CODEWITHSUNDEEP
javascriptwhile-loop
Wyldbrian
Wyldbrian

Reputation: 29

Bouncing ball: number of bounces and tracelled distance

I have received an interesting javascript practice exercise that I'm trying to understand. The problem below (screenshot) needs to be solved with a while loop.

The coefficient of restitution of a ball, a number between 0 and 1, specifies how much energy is conserved when a ball hits a rigid surface. A coefficient of .9, for instance, means a bouncing ball will rise to 90% of its previous height after each bounce.

Write a program to input a coefficient of restitution and an initial hight in meters, and report how many times a ball bounces when dropped from its initial height before it rises to a height of less than 10 centimeters. Also report the total distance traveled by the ball before this point.

The coefficients of restitution for a tennis ball, basketball, super ball, and softball are .7, .75, .9, and .3, respectively.

I'm trying to use the code below to complete this, but it simply hangs. 

 function code1() {
  var heightM = getInputOne();
  var heightCm = heightM * 100;
  var coefficient = getInputTwo();
  var distance = getInputOne();
  var bounce = 0;

  while (heightCm >= 10) {
    bounce = bounce + 1;
    distance = distance + (heightM * coefficient * 2);
    heightCm = heightCm * coefficient;
  }
  console.log(bounce);
  console.log(distance);

}

Here are the functions being called within it

// Return the text in the 'In 1:' box
function getInputOne() {
  var input = getElement("inOne");
  return input.value;
}

// Return the text in the 'In 2:' box
function getInputTwo() {
  var input = getElement("inTwo");
  return input.value;
}

Any help with this would be appreciated. Also, let me know what other data might be useful.

Upvotes: 0

Views: 2074

Answers (1)

trincot
trincot

Reputation: 350147

There are a few issues:

  • The value read from the input.value property is a string, not a number. This effects how the + operator works in the following expression:

    distance = distance + (heightM * coefficient * 2);

    As distance is a string, the + is a concatenation, not an addition. And so distance will just be growing string of digits.

  • although you adapt heightCm in every iteration, you don't do the same with heightM. It maintains its original value, and so the distance calculation is wrong.

  • You should check that the input value for coefficient is within the limits. If you allow a value of 1 or more, then the calculated height will increase in every iteration of the loop, which will make it hang.

So I would suggest this code:

function getBounces(heightM, coefficient) {
  var distance = heightM; // avoid reading input twice
  var bounce = 0;

  while (heightM >= 0.10) { // just compare with meters...
    bounce = bounce + 1;
    heightM = heightM * coefficient;
    distance = distance + heightM * 2;
    if (bounce > 100) throw "error";
  }
  return [bounce, distance];
}

document.getElementById('calc').addEventListener('click', function() {
  var heightM = getInputOne();
  var coefficient = getInputTwo();
  if (coefficient >= 1 || coefficient < 0) {
    alert('invalid input for coefficient');
    return;
  }
  var result = getBounces(heightM, coefficient);
  outputResults(result[0], result[1]);
});


// Return the text in the 'In 1:' box
function getInputOne() {
  var input = document.getElementById("inOne");
  return +input.value; // add the + for conversion to number
}

// Return the text in the 'In 2:' box
function getInputTwo() {
  var input = document.getElementById("inTwo");
  return +input.value; // add the + for conversion to number
}

function outputResults(bounce, distance) {
  var output = document.getElementById("bounce");
  output.textContent = bounce;
  output = document.getElementById("dist");
  output.textContent = distance.toFixed(2);
}
Height: <input id="inOne">m<br>
Coefficient: <input id="inTwo"> (0 ... 0.99)<br>
<button id="calc">Calculate</button>
<br>
Bounces: <span id="bounce"></span><br>
Distance: <span id="dist"></span>m<br>

Upvotes: 2

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