Project Euler #52 - Permuted multiples

Question

It can be seen that the number, 125874, and its double, 251748, contain exactly the same digits, but in a different order.

Find the smallest positive integer, x, such that 2x, 3x, 4x, 5x, and 6x, contain the same digits.

The following code works on SpiderMonkey 1.8.5. But didn't work on my nodejs.
It takes about two seconds, and gives the correct answer.

My two major concerns are my style. And my choice in functions.
Personally I think str_sorted is horrible, int -> str -> array -> str.

// Like Python's range.
function range(start, stop, step){
    if (typeof(stop)==='undefined'){
        stop = start;
        start = 0;
    }
    if (typeof(step)==='undefined') step = 1;
    if (stop === null){
        while (true){
            yield start;
            start += step;
        }
    }else{
        for (number = start; number < stop; number += step){
            yield number;
        }
    }
}


function str_sorted(num){
    num = num.toString();
    arr = Array();
    for (index in num){
        arr.push(num[index]);
    }
    return arr.sort().join('');
}


function permuted(num){
    tmp = str_sorted(num);
    return tmp == str_sorted(num * 2) &&
           tmp == str_sorted(num * 3) &&
           tmp == str_sorted(num * 4) &&
           tmp == str_sorted(num * 5) &&
           tmp == str_sorted(num * 6);
}


function find(iterable, fn){
    for (num in iterable){
        if (fn(num)){
            return num;
        }
    }
}


print(find(range(1, null), permuted));

---

The way that I run this code is via the command line.

$ js p52.js
142857
$

This runs the the SpiderMonkey 1.8.5 interpreter. I got it from the Arch repository at some point.

Answer 1

Overall, your code looks pretty decent. The indentation is spot-on, the variable names make sense, the code is clear and easy to read. But there are some big issues with your code.

1. The first thing that popped in my eyes was this:

   if (typeof(stop)==='undefined'){
   

   Please, don't use typeof to check if it is defined or not. You have a very interesting object called arguments.

   Try this instead:

   if (arguments.length < 3){
   

2. A few lines below, you have this:

   if (stop === null){
   

   What if I pass undefined or false?

   You need to predict those, since they are valid values and, somewhat, in context. And yes, it is possible to pass undefined to a function, in one of these ways:

   range(1, undefined, 3); //simply pass undefined
   range.call({}, 1, undefined, 3); //mostly the same
   range.apply({}, [1,,3]); //it may happen!
   //... more similar variants ...
   

   You need to take care of those.

3. This is one of a few examples.

   You have the following function:

   function str_sorted(num){
       num = num.toString();
       arr = Array();
       for (index in num){
           arr.push(num[index]);
       }
       return arr.sort().join('');
   }
   

   Let's focus on the following bit:

   arr = Array();
   

   There are 2 wrong things here:

   • This is a local variable without var
   • You are using the Array constructor as a function
     You either use [] or use new Array()

   The fix is simple:

   function str_sorted(num){
       var arr = [];
       num = num.toString();
       for (index in num){
           arr.push(num[index]);
       }
       return arr.sort().join('');
   }
   

   But you still have something there that's bothering me:

       num = num.toString();
       for (index in num){
           arr.push(num[index]);
       }
   

   Why are you converting it to a string and then iterating it? Is it to sort the numbers in the string? If that's the case, here's the whole function:

   function str_sorted(num){
       return num.toString().split('').sort().join('');
   }
   

   Using string.split('') will split a string by each character, making an array of numbers.

4. Let's check your permuted() function.

   It also suffer from a tiny flaw:

   function permuted(num){
       tmp = str_sorted(num);
       return tmp == str_sorted(num * 2) &&
              tmp == str_sorted(num * 3) &&
              tmp == str_sorted(num * 4) &&
              tmp == str_sorted(num * 5) &&
              tmp == str_sorted(num * 6);
   }
   

   I have no idea what you are trying to do, but here is the mistake:

   tmp = str_sorted(num);
   

   There, you forgot a var.

   Change it to this:

   var tmp = str_sorted(num);
   

5. Lets take a look at the find function.

   This one isn't too bad:

   function find(iterable, fn){
       for (num in iterable){
           if (fn(num)){
               return num;
           }
       }
   }
   

   Just more of the same:

   for (num in iterable){
   

   You forgot a var there.

   Change it to this:

   for (var num in iterable){
   

6. I'm not sure if this is part of the code to be reviewed or not.

   But let's analyze the last line:

   print(find(range(1, null), permuted));
   

   Please, don't use print. It isn't even a standard! It's just supported in some consoles (like Chrome). Use console.log() instead.

   For now, I will leave the review as is. If I find any more issues, in the future, I will check on them.