Questions tagged [while-loop]

A while loop is a control structure used in many programming languages to continuously execute a set of instructions as long as a particular condition is met.

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How to emulate a do-while loop?

I need to emulate a do-while loop in a Python program. Unfortunately, the following straightforward code does not work: list_of_ints = [ 1, 2, 3 ] iterator = list_of_ints.__iter__() element = None ...
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Syntax for a single-line while loop in Bash

I am having trouble coming up with the right combination of semicolons and/or braces. I'd like to do this, but as a one-liner from the command line: while [ 1 ] do foo sleep 2 done
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Why is “while ( !feof (file) )” always wrong?

What is wrong with using feof() to control a read loop? For example: #include <stdio.h> #include <stdlib.h> int main(int argc, char **argv) { char *path = "stdin"; FILE ...
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23answers
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Which is faster: while(1) or while(2)?

This was an interview question asked by a senior manager. Which is faster? while(1) { // Some code } or while(2) { //Some code } I said that both have the same execution speed, as the ...
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369votes
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Else clause on Python while statement

I've noticed the following code is legal in Python. My question is why? Is there a specific reason? n = 5 while n != 0: print n n -= 1 else: print "what the..."
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312votes
13answers
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How do I plot in real-time in a while loop using matplotlib?

I am trying to plot some data from a camera in real time using OpenCV. However, the real-time plotting (using matplotlib) doesn't seem to be working. I've isolated the problem into this simple ...
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304votes
6answers
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How can you run a command in bash over and over until success?

I have a script and want to ask the user for some information, but the script cannot continue until the user fills in this information. The following is my attempt at putting a command into a loop to ...
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285votes
34answers
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Are loops really faster in reverse?

I've heard this quite a few times. Are JavaScript loops really faster when counting backward? If so, why? I've seen a few test suite examples showing that reversed loops are quicker, but I can't find ...
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272votes
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How to check if all elements of a list match a condition?

I have a list consisting of like 20000 lists. I use each list's 3rd element as a flag. I want to do some operations on this list as long as at least one element's flag is 0, it's like: my_list = [["a"...
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265votes
20answers
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Declaring variables inside or outside of a loop

Why does the following work fine? String str; while (condition) { str = calculateStr(); ..... } But this one is said to be dangerous/incorrect: while (condition) { String str = ...
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245votes
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A variable modified inside a while loop is not remembered

In the following program, if I set the variable $foo to the value 1 inside the first if statement, it works in the sense that its value is remembered after the if statement. However, when I set the ...
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Java method with return type compiles without return statement

Question 1: Why does the following code compile without having a return statement? public int a() { while(true); } Notice: If I add return after the while then I get an Unreachable Code Error. ...
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225votes
21answers
201kviews

Are "while(true)" loops so bad? [closed]

I've been programming in Java for several years now, but I just recently returned to school to get a formal degree. I was quite surprised to learn that, on my last assignment, I lost points for using ...
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190votes
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How can I make sense of the `else` clause of Python loops?

Many Python programmers are probably unaware that the syntax of while loops and for loops includes an optional else: clause: for val in iterable: do_something(val) else: clean_up() The body ...
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190votes
2answers
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How to break out of a loop in Bash?

I want to write a Bash script to process text, which might require a while loop. For example, a while loop in C: int done = 0; while(1) { ... if(done) break; } I want to write a Bash script ...
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