• Create a new C++ file called xy.cpp.
• In this assignment, we are going to write another program that uses a for loop. 
• Within the loop, make the variable x go from -5 to 5, counting by 0.5. (This means that x can't be an int.)
• Inside the body of the loop, set another variable y to be the current value of x raised to the fifth power. 
• For output, you will need to display the current values of both x and y in a chart, as shown below.
• You will need to use tab, fixed and setprecision(n) and the pow(base,exp) function here. (What library(ies) do you need?)
• When you are finished, upload your source code to Canvas.

Read About and Run the Code for These Loops:

1. Indefinite Loops

Recall our looping application that simulated the play of an exciting game

#include <iostream>
using namespace std;

int main() {
    string repeat = "y";
    while ("y" == repeat) {
        cout << "\nPlaying an exciting game!\n";
        cout << "Do you want to play again? (y/n) ";
        cin >> repeat;
    }
    cout << "\nThanks for playing!\n";

    return 0;
}

• Loops of this type are called indefinite loops because you do not know in advance how many time the loop will execute
• This behavior is different from a counting loop where you know how many times the loop will execute before the loop starts
• With an indefinite loop we can solve a new set of problems
• Most problems solved with indefinite loops make use of while statements

• Another common use for indefinite loops is to process a sequence of inputs
• As an example, let us add up (sum) a series of numbers
• Every number is added to the sum
• We use a loop to repeat the input until the user decides to stop
• Since we do not know how many number the user will enter, we use an indefinite loop as shown below

#include <iostream>
using namespace std;

int main() {
    double input = 1;
    double sum = 0;
    string repeat = "y";

    cout << "I will add up numbers for you\n\n";
    while ("y" == repeat) {
        cout << "So far, sum = " << sum << endl;
        cout << "Enter a number: ";
        cin >> input;
        sum = sum + input;

        cout << "Another number? (y/n) ";
        cin >> repeat;
    }
    cout << "Ending sum: " << sum << endl;

    return 0;
}

3. Terminating the Input with a Sentinel

• Whenever we read a sequence of input values, we need to have some way of terminating the input loop
• We could use a separate variable and input statement as we have done before:

  string repeat = "y";
  while ("y" == repeat) {
      // ... statements to repeat
      cin >> repeat;
  }
  

• However, when entering numbers (or other data) repeatedly, answering an extra question each time through the loop becomes annoying
• One way to avoid asking an extra question is to use a sentinel value
• A sentinel is guard who watches for something to happen

        image source

• Similarly, a sentinel in a program watches for a specific sentinel value that signals termination of a loop
• To use a sentinel value, we must have a special value in the input
• Some commonly used sentinel values for numeric input are 0 or -1
• The following program is an update of the previous program to use a sentinel value to end the loop
• Question: What is the sentinel value for this loop?
  

Example Application Using a Sentinel Value for the Loop Test

#include <iostream>
using namespace std;

int main() {
    double input = 1;
    double sum = 0;

    cout << "I will add up numbers for you\n\n";
    while (input != 0) {
        cout << "So far, sum = " << sum << endl;
        cout << "Enter a number or 0 to exit: ";
        cin >> input;
        sum = sum + input;
    }
    cout << "Ending sum: " << sum << endl;

    return 0;
}

4. Error Checking: Input Validation

• Another common use for indefinite loops is input validation
• Input validation combines a loop with one or more if statements
• The input statement is placed inside the loop
• The if-statement tests for an incorrect input value
• The loop repeats while the user's input contains an error
• Since we do not know how many times the loop must execute ahead of time, the loop is indefinite
• For example, the following program uses a loop to ensure a user enters a positive number
• The if statement is used to decide when to output an error message

Example of Input Validation Using a Loop

#include <iostream>
using namespace std;

int main() {
    double input = 0.0; // initialize value
    while (input <= 0) {
        cout << "Enter a positive number: ";
        cin >> input;
        if (input <= 0.0) {
            cout << "You must enter a positive number\n";
        }
    }
    cout << "You entered: " << input << endl;

    return 0;
}

• We will explore all of these applications in our programs as we continue to write loops.