Sentinel Value in Programming

Sentinel Value

Hearing the word “sentinel value” for the first time, what comes to your mind? If you did a little programming in school, it would make sense. Computer programming refers to a special value within an algorithm that uses its presence to terminate a condition, usually in a recursive or looping algorithm. Sentinel value is referred to in many ways, such as dummy data, signal value, trip value, flag value, and rogue value. 

Sentinel Value in Programming

If that description doesn’t paint the picture clearly, perhaps this might help. At times, we want to repeat a particular process without knowing how often the process should continue. In other words, before starting the repetition structure, we don’t know the number of repetitions needed. Generally, in such a situation, the repetition would continue as long as there is a value to continue the process. 

For instance, you are working in a paper-based environment; there might be a job that involves processing a stack of order forms pending when the bottom is reached; it means you would look at the top of the order and process it until it gets to the end. You only stop when there is no more order form from the desk. 

In the example above, the desktop acts as a signal, which is a sentinel value. It would continue the repetitive process until the last order is processed. Algorithmically, we can express it in this format:

Look at the top of order forms
While not looking at the desk surface
Process the order forms
Look at the next order form
end while

Nevertheless, in a computer programming environment, the sentinel value would be a special value when the input signals aren’t valid, items from left to be processed. However, the value will not be taken as a valid item for processing. Before going further, let’s look at what loops are in programming languages.

What are Loops?

Loops are a sequence of instructions or statements to be executed several times before the instruction turns false. It comprises of two parts – the body and the control statement or instruction. The control statement comprises a series of conditions, which tells the loop’s body to execute pending when the specified instruction of condition becomes false. The essence of the loop is to repeat a particular code several times before ending the program. This definition is true for all programming languages. Virtually all programming languages perform some loops for programs to execute.

Types of loops in C Programming Language

C is a popular programming language used for creating various applications. It is among the most straightforward and beginner programming language to learn. We decide to discuss the types of loops in the C programming language because they are easy to understand. Additionally, it also serves as a foundation for you to understand looping in different languages.

Depend on the control statement position in a program, there are two types of looping in C. These are the entry and exit controlled loop.

The difference between these loops is that in an entry-controlled loop, a statement’s condition is checked before executing the loop’s body. You can also call it a pre-checking loop. Now you know that an exit-controlled loop is always checked after the execution of the body loop. Another name for it is a post-checking loop.

Endless Loop

We talked about the body and the control instruction or statement. The control statement must be clearly defined and specified else the loop will execute without stopping. In programming, this situation is known as an endless or infinite loop. An endless loop continues executing and never ends because no condition is specified on what next to do.

An endless loop has the following features:

  • No termination conditions
  • Specified conditions never met.

The specified condition determines if to execute the body of the loop or not. In programming, there are three types of loop constructs when writing programs. We will explore these loops and provide examples to show how to implement them in your programming. The three-loop structures are:

  • While loop
  • Do-while loop
  • For loop

While Loop

It is the simplest and most straightforward looping structure in a programming language. If you don’t understand how it works, then programming shouldn’t be your field. Each programming language has its syntax, but in C language, the syntax is as follows:

while (condition) {

             statements;

}

This looping is an example of an entry-controlled looping structure. The condition will first be evaluated before the loop’s body. The only time the body of the loop is executed is if the condition is true. Once the loop’s body gets executed, the programs go back to the beginning to test the following condition. The process would continue pending when the condition becomes false before exiting the loop. 

Immediately it exits the loop; the program passes the control to the statement following the loop. Usually, the loop’s body can comprise more than one statement. Peradventure contains only one statement; you don’t have to use the curly braces shown in the syntax. Nevertheless, it is preferable to include the curly braces as a programming etiquette, notwithstanding the number of statements in a loop.

Furthermore, if the condition of a while loop is not true, the body of the loop won’t be executed. You would understand this when we explain the “do-while loop” structure. Nevertheless, here is a simple example of a while loop.

#include<stdio.h>#include<conio.h>int main(){ int FootballTeam=1; //initializing the variable while(FootballTeam<=11) //while loop with condition { printf(“%d\n”,FootballTeam); FootballTeam++; //incrementing operation } return 0;}

The output for this program will be as follows

1234567891011

In the program above, we initial a variable called “football team” and assigned the value 1 to it. We will print the value from 1 to 11, so we initialize the value to 1. Peradventure you want to start printing from 0, you need to assign the value 0 during the initialization stage. 

In the while loop, we gave the condition (FootballTeam<=11). It means the loop will continue executing pending when the value becomes 11, after which control is passed to the next condition outside the loop. In the loop, we increased the initial value by 1 pending when it gets to 11.

Do While Loop

The do-while loop is similar to the previous loop. However, the condition gets executed after the body of the loop. It is an example of an exit-controlled loop. The syntax is as follows:

do {

  statements

} while (expression);

In this type of loop structure, the loop’s body is executed at least once before ending. Once the body is executed, it tests the condition again for the next action. If the condition is true, then the loop will execute the body again. However, once the condition is false, it transfers operation to the statement outside the loop.

Here is an example of a do while loop

#include<stdio.h>#include<conio.h>int main(){ int Table=1; //initializing the variable do //do-while loop  { printf(“%d\n”,3*Table); Table++; //incrementing operation }while(Table<=10); return 0;}

Our output would be as follows:

36912151821242730

If you observed carefully, the program is a multiplication table of 3 using the “do-while loop.”

For Loop

The last loop structure is the “for loop” condition that is more efficient in programming. Here is the syntax.

for (starting value; condition; increment or decrement) 

{

  statements;

}

Looking at the syntax, it cannot be easy to understand, but here is a simple explanation.

  • The starting value of the loop is performed at least once
  • The condition is a Boolean expression to test and compare the counter with a fixed value after each repetition. It only stops when the loop turns false.
  • The increment or decrement decreases or increases the counter by a set value 

Let’s use an example to illustrate how it works.

#include<stdio.h>int main()        {              int score;             for(score=1;score<=10;score++) //for loop to print 1-10 numbers             {             printf(“%d\n”,score); //to print the score            }            return 0;}

Output will be

12345678910

In the program above, what we did was declare a variable with the int data type. If you don’t know about int data type, you need to brush yourself on data types in a programming language. In the initialization part of the loop, we assign 1 to the variable “score.” We also specified our condition for the score to increase. The body of the loop has a print function that prints the score on a new line. After printing the first one, it goes to the loop and increases the value pending when it gets to 10 before terminating since the condition is for it to stop when it reaches 10.

Sentinel Value in Different Programming Languages

This part of this article would look at some sentinel values in a few programming languages. We will begin with Sentinel values in Java. What is Java sentinel value, and why do we use them.

Here are few points about Sentinel Value in Java

  • You can use a sentinel value to notify the program to stop accepting input. You would need this when you don’t know the ending point of input data.
  • It is mostly used in connection with while and do-while loops.
  • The sentinel value is needed when you want to terminate the while or do-while loop and stop getting inputs from the program.

Uses of Sentinel Values

  • For indicating when an individual is done entering values
  • It enables users to inform the program that no more input is required
  • It should be something that is not valid but of the same data type
  • It allows different input to be accepted each time
  • The number of repetitions allows the users to determine how frequently the loop would run and stop. 

Java Sentinel Value

To make it easier, we would use a java sentinel value example to show how looping works. In the example below, we will use the While loop that accepts a series of numbers from the users pending when they enter zero. The program will generate the output of all the sum of the number once zero is entered. Here is the example.

Import java.util. Scanner;
Public class SentinelValueDemo {Public static void main (String [] args ) {
Int number, sum, count;Count = 0;sum = 0 ;scanner input = new Scanner (System.in); // Creating object of Scanner class to take keyboard inputSystem.out.println (“Enter any number, or 0 to stop”);Number = input.nextInt(); // taking input from keyboard//number zero (0) is the sentinel valueWhile (number !=0){Sum=sum+number;Count++;
System.out.println(“Enter another number, or 0 to stop”);Number=input.nextInt();
}System.out.println(“The sum of numbers = “+sum);//Printing sum of all numbers inputted}}

If you inputted everything correctly, you should have the following output:

Enter any number or 0 to stop12Enter another number, or 0 to stop10Enter another number, or 0 to stop70Enter another number, or 0 to stop66Enter another number, or 0 to stop54Enter another number, or 0 to stop0The sum of numbers = 212

From the program’s output, you can see the loop continued pending when the number zero (0) is entered. 

Is there any difference between a count-controlled loop and a sentinel-controlled loop?

For newbies, it may be hard differentiating between these two. However, here is the simple difference that exists between a sentinel-controlled loop and a count-controlled loop. In the count-controlled loop, we know the number of times the loop will execute, whereas, in the sentinel-controlled loop, we don’t know the number of execution times. Furthermore, the sentinel-controlled loop can run an unknown number of times depending on the program’s requirement or condition.

Is there any benefit for a user to use a sentinel-controlled loop?

One unique benefit of using a sentinel-controlled loop is that there is no guarantee and restrain on the number of times the loop will continue. The loop only ends when the user inputs the value, they want the program to end.

Other Words for Sentinel Loop

At times, you may find the following words used in place of sentinel loop

  • Dummy data
  • Rogue value
  • Flag value
  • Signal value
  • Trip value

Also read Is Python a Scripting Language or Programming Language?

Sentinel Value in Programming

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