How Does a Sentinel Value Work in an Algorithm?

[jackson6612]

Hi

Please have a look on this scanned page:
http://img257.imageshack.us/img257/8175/chap11page191.jpg

I humbly request you to be precise and to the point in your replies. I need you help and need to understand this as soon as possible. Thank a lot.

Suppose there were 100 students and once it has processed the percentage of the last student (100th) it writes output data and then goes back to first step "Read Input Data" as the 'arrow line' suggests. Well, there is any more Input Data so the program will keep on looking for the Input Data. How is it suppose to lead to the step "Is Roll no. = 0000000" - a sentinel value? I don't understand it.

Please help me.

 

[Mark44]

Using a sentinel or trailer value, the program doing the processing doesn't need to know how much data there is.

Basically the program reads a student ID. If the ID is 0000000, the program stops. If the ID is not 0000000, it reads in data about that student, does some calculations, and then goes back to read another student ID.

 

[jtbell]

In order for the algorithm to work properly, the input data file must be constructed so that the last line or data set contains 0000000 in the position that normally contains the roll number. This last line is not intended to be used as "real data." It exists only as a "sentinel" which signals that there is no more "real data."

 

[jackson6612]

I understand it now. Thank you, both of you, Mark, JT. These are the answers I call concise and to the point. Or, as you say in English, don't give me what I ask for, give me what I need.

Best wishes
Jackson

 

[LudusRex]

Hello,

Thank you for reaching out for assistance with understanding the concept of sentinel value in algorithms. I will do my best to provide a clear and concise explanation for you.

A sentinel value is a special value used in programming algorithms to indicate the end of a data set or a specific condition. In the example you provided, the sentinel value is the roll number 0000000. This value serves as a marker to signify that there is no more input data to be processed.

In the algorithm, the program will continue to read input data and process it until it reaches the last student (100th) and writes the output data. At this point, the program will encounter the sentinel value, indicating that there is no more input data to be processed. This prompts the program to go back to the first step of reading input data, as shown by the arrow line in the scanned page.

The program will continue to loop through this process until it reaches the sentinel value again, indicating that all the input data has been processed. This is how the sentinel value leads to the step "Is Roll no. = 0000000" - it serves as a condition for the program to check for in order to determine when to stop processing input data.

I hope this helps to clarify the concept of sentinel value for you. If you have any further questions, please don't hesitate to ask. Thank you.