Variables and values can have multiple roles, but it is useful to mention three different roles in the context of loops:
Counter
: Variable that is incremented every time a given event occurs.
int i = 0; // i is a counter
while (i < 10){
Console.WriteLine($"{i}");
i++;
}Sentinel Value
: A special value that signals that the loop needs to end.
Console.WriteLine("Give me a string.");
string ans = Console.ReadLine();
while (ans != "Quit") // The sentinel value is "Quit".
{
Console.WriteLine("Hi!");
Console.WriteLine("Enter \"Quit\" to quit, or anything else to continue.");
ans = Console.ReadLine();
}Accumulator
: Variable used to keep the total of several values.
int i = 0, total = 0;
while (i < 10){
total += i; // total is the accumulator.
i++;
}
Console.WriteLine($"The sum from 0 to {i} is {total}.");We can have an accumulator and a sentinel value at the same time:
Console.WriteLine("Enter a number to sum, or \"Done\" to stop and print the total.");
string enter = Console.ReadLine();
int sum = 0;
while (enter != "Done")
{
sum += int.Parse(enter);
Console.WriteLine("Enter a number to sum, or \"Done\" to stop and print the total.");
enter = Console.ReadLine();
}
Console.WriteLine($"Your total is {sum}.");You can have counter, accumulator and sentinel values at the same time:
int a = 0;
int sum = 0;
int counter = 0;
Console.WriteLine("Enter an integer, or N to quit.");
string entered = Console.ReadLine();
while (entered != "N") // Sentinel value
{
a = int.Parse(entered);
sum += a; // Accumulator
Console.WriteLine("Enter an integer, or N to quit.");
entered = Console.ReadLine();
counter++; // counter
}
Console.WriteLine($"The average is {sum / (double)counter}");We can distinguish between three “flavors” of loops (that are not mutually exclusive):
Sentinel controlled loop
: The exit condition tests if a variable has (or is different from) a specific value.
User controlled loop
: The number of iterations depends on the user.
Count controlled loop
: The number of iterations depends on a counter.
Note that a user-controlled loop can be sentinel-controlled (that is the example we just saw), but also count-controlled (“Give me a value, and I will iterate a task that many times”).