A function which should be repeating every second is instantly happening

user_13102310 0
Viewed 38

I'm making a kind of physics thing where you can place black squares and they will fall. In the demo below, you can type in a location and press go (e.g "b6") and it will place a square there. However, it should fall one square at a time, because of the setinterval, but it just teleports to the bottom instead. What have I done wrong in my code?

var cells = document.getElementsByTagName("td");
var alphabet = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"];
setInterval(grav, 1000);
for (var i = 0; i < 625; i++) {
    cells[i].id = String(alphabet[Math.floor(i / 25)]) + String((i % 25) + 1);
}
function place() {
  document.getElementById(document.getElementById("input").value).classList.add("on");
}
function grav() {
  for (var o = 0; o < 625; o++) {
    if (cells[o].classList.contains("on")) {
      document.getElementById((alphabet[alphabet.indexOf(cells[o].id.charAt(0)) + 1] + cells[o].id.substring(1))).classList.add("on");
      document.getElementById(cells[o].id).classList.remove("on");
    }
  }
}
td {
  border: 1px solid black;
  width: 20px;
  height: 20px;
}
td.on {
  background-color: black;
}
table {
  border-spacing: 0px;
}
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<input type="text" placeholder="Location" id="input"><button onclick="place()">Go</button>
And yes, I know it gives errors when it hits the bottom, I will make it just stop instead of doing that.

html javascript
3 Answers

Calling grav just once is enough to move the block all the way to the bottom, because you're looping through the blocks in order, left-to-right, top-to-bottom, and so when you move b6 to b7, it gets found again when the function keeps going and moves b7 to b8, and then again to b9, etc.

If you loop backward, that won't happen, but it might make your animation not work the way you want it to when there are multiple blocks:

function grav() {
  for (var o = 624; o >= 0; --o) {
    if (cells[o].classList.contains("on")) {
      document.getElementById((alphabet[alphabet.indexOf(cells[o].id.charAt(0)) + 1] + cells[o].id.substring(1))).classList.add("on");
      document.getElementById(cells[o].id).classList.remove("on");
    }
  }
}

Live Example:

var cells = document.getElementsByTagName("td");
var alphabet = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"];
setInterval(grav, 1000);
for (var i = 0; i < 625; i++) {
    cells[i].id = String(alphabet[Math.floor(i / 25)]) + String((i % 25) + 1);
}
function place() {
  document.getElementById(document.getElementById("input").value).classList.add("on");
}
function grav() {
  for (var o = 624; o >= 0; --o) {
    if (cells[o].classList.contains("on")) {
      document.getElementById((alphabet[alphabet.indexOf(cells[o].id.charAt(0)) + 1] + cells[o].id.substring(1))).classList.add("on");
      document.getElementById(cells[o].id).classList.remove("on");
    }
  }
}
td {
  border: 1px solid black;
  width: 20px;
  height: 20px;
}
td.on {
  background-color: black;
}
table {
  border-spacing: 0px;
}
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</table>
<input type="text" placeholder="Location" id="input"><button onclick="place()">Go</button>

If you want multiple blocks to be handled in left-to-right, top-to-bottom order, you'll need to keep track of the fact you've moved a block during the current call to grav and not move it again. That's easily done with a Set:

function grav() {
  const seen = new Set();
  for (var o = 0; o < 625; o++) {
    const cell = cells[o];
    if (cell.classList.contains("on") && !seen.has(cell)) {
      const next = document.getElementById((alphabet[alphabet.indexOf(cell.id.charAt(0)) + 1] + cell.id.substring(1)));
      next.classList.add("on");
      seen.add(next);
      document.getElementById(cell.id).classList.remove("on");
    }
  }
}

Live Example:

var cells = document.getElementsByTagName("td");
var alphabet = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"];
setInterval(grav, 1000);
for (var i = 0; i < 625; i++) {
    cells[i].id = String(alphabet[Math.floor(i / 25)]) + String((i % 25) + 1);
}
function place() {
  document.getElementById(document.getElementById("input").value).classList.add("on");
}
function grav() {
  const seen = new Set();
  for (var o = 0; o < 625; o++) {
    const cell = cells[o];
    if (cell.classList.contains("on") && !seen.has(cell)) {
      const next = document.getElementById((alphabet[alphabet.indexOf(cell.id.charAt(0)) + 1] + cell.id.substring(1)));
      next.classList.add("on");
      seen.add(next);
      document.getElementById(cell.id).classList.remove("on");
    }
  }
}
td {
  border: 1px solid black;
  width: 20px;
  height: 20px;
}
td.on {
  background-color: black;
}
table {
  border-spacing: 0px;
}
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<input type="text" placeholder="Location" id="input"><button onclick="place()">Go</button>

Side note: When you already have an element, there's no need to look it up again. So this:

document.getElementById(cells[o].id).classList.remove("on");

should be just

cells[o].classList.remove("on");
user_1572470

As in my comment: You make it go one line lower, then check it again...
Loop upwards instead:

function grav() {
  for (var o = 624; o >=0; o--)...


var cells = document.getElementsByTagName("td");
var alphabet = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"];
setInterval(grav, 1000);
for (var i = 0; i < 625; i++) {
    cells[i].id = String(alphabet[Math.floor(i / 25)]) + String((i % 25) + 1);
}
function place() {
  document.getElementById(document.getElementById("input").value).classList.add("on");
}
function grav() {
  for (var o = 624; o >= 0; o--) {
    if (cells[o].classList.contains("on")) {
      document.getElementById((alphabet[alphabet.indexOf(cells[o].id.charAt(0)) + 1] + cells[o].id.substring(1))).classList.add("on");
      document.getElementById(cells[o].id).classList.remove("on");
    }
  }
}
td {
  border: 1px solid black;
  width: 20px;
  height: 20px;
}
td.on {
  background-color: black;
}
table {
  border-spacing: 0px;
}
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<input type="text" placeholder="Location" id="input"><button onclick="place()">Go</button>

user_57925090

It's due to your algorithm coded in grav, try to put a debugger in the condition "if (cells[o].classList.contains("on")) {" and you will figure out that at index N you move the block to next line, then later in the loop you process this line and moving it again to the next line. As a result, in this single loop you move your block to the end, it's not a issue with the interval.

user_32467700
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