I’m trying to use an increment loop but I want it to increment at the end of the loop. Sadly, whenever I simply put the i++ at the end of the loop it doesn’t behave like I’d expect or want it to. Anyone mind showing me the proper way of doing it?
The referred increment loop:
for (i = 1; i < 15; i++) {
// do somthing here
}
Here is the loop I’m working with:
for (i = 1; i < 15; i++) {
for (x = 1; x < 15; x++) {
var take = document.getElementById("row" + i + "sm" + x);
Tesseract.recognize(take)
.then(function(result) {
console.log(result.text);
// rows[i][x] = result.text;
})
}
}
What I’d like it to do:
for (i = 1; i < 15) {
for (x = 1; x < 15) {
var take = document.getElementById("row" + i + "sm" + x);
Tesseract.recognize(take)
.then(function(result) {
console.log(result.text);
//rows[i][x] = result.text;
x += 1;
})
i += 1;
}
}
I am using the for loop because I need to iterate over something one by one. How do I properly increment i at the end of the loop?
Here is a video explaining my problem with context and explanation why it is not an ASYNC problem. Sorry if it is hard to follow, ill update it with audio soon so I can explain it propperly.
https://drive.google.com/file/d/1n1ZwNJif5Lb5zfLb2GPpBemObwpOqNf7/view
The problem is that the second one doesnt wait until first one is complete.
You can try with recursion inside then. There maybe some mistake with i,x but you get the point.
You execute first with i=1 and x=1, after the operation is done (then) you call the next until all elements are executed.
function execItem(i, x) {
var take = document.getElementById("row" + i + "sm" + x);
Tesseract.recognize(take)
.then(function(result){
rows[i][x] = result.text;
if (i < 15 && x < 15) {
if (i > 15) {
x += 1
i = 1
} else {
i += 1
}
execItem(i, x)
}
})
}
execItem(1, 1)
As a comment suggests this actually seems likely to be a problem with an asynchronous call (Tesseract...then) inside a loop. By the time the function inside then is called, your values of x and i have already moved on, so you don't get the result you expect.
One way around this would be to use a 'closure' - making a function that creates another function based on the value of i and x.
function getDisplayFunc(row, col) {
function displayRecognisedText(result) {
console.log(row, col, result.text);
//rows[row][col] = result.text;
}
return displayRecognisedText;
}
for (i = 1; i < 15; i++) {
for (x = 1; x < 15; x++) {
var take = document.getElementById("row" + i + "sm" + x);
Tesseract.recognize(take).then(getDisplayFunc(i, x));
}
}
I guess #Mike spot the error on: your code is asynchronous. What does it mean?
So, let's suppose you have this loop:
for (i = 0; i < 10; i++) {
console.log(i);
}
It will print this, right?
0
1
2
3
4
5
6
7
8
9
However, you do not print your value inside the loop directly, but as a follow-up operation to a promise. This makes this code asynchronous. It means that it does not have to execute at the exact moment you call it. I do not have Tesseract here so I will make my loop asynchronous using another very old trick, setTimeout():
for (i = 0; i < 10; i++) {
setTimeout(function() {
console.log(i);
}, 0);
}
If I run it, I get this:
10
10
10
10
10
10
10
10
10
10
What happens is, when I pass the operation we want to do (in this case, printing the i value) to an asynchronous function (recognize().then() in your case, setTimeout() in my case) through a callback (function() {console.log(i);} in my example) the asynchronous function "schedules" the operation to execute as soon as possible, but this "soon" is not faster than the loop. So, the loop finishes executing but our callback is not called, not even once! Since you are not declaring i with let, it is a global variable, so there exists only one i. And since the loop finished, the value of the i variable is 10 already.
It used to be a hard thing to solve, but with ES6 it is quite straightforward: declare i with let!
for (let i = 0; i < 10; i++) {
setTimeout(function() {
console.log(i);
}, 0);
}
The let-ed variable has a new binding at each iteration of the loop, so in practice you have 10 variables called i. The closure of your function will have access only to the one with the right value!
Maybe you should try to use while loop.
Like this:
while i < 15:
//do something
i += 1
For two variables: x, i with embeding:
while x < 15:
//do something
while i < 15:
//do something2
i += 1
x += 1
Hope I understand the problem correctly.
Related
I'm building a simon game. And after each round the player should see the moves he must play in the next round. So i created a function showMoves which flashes the square he has to play. The problem is that the function is not showing anything. Can anyone tell me what did i miss?
// the effect
function flasher(index) {
$(moves[index]).fadeIn(50).fadeOut(50).fadeIn(50).fadeOut(50).fadeIn(100);
}
var interval2;
// show the moves that supposed to be played
function showMoves() {
for (var i = 0; i < moves; i++) {
if (i === 0) {
interval2 = setTimeout(flasher(i), 1000);
} else {
interval2 = setTimeout(flasher(i), (i+1) * 1000);
}
}
}
setTimeout accepts a function as a first parameter. I assume that by calling flasher you tried to avoid this situation. In you case, this should be done like this:
function showMoves() {
for (var i = 0; i < moves; i++) {
if (i === 0) {
interval2 = setTimeout(function(i) {return function() {flasher(i)}}(i), 1000);
} else {
interval2 = setTimeout(function(i) {return function() {flasher(i)}}(i), (i+1) * 1000);
}
}
}
The setTimeout and setInterval are a little diffrent than we think about them.
They are save some event on specified times that will be fired in its times. Because of this they has a problem with loops:
for(i=0;i<3;i++)
{
setTimeout(function(){alert(i)}, i*1000);
}
after ending the loop the browser has 3 jobs to do:
alert(i) after 1 second
alert(i) after 2 seconds
alert(i) after 3 seconds
But what is the value of 'i'. If you are in c# programming after ending the loop 'i' will be disposed and we have not that.
But javascript does not dispose 'i' and we have it yet. So the browser set the current value for i that is 3. because when 'i' reaches to 3 loop goes end. Therefor Your browser do this:
alert(3) after 1 second
alert(3) after 2 seconds
alert(3) after 3 seconds
That is not what we want. But if change the above code to this:
for(i=0;i<3;i++){
(function (index)
{
setTimeout(function () { alert(index); }, i * 1000);
})(i);
}
We will have:
alert(0) after 1 second
alert(1) after 2 seconds
alert(2) after 3 seconds
So as Maximus said you mast make the browser to get value of i currently in loop. in this way:
setTimeout(function(i) {return function() {flasher(i)}}(i), (i+1) * 1000);
i does not leave out until end of loop and must be get value just now.
What I can derive from your code is that moves is an array, but you're using it as if it's an integer in the for loop. And that's why nothing happens at all.
Replace:
for (var i = 0; i < moves; i++) {
With:
for (var i = 0; i < moves.length; i++) {
And you should see things happening.
But you will notice flasher is called immediately, without timeout. And that's because the result of flasher is set to be called, instead of flasher itself.
Other answers here suggest using an wrapper function, but this requires workarounds to correctly pass the index to the function called by setTimeout.
So assuming that it doesn't have to run in IE8 and below, the following is the most concise solution:
setTimeout(flasher.bind(null, i), (i+1) * 1000)
Full working example:
var moves = [1, 2, 3, 4];
function flasher(index) {
console.log('move', moves[index]);
}
var interval2;
// show the moves that supposed to be played
function showMoves() {
for (var i = 0; i < moves.length; i++) {
interval2 = setTimeout(flasher.bind(null, i), (i+1) * 1000);
}
}
showMoves()
I'm trying to make a faux loading screen, and I need delays between loading messages of about 20-50ms or so so that people can actually see what's going on before it cuts to the initialized screen. The button that activates this goes to the following function:
function gameinit() {
for (k = 0; k <=1; k += 0.125) {
setTimeout(function () {
var nexttxt = "Loading... " + toString(100 * k) + "%"
}, 20);
displayupdate(nexttxt);
}
}
However this comes up as an incorrect syntax (on JSfiddle - https://jsfiddle.net/YoshiBoy13/xLn7wbg6/2/) when I use JShint - specifically lines four and five. I've looked at the guides for this and everything seems to be in order. What am I doing wrong?
(Note: displayupdate(nexttxt) updates the <p> tags with the next line of text)
When executing the script, nothing happens - the sixteen lines of text on the HTML move up as normal, the top eight being replaced with the eight generated by the gameinit() function, but the gameinit() only generates blank. If the script is executed again, it just outputs eight lines of 112.5% (as if it was the 9th iteration of the for loop).
I'm almost certain it's something elementary that I've missed, could someone please tell me what I've done wrong?
Use setInterval() instead, you can clear interval using clearInterval()
function gameinit() {
displayupdate("Loading... 0%");
var k = 0;
var inter = setInterval(function() {
if (k < 1) {
k += .25;
displayupdate("Loading... " + 100 * k + "%")
} else {
clearInterval(inter);
}
}, 2000);
}
function displayupdate(d) {
console.log(d);
}
gameinit();
here is another function can do this better ---- setInterval
var txt = '';
var time = 0;
var id = setInterval(function(){
console.log("loading..."+time/8*100+"%");
if(time++>7)
clearInterval(id);
},1000);
setTimeout doesn't work as you would expect it to work inside loops. You have to create a closure for each loop variable passed on to setTimeout, or create a new function to execute the setTimeout operation.
function gameinit() {
for (var k = 0; k <= 1; k += 0.125) {
doSetTimeOut(k);
}
}
function doSetTimeOut(k) {
setTimeout(function() {
var nexttxt = "Loading... " + toString(100 * k) + "%"
}, 20);
displayupdate(nexttxt);
}
This question already has answers here:
JavaScript closure inside loops – simple practical example
(44 answers)
Closed 1 year ago.
I have a very trivial question. For a simple loop with setTimeout, like this:
for (var count = 0; count < 3; count++) {
setTimeout(function() {
alert("Count = " + count);
}, 1000 * count);
}
console gives an output like this:
Count = 3
Count = 3
Count = 3
Not sure why the output like this. Anyone could explain, please?
This has to do with how scoping and hoisting is being treated in JavaScript.
What happens in your code is that the JS engine modifies your code to this:
var count;
for (count = 0; count < 3; count++) {
setTimeout(function() {
alert("Count = " + count);
}, 1000 * count);
}
And when setTimeout() is being run it will first look in it's own scope after count but it won't find it so then it'll start looking in the functions that closes (this is called closures) over the setTimeout function until it finds the var count statement, which will have the value 3 since loop will have finished before the first timeout function has been executed.
More code-ily explained your code actually looks like this:
//first iteration
var count = 0; //this is 1 because of count++ in your for loop.
for (count = 0; count < 3; count++) {
setTimeout(function() {
alert("Count = " + 1);
}, 1000 * 1);
}
count = count + 1; //count = 1
//second iteration
var count = 1;
for (count = 0; count < 3; count++) {
setTimeout(function() {
alert("Count = " + 2);
}, 1000 * 2);
}
count = count + 1; //count = 2
//third iteration
var count = 2;
for (count = 0; count < 3; count++) {
setTimeout(function() {
alert("Count = " + 3);
}, 1000 * 3);
}
count = count + 1; //count = 3
//after 1000 ms
window.setTimeout(alert(count));
//after 2000 ms
window.setTimeout(alert(count));
//after 3000 ms
window.setTimeout(alert(count));
think about it like that:
AFTER the 1000*n miliseconds are over, what will be the value of count?
of course it will be 3, because the foor loop ended way earlier than the timeout of 1000*n ms.
in order to print 1,2,3 you'll need the following:
for (var count = 0; count < 3; count++) {
do_alert(num);
}
function do_alert(num) {
setTimeout(function() {
alert("Count = " + num);
}, 1000 * num);
}
a different approach is to make it a closure function (explained well in JavaScript closures vs. anonymous functions)
for (var count = 0; count < 3; count++) {
(function(num){setTimeout(function() {
alert("Count = " + num);
}, 1000 * num)})(count);
}
these two code samples will actually work similarly.
the first sample calls a named function (do_alert) each iteration.
the second sample calls a CLOSURE anonymous function (which is just like do_alert) each iteration.
it's all a matter of SCOPE.
hope that helps.
This is to do with closure scoping. The same variable count is available in the scope for each of the setTimeout callback functions. You are incrementing its value and creating a function, but each instance of the function has the same variable count in its scope, and by the time the callback functions execute it will have the value 3.
You need to create a copy of the variable (e.g. var localCount = count) inside a new scope in the for loop to make this work. As for doesn't create a scope (which is the cause of the whole thing) you need to introduce one with a function scope.
e.g.
for (var i = 0; i < 5; i++) {
(function() {
var j = i;
setTimeout(function() {
console.log(j)
},
j*100);
})();
}
Easy fix here is to utilize es6 let local variable. Your code will look almost the same except it will do what you expect :)
for (let count = 0; count < 3; count++) {
setTimeout(function() {
alert("Count = " + count);
}, 1000 * count);
}
Or you could create a recursive function to get that job done, as following:
function timedAlert(n) {
if (n < 3) {
setTimeout(function() {
alert("Count = " + n);
timedAlert(++n);
}, 1000);
}
}
timedAlert(0);
Think about it:
The code executes a loop, in that loop it sets some code to run later.
The loop finishes.
The setTimeout code executes. What's the value of count going to be? The loop finished ages ago...
First, setTimeout(function, milliseconds) is a function which takes a function to execute after "milliseconds" milliseconds.
Remember, JS treats functions as objects, so the for(...) loop will initially produce something like:
setTimeout( ... )
setTimeout( ... )
setTimeout( ... )
Now the setTimeout() functions will execute one by one.
The setTimeout() function will try to find the count variable in the current scope. Failing that, it will go to the outer scope and will find count, whose value is already incremented to 3 by the for loop.
Now, starting execution....The first alert shows immediately, as the milliseconds is 0, the second alert shows after 1000 ms, and then the third alert shows after 2000 ms. All of them shows Count = 3
That's because all the timeouts are run when the loop finished.
The timeout functions then take the current value of count.
And thats always 3 because the for loop has finished.
That is because by the time the for loop completes its execution the count is 3, and then the set timeout is called.
Try this:
var count = 0;
setTimeout(function() {
for (count = 0; count < 3; count++) {
alert("Count = " + count);
}
}, 1000* count);
Better solution IS "Forget both Loops and Recursion" in this case and use this combination of "setInterval" includes "setTimeOut"s:
function iAsk(lvl){
var i=0;
var intr =setInterval(function(){ // start the loop
i++; // increment it
if(i>lvl){ // check if the end round reached.
clearInterval(intr);
return;
}
setTimeout(function(){
$(".imag").prop("src",pPng); // do first bla bla bla after 50 millisecond
},50);
setTimeout(function(){
// do another bla bla bla after 100 millisecond.
seq[i-1]=(Math.ceil(Math.random()*4)).toString();
$("#hh").after('<br>'+i + ' : rand= '+(Math.ceil(Math.random()*4)).toString()+' > '+seq[i-1]);
$("#d"+seq[i-1]).prop("src",pGif);
var d =document.getElementById('aud');
d.play();
},100);
setTimeout(function(){
// keep adding bla bla bla till you done :)
$("#d"+seq[i-1]).prop("src",pPng);
},900);
},1000); // loop waiting time must be >= 900 (biggest timeOut for inside actions)
}
PS: Understand that the real behavior of (setTimeOut): they all will start in same time "the three bla bla bla will start counting down in the same moment" so make a different timeout to arrange the execution.
PS 2: the example for timing loop, but for a reaction loops you can use events, promise async await ..
I have received this comment:
On the server side of Node.js when handling incoming data if you want to use a for loop you have to create i inside an anonymous function or you will pull your hair out wondering how the hell your variable i is greater than what you limit it to be inside your loop.
Here is the bugfix that was recommended:
var i = 0,
len = that.users.length;
(function(i) {
while(i < len) {
console.log(' - - - - - debug - - - - -');
console.log('i = ' + i );
i++;
}
})(i);
Can someone explain to me why using an anonymous function is necessary?
The problem only shows up when you have asynchronous code inside the loop. For example (I changed to a for loop for simplicity):
var i = 0,
len = that.users.length;
for(i = 0;i < len;i++) {
setTimeout(function() {
console.log('i = ' + i );
}, 500);
}
You will find that running this code causes the value of len to be printed len times, instead of a count up to len. This is because the for loop finishes before any of the print statements run, so the loop has exited because i == len.
The fix for this is to lock i in to each value with an Immediately-Invoked Function Expression (IIFE):
var i = 0,
len = that.users.length;
for(i = 0;i < len;i++) {
(function(i) {
setTimeout(function() {
console.log('i = ' + i );
}, 500);
})(i);
}
This isn't exactly the bugfix you presented, but it is the closest I can think of that makes much sense. Given more context, I could be more sure about what problem it is supposed to solve.
This is my code. What I want it to do is write 0, wait one sec, write 1, wait one sec, write 2, wait one sec, etc. Instead it writes 5 5 5 5 5
for(i = 0; i < 5; i++) {
setTimeout("document.write(i + ' ')", 1000);
}
http://jsfiddle.net/Xb7Eb/
1) You set all the timeouts to last 1 second at the same time. The loop doesn't wait for the timeout to occur. So you have 5 timeouts that all execute at the same time.
2) When the timeouts execute, the loop is long since complete and i has become 5. So once they execute, they all print "5"
3) document.write() writes somthing onto the page, in the same place it executes. I.e. if you have <script>document.write("xyz")</script> in the middle of a piece of text, it'll write "xyz" in the middle of the text. The timeouts, however, are not necessarily anywhere on the page. They exist only in code.
Here's a solution that's as close to yours as possible: http://jsfiddle.net/rvbtU/1/
var container = document.getElementById("counter");
for(i = 0; i < 5; i++) {
setTimeout("container.innerHTML += '" + i + " ';", 1000 * i);
}
However, that solution uses setTimeout's ability to evaluate a string as javascript, which is never a good idea.
Here's a solution that uses an anymous function instead: http://jsfiddle.net/YbPVX/1/
var container = document.getElementById("counter");
var writer = function(number) {
return function() { container.innerHTML += String(number) + " "; };
}
for(i = 0; i < 5; i++) {
setTimeout(writer(i), 1000 * i);
}
Edit: Forgot to save the 2nd fiddle. Whoops. Fixed now.
Most of the answers available are giving bad advice.* Specifically, you shouldn't be passing a string to setTimeout anymore (it still works, but it's discouraged), it's no longer 2000, there are better ways to do this.
setTimeout takes a function as the first parameter, and that's what you should do, however there are some issues when calling setTimeout in a loop.
This looks like it should work:
var i;
for ( i = 0; i < 5; i++ )
{
setTimeout(function(){
document.write( i + ' ' );
}, 1000 * (i + 1) );
}
But it doesn't. The issue is that by the time setTimeout executes the function, the loop will have incremented i to 5, so you'll get the same value repeated.
There are a few fixes. If you're willing to risk a with statement, you could try the following:
var i;
for ( i = 0; i < 5; i++ )
{
with( { i:i } )
{
setTimeout(function(){
document.write( i + ' ' );
}, 1000 * (i+1) );
}
}
Note that with is typically discouraged just like passing string values to setTimeout, so I don't really suggest this method of doing things.
The better way is to use a closure:
var i;
for ( i = 0; i < 5; i++ )
{
(function(i){
setTimeout(function(){
document.write( i + ' ' );
}, 1000 * (i+1) );
})(i);
}
To explain what's going on, the anonymous function wrapper (function(i){...code...}) executes immediately because it's wrapped in parens and passed i as a value:
(function(i){...code...})(i);
This forces the i variable that document.write uses to be a different one than what's being used in the for loop. You could even change the parameter used in the anonymous function wrapper if the difference gets too confusing:
(function(a){document.write(a+' ')})(i);
* when I started writing this question there were a number of answers describing how to fix the string to work with setTimeout, although they would technically work, they didn't include why they would work (because 'document.write("' + i + ' ");' evaluates i at the time of calling due to string concatenation, versus evaluating i at runtime like the previous version did), and they most certainly didn't mention that it's the bad old way of calling setTimeout.
try
var i = 1;
function timeout(){
document.write(i + ' ');
i++;
if (i == 5) return;
setTimeout(timeout, 1000);
}
timeout();
http://jsfiddle.net/nnJcG/1/
You have a problem with clousures, you can try this:
var timeout = function(){
var i = 0;
return function(){
document.write(i+ ' ');
i++;
if(i!==5)
setTimeout(timeout,1000);
};
}();
setTimeout(timeout,1000);
Here is the example in jsBin http://jsbin.com/uloyuc/edit
First of all, NEVER pass a string to setTimeout. Use a function, it's much cleaner.
Second, you have to "close over" the loop value. I bet this is what you want.
for(var i = 0; i < 5; i++) {
(function(i) {
setTimeout(function() {
document.write(i + ' ')
}, i * 1000);
}(i));
}
See more about you a self executing function to close over a loop value here http://www.mennovanslooten.nl/blog/post/62
And just cause I love it, here is the equivalent in CoffeeScript whihc has the do keyword to help out with just this case.
for i in [0..4]
do (i) ->
setTimeout ->
document.write "#{ i } "
, i * 1000
You can also work with setInterval and clearInterval:
var i = 0;
var f = setInterval(function() {
if(i == 4) clearInterval(f);
document.write(++i + ' ');
}, 1000);
I think this code is very readable.
You could try like this:
var tick_limit = 5; // Or any number you wish representing the number of ticks
var counter = 0; // Or any number you wish
var timer_interval = 1000; // Interval for the counter
var timer;
function timerTick()
{
if(counter < tick_limit)
{
// Execute code and increase current count
document.body.innerHTML+=(counter + ' '); // Append the counter value to the body of the HTML page
counter++;
timer = setTimeout(timerTick,timer_interval);
}
else
{
// Reset everything
clearTimeout(timer);
counter = 0;
}
}
function startCounter()
{
clearTimeout(timer); // Stop current timer
timer = setTimeout(timerTick,timer_interval); // Start timer with any interval you wish
}
...
// Start timer when required
startCounter();
...
This way, calling the startCounter a number of times will result in a single timer executing the code
You're triggering five timeouts at the same time.
I like Pindatjuh's answer, but here's another fun way to do it.
This way starts the next timeout when the previous one is finished:
// Wrap everything in a self executing anonymous function so we don't pollute
// the global namespace.
//
// Note: Always use "var" statments or you will pollute the global namespace!
// For example "for(i = 0; i < 5; i++)" will pollute the global namespace
// unless you have "var i; for(i = 0; i < 5; i++)" or
// "for(var i = 0; i < 5; i++)" & all of that is not in the global namespace.
//
(function() {
// "i" will be available within doThis()
// you could also pass "i" as an argument
var i = 0,
doThis = function() {
// setTimeout can take an anonymous function
// or a regular function. This is better than
// eval-ing a string.
setTimeout(function() {
document.write(i + ' ');
++i;
// Do the function again if necessary
if (i < 5) doThis();
}, 1000);
}
// Let's begin!
doThis();
})();
Working Example