JMUConsole.readInt();
        }