Infinite loop - Wikipedia, the free encyclopedia. An infinite loop (or endless loop) is a sequence of instructions in a computer program which loops endlessly, either due to the loop having no terminating condition, having one that can never be met, or one that causes the loop to start over. In older operating systems with cooperative multitasking, infinite loops normally caused the entire system to become unresponsive. With the now- prevalent preemptive multitasking model, infinite loops usually cause the program to consume all available processor time, but can usually be terminated by the user. Busy wait loops are also sometimes called . One possible cause of a computer . An infinite loop occurs when the condition will never be met, due to some inherent characteristic of the loop. Intentional looping. For example, the games on cartridge- based game consoles typically have no exit condition in their main loop, as there is no operating system for the program to exit to; the loop runs until the console is powered off. Modern interactive computers require that the computer constantly be monitoring for user input or device activity, so at some fundamental level there is an infinite processing idle loop that must continue until the device is turned off or reset. In the Apollo Guidance Computer, for example, this outer loop was contained in the Exec program, and if the computer had absolutely no other work to do it would loop run a dummy job that would simply turn off the . Instead they fall back to an error condition displaying messages to the operator, and enter an infinite loop waiting for the user to either respond to a prompt to continue, or to reset the device. Unintentional looping. Such errors are most common among novice programmers, but can be made by experienced programmers as well, because their causes can be quite subtle. One common cause, for example, is that the programmer intends to iterate over a collection of items such as a linked list, executing the loop code once for each item. I have finally managed to boil down a problem I have been struggling with for a few weeks. I use SSH with 'authorized keys' to run commands remotely. The while loop enables you to execute a set of commands repeatedly until some condition occurs. It is usually used when you need to manipulate the value of a variable. Say I have this file: hello world hello world This program #!/bin/bash for i in $(cat $1); do echo 'tester: $i' done outputs. I am having a bash script which is something like following, cat filename In the example above, the while loop will run, as long i is smaller then twenty. In the while loop there is an if statement that states that if i equals ten the while. In most computer programming languages, a while loop is a control flow statement that allows code to be executed repeatedly based on a given boolean condition. Improperly formed links can create a reference loop in the list, where one list element links to one that occurred earlier in the list. This joins part of the list into a circle, causing the program to loop forever. While most infinite loops can be found by close inspection of the code, there is no general method to determine whether a given program will ever halt or will run forever; this is the undecidability of the halting problem. Interruption. This can be done in a task manager, in a terminal with the Control- C command, or by using the kill command or system call. However, this does not always work, as the process may not be responding to signals or the processor may be in an uninterruptible state, such as in the Cyrix coma bug (caused by overlapping uninterruptible instructions in an instruction pipeline). In some cases other signals such as SIGKILL can work, as they do not require the process to be responsive, while in other cases the loop cannot be terminated short of system shutdown. Language support. Most commonly, in unstructured programming this is jump back up (goto), while in structured programming this is an indefinite loop (while loop) set to never end, either by omitting the condition or explicitly setting it to true, as while (true) .. Some languages have special constructs for infinite loops, typically by omitting the condition from an indefinite loop. Examples include Ada (loop .. This could be fixed by moving the x = 1 instruction outside the loop. Essentially what this infinite loop does is to instruct a computer to keep on adding 1 to 1 until 5 is reached. Since 1+1 always equals 2, this will never happen. In some languages, programmer confusion about the mathematical symbols may lead to an unintentional infinite loop. For example, here is a snippet in C: #include< stdio. However, in the line . Instead, this will assign the value of 5 to a at this point in the program. Thus, a will never be able to advance to 1. Variable handling errors. Here is an example (in C): floatx=0. The problem is that the loop terminating condition (x != 1. The same can happen in Python: x=0. Because of the likelihood of tests for equality or not- equality failing unexpectedly, it is safer to use greater- than or less- than tests when dealing with floating- point values. For example, instead of testing whether x equals 1. Another way to fix this particular example would be to use an integer as a loop index, counting the number of iterations that have been performed. A similar problem occurs frequently in numerical analysis: in order to compute a certain result, an iteration is intended to be carried out until the error is smaller than a chosen tolerance. However, because of rounding errors during the iteration, the specified tolerance can never be reached, resulting in an infinite loop. Multi- party loops. Consider a server that always replies with an error message if it does not understand the request. Apparently, there is no possibility for an infinite loop in the server, but if there are two such servers (A and B), and A receives a message of unknown type from B, then A replies with an error message to B, B does not understand the error message and replies to A with its own error message, A does not understand the error message from B and sends yet another error message, and so on ad infinitum. One common example of such situation is an email loop. Pseudo- infinite loops. While Loop Program In Unix A ProcessConvert this while-loop to a function that you can call, and replace 6 in the test (i < 6) with a variable. Use this function to rewrite the script to. The actual limit of i depends on the details of the system and compiler used. With arbitrary- precision arithmetic, this loop would continue until the computer's memory could no longer contain i. If i was a signed integer, rather than an unsigned integer, overflow would be undefined. In this case, the loop could be optimized into an infinite loop. Infinite recursion. When using structural recursion, infinite recursions are usually caused by a missing base case or by a faulty inductive step. An example of such a faulty structural recursion: (define (sum- from- 1- ton)(+ n(sum- from- 1- to(sub. The function sum- from- 1- to will run out of stack space, because the recursion never stops . To correct the problem, a base case is added.(define (sum- from- 1- to'n)(cond. For information on recursive functions which never run out of stack space, see tail recursion. See also: Recursion, for an alternate explanation of infinite recursion. Break statement. Example in PHP: while(true). While Loop Program In Unix\/linuxThese are most common and visible while debugginguser interface code. A C- like pseudocode example of an Alderson loop, where the program is supposed to sum numbers given by the user until zero is given, but where the programmer has used the wrong operator: sum=0; while(true).
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