| Sentinel loop | |
|---|---|
| Differential diagnosis | acute cholecystitis, acute pancreatis |
The specified condition determines whether to execute the loop body or not. 'C' programming language provides us with three types of loop constructs: 1. The do-while loop. The for loop While Loop in C. A while loop is the most straightforward looping structure. Syntax of while loop in C programming language is as follows.
A sentinel loop is a sign seen on a radiograph that indicates localized ileus from nearby inflammation[1]. Simply put, it is the dilatation of a segment of small intestine. to be differentiated from colonic cutoff sign which is a dilatation of a segment of large bowel.[citation needed]
Sentinel-controlled repetition is sometimes called indefinite repetition because it is not known in advance how many times the loop will be executed. It is a repetition procedure for solving a problem by using a sentinel value (also called a signal value, a dummy value or a flag value) to indicate 'end of data entry'. Sentinel Controlled Loop When we don't know exactly know how many times loop body will be executed known as Sentinel Controlled Loop, for example - Reverse a given number, such kind of problem will be solved using sentinel controlled loop. Consider the code snippet. Beware the endless loop! When a C program enters an endless loop, it either spews output over and over without end or it sits there tight and does nothing. Well, it's doing what you ordered it to do, which is to sit and spin forever. Sometimes, this setup is done on purpose, but mostly it.
An isolated distended loop of bowel is seen near the site of injured viscus or inflamed organ. This loop is called a 'sentinel loop.' It arises from the body's efforts to localize traumatic or inflammatory lesions. The local distention of that intestinal loop is due to local paralysis and accumulation of gas in the intestinal loop.In acute pancreatitis, the sentinel loop is usually seen in left hypochondrium, while in acute cholecystitis, it is seen in the right hypochondrium. In acute appendicitis, the sentinel loop is seen in right iliac fossa.[citation needed]
C++ Sentinel While Loop
References[edit]
- ^Brant, William E.; Helms, Clyde A. (2007). Fundamentals of Diagnostic Radiology. Lippincott Williams & Wilkins. p. 742. ISBN9780781761352. Retrieved 18 September 2019.
In computer programming, a sentinel value (also referred to as a flag value, trip value, rogue value, signal value, or dummy data)[1] is a special value in the context of an algorithm which uses its presence as a condition of termination, typically in a loop or recursive algorithm.
The sentinel value is a form of in-band data that makes it possible to detect the end of the data when no out-of-band data (such as an explicit size indication) is provided. The value should be selected in such a way that it is guaranteed to be distinct from all legal data values since otherwise, the presence of such values would prematurely signal the end of the data (the semipredicate problem). A sentinel value is sometimes known as an 'Elephant in Cairo,' due to a joke where this is used as a physical sentinel. In safe languages, most sentinel values could be replaced with option types, which enforce explicit handling of the exceptional case.
Examples[edit]
Some examples of common sentinel values and their uses:
- Null character for indicating the end of a null-terminated string
- Null pointer for indicating the end of a linked list or a tree.
- A set most significant bit in a stream of equally spaced data values, for example, a set 8th bit in a stream of 7-bit ASCII characters stored in 8-bit bytes indicating a special property (like inverse video, boldface or italics) or the end of the stream
- A negative integer for indicating the end of a sequence of non-negative integers
Variants[edit]
A related practice, used in slightly different circumstances, is to place some specific value at the end of the data, in order to avoid the need for an explicit test for termination in some processing loop, because the value will trigger termination by the tests already present for other reasons. Unlike the above uses, this is not how the data is naturally stored or processed, but is instead an optimization, compared to the straightforward algorithm that checks for termination. This is typically used in searching.[2][3]
For instance, when searching for a particular value in an unsorted list, every element will be compared against this value, with the loop terminating when equality is found; however, to deal with the case that the value should be absent, one must also test after each step for having completed the search unsuccessfully. By appending the value searched for to the end of the list, an unsuccessful search is no longer possible, and no explicit termination test is required in the inner loop; afterward, one must still decide whether a true match was found, but this test needs to be performed only once rather than at each iteration.[4]Knuth calls the value so placed at the end of the data, a dummy value rather than a sentinel.
Examples[edit]
Array[edit]
For example, if searching for a value in an array in C, a straightforward implementation is as follows; note the use of a negative number (invalid index) to solve the semipredicate problem of returning 'no result':
However, this does two tests at each iteration of the loop: whether the value has been found and whether the end of the array has been reached. This latter test is what is avoided by using a sentinel value. Assuming the array can be extended by one element (without memory allocation or cleanup; this is more realistic for a linked list, as below), this can be rewritten as:
The test for i < len is still present, but it has been moved outside the loop, which now contains only a single test (for the value), and is guaranteed to terminate due to the sentinel value. There is a single check on termination if the sentinel value has been hit, which replaces a test for each iteration.
C++ Counting Loop
It is also possible to temporarily replace the last element of the array by a sentinel and handle it, especially if it is reached:
See also[edit]
References[edit]
Sentinel Controlled Loop
- ^Knuth, Donald (1973). The Art of Computer Programming, Volume 1: Fundamental Algorithms (second edition). Addison-Wesley. pp. 213–214, also p. 631. ISBN0-201-03809-9.
- ^Mehlhorn, Kurt; Sanders, Peter (2008). Algorithms and Data Structures: The Basic Toolbox 3 Representing Sequences by Arrays and Linked Lists(PDF). Springer. ISBN978-3-540-77977-3. p. 63
- ^McConnell, Steve (2004). Code Complete (2nd ed.). Redmond: Microsoft Press. p. 621. ISBN0-7356-1967-0.
- ^Knuth, Donald (1973). The Art of Computer Programming, Volume 3: Sorting and searching. Addison-Wesley. p. 395. ISBN0-201-03803-X.
Sentinel While Loop
