Why can't my algorithm find the index in my array?

Question

Why is my algorithm returning “-1” meaning that target value 73 isn’t in the array? (When clearly 73 is in the array). [this is from Khan Academy, but isn't helping]

It's supposed to return either the index of the location in the array, or "-1" if the array did not contain the targetValue

Var doSearch = function(array, targetValue) {
    var min = 0;
    var max = array.length - 1;
    var guess;
    while(max >= min) {
        guess = floor((max*1 + min*1) / 2);
        if (guess === targetValue) {
            return guess;
        } else if (guess < targetValue) {
            min = guess + 1;
        } else {
            max = guess - 1;
        }
    }
    return -1;
};

var primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 
    41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97];

var result = doSearch(primes, 73);
println("Found prime at index " + result);

guess = floor Can you post your floor function? (assuming it's different from Math.floor) — CertainPerformance
– CertainPerformance, Commented Jun 15, 2018 at 0:29
What's the point of writing max*1 and min*1. Multiplying by 1 doesn't change a number. — Barmar
– Barmar, Commented Jun 15, 2018 at 0:29
You're not even iterating through the array. Your function will always return -1. — reisdev
– reisdev, Commented Jun 15, 2018 at 0:31
@HoCo_ you may have missed the other return statement. I've formatted the question to make them clearer — Phil
– Phil, Commented Jun 15, 2018 at 0:32

Barmar · Accepted Answer · 2018-06-15 00:32:55Z

5

You're setting guess to an array index, but then you're comparing it with targetValue. You need to compare the array element at that index.

var doSearch = function(array, targetValue) {
  var min = 0;
  var max = array.length - 1;
  var guess;
  var guessvalue;
  while (max >= min) {
    guess = Math.floor((max * 1 + min * 1) / 2);
    guessValue = array[guess];
    if (guessValue === targetValue) {
      return guess;
    } else if (guessValue < targetValue) {
      min = guess + 1;
    } else {
      max = guess - 1;
    }
  }
  return -1;
};

var primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37,
  41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97
];

var result = doSearch(primes, 73);
console.log("Found prime at index " + result);

answered Jun 15, 2018 at 0:32

Barmar

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Comments

reisdev · Accepted Answer · 2018-06-15 00:34:18Z

Your function doSearch() doesn't compare any value of the array to the targetValue. You need to access the guess position and compare it, like this:

var doSearch = function(array, targetValue) {
    var min = 0;
    var max = array.length - 1;
    var guess;
    while(max >= min){
        guess = Math.floor((max*1 + min*1) / 2);
        if (array[guess] === targetValue) {
            return guess;
        } else if (array[guess] < targetValue) {
            min = guess + 1;
        } else {
            max = guess - 1;
        }
    }
    return -1;
};

var primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 
41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97];

var result = doSearch(primes, 73);
console.log("Found prime at index " + result);

Warning: The function floor() doesn't exist in JavaScript. You should use Math.floor(). Also, the function println(). Use console.log() instead.

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