why is this while loop causing an infinite loop?

Question

For some reason I'm having difficulty getting this while loop to work. It keeps crashing my browser whenever I try to test it out, and in the one case that I was able to see the results of the loop in the console, all I saw was NaN printed several times. Is there something I've forgotten in my code?

<div id="output"></div>

<script>

var starting = prompt("What is your starting balance?");
var target = prompt("What is your target balance?");
var interest = prompt("What is your interest rate?");
var periods = 0;

var current = starting;
var greaterThan = false;

while (greaterThan === false) {
  if (current < target) {
    current = current + (current * interest);
    periods++;
  } else {
    greaterThan = true;
    alert("it took " + periods + " periods to make your starting balance greater than your target balance.");
    document.querySelector('#output').textContent = "to grow an initial investment of " + starting + " to " + target + " at a " + interest + " interest rate will require " + periods + " investment periods.";
  }
}

</script>

Did you try and set a breakpoint at greaterThan = true and verify the code reaches this far ? — elad.chen
– elad.chen, Commented Sep 30, 2015 at 5:07

Arun P Johny · Accepted Answer · 2015-09-30 05:13:21Z

5

The one problem I could see is, all your input values are string, not numbers so they are doing string comparison not numeric

var starting = +prompt("What is your starting balance?") ||0;
var target = +prompt("What is your target balance?")||0;
var interest = +prompt("What is your interest rate?")||1;

The + in front of prompt() is the unary plus operator

edited Sep 30, 2015 at 5:13

answered Sep 30, 2015 at 5:10

Arun P Johny

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3 Comments

user3794166 Over a year ago

+null returns 0. Much better than parseFloat since it wont return NaN.

Ruan Mendes Over a year ago

@hani only if that's your desired behavior. One may not want to convert null to zero because null means they hit cancel. How are you supposed to tell the difference between typing 0 and hitting cancel?

user3794166 Over a year ago

@Arun P Johny In this case you are doing an OR which means if they hit cancel they still get 0 (null || 0). There is also a plus before prompt, which again makes it 0 if they hit cancel (+null)

Ruan Mendes · Accepted Answer · 2015-09-30 05:10:27Z

4

You are forgetting to convert the result from prompt from a string into a number.

var starting = parseFloat(prompt("What is your starting balance?"));

Do the same thing to the other numbers that are input by the user from the prompt.

answered Sep 30, 2015 at 5:10

Ruan Mendes

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Comments

Vivek Tiwari · Accepted Answer · 2015-09-30 06:14:32Z

First you need to convert your input into an integer value. The input from the prompt is a string. even if you enter 1 or 10 or any number. You can use parseInt() for that. and because you are asking for interest rate, i think any user would enter something like 2. 5, or 10 as a percentile. not 0.1, or 0.05. Even if he does, the parseInt() function can't get it right because 0.05 is not an integer value. You can use parseFloat for that. so i suggest you look at my implementation of your code below. also, i have omitted the if else statements because they weren't necessary and would only make the code more complex.

<div id="output"></div>

<script type="text/javascript">

var starting = parseInt(prompt("What is your starting balance?"));
var target = parseInt(prompt("What is your target balance?"));
var interest = parseInt(prompt("What is your interest rate?"));
var periods = 0;
var intrate = interest/100;
var current = starting;

while (current< target) {

    current += (current*intrate);
    periods += 1;
  } 
    alert("it took " + periods + " periods to make your starting balance greater than your target balance.");
    document.querySelector('#output').textContent = "to grow an initial investment of " + starting + " to " + target + " at a " + interest + " interest rate will require " + periods + " investment periods.";
</script>

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