Question 1

I am having trouble with the second step of he implement swap challenge.
the only way to get the assertions to pass is to delete on of the igniters but leave a comma. This is only necessary in one of the assertions not the other and not in the first that challenge previously provided.

the code as follows is considered correct.
If i reinsert an igniter into *Program.assertEqual(testArray, [ , 4, 9]);* before the first comma the program does not pass.
*Program.assertEqual(testArray, [ 7, , 9]);* however is also a correct solution according to the challenge computer.

```
var swap = function(array, firstIndex, secondIndex) {
	var temp = array[firstIndex];
	array[firstIndex] = array[secondIndex];
	array[secondIndex] = temp;
};

var testArray = [7, 9, 4];
swap(testArray, 0, 1);

println(testArray);

Program.assertEqual(testArray, [9, 7, 4]);

swap(testArray, 1, 2);

Program.assertEqual(testArray, [ , 4, 9]);

swap(testArray, 0, 2);

Program.assertEqual(testArray, [4, 9, 7]);
```

Accepted Answer

var swap = function(array, firstIndex, secondIndex) {
var temp = array[firstIndex];
array[firstIndex] = array[secondIndex];
array[secondIndex] = temp;
};

var testArray = [7, 9, 4];
swap(testArray, 0, 1);

println(testArray);

Program.assertEqual(testArray, [9, 7, 4]);

swap(testArray, 1, 2);

Program.assertEqual(testArray, [9, 4, 7]);

swap(testArray, 0, 2);

Program.assertEqual(testArray, [7 , 4, 9]);

I don't want to give you the answer, but this code worked. It has to do with identifying the array index and then replacing them in the second part to the corresponding values. If that doesn't make sense, keep looking, and break the code into parts. Look at the first 
swap, and compare it with program assertion. You have to remember the goal is to act like a swapping program, you are swapping values and checking to see if they match the correct position.

Question 2

in the challenge problem why this code is not working ??

var swap = function(array, firstIndex, secondIndex) {
	array[firstIndex] = array[secondIndex];
	array[secondIndex] = array[firstIndex];
};

it all seems to be reasonable so why it is not working ?

Accepted Answer

The code above will write over the value in the first index with the value at the second index. So, when you try to copy the value from the first index into the second index, it is no longer contains the original value. So, you end up with two copies of the value that was in the second index.

You need to store the value from the first index into a variable before you overwrite it, so that you can use it to copy into the second index. Have a look at the hint code provided in the challenge.

Question 3

Hi!

Just out of curiosity, why does everything in the next challenge have to be in a specific order? After I finished step one, I tried doing _temp = array[secondIndex]_ instead of _array[secondIndex] = temp_ for fun and it didn't work. Can someone please explain?

Thanks!

Accepted Answer

In JavaScript, the equals sign, "=", is used for assignment. When you type temp = array[secondIndex];, you are assigning that value in the array to the variable temp. If you try it the other way, then you will be assigning the value of temp to that position in the array.

The later is desired because it modifies the array. The former only reads the array. Does that help?

The later is desired because it modifies the array.  The former only reads the array.  Does that help?

Question 4

just wondering if anyone found an algorithmic way to order those cards in the example without moving a card more than once?

Accepted Answer

No algorithm can be guaranteed to do it if you use swaps. e.g. If I have 2,3,1 it requires at least two swaps to sort it. But each swap involves 2 numbers. Which means 2 * 2 = 4 numbers were moved. Since 4 > 3, at least one number must be moved twice.

However, you can do it with a cycle sort:
https://en.wikipedia.org/wiki/Cycle_sort
The cycle sort doesn't swap the cards, instead it puts the card in the position it would appear in if the cards were sorted, and repeats that with the card currently sitting in that position. Eventually, all of the cards in the cycle will be in their sorted position by only moving them once. If you repeat that for all the cycles, the cards will be sorted.

Question 5

Why were there smaller numbers on the bottom of the cards like the card number 2 had a little 5 underneath it? Maybe you had the same question.

Accepted Answer

@clindsey737 I'm pretty sure the numbers underneath the cards showed the position of the card within the array. Javascript arrays start at 0, and go upward from there, and so do these cards.

You can find out more here:
https://www.khanacademy.org/computing/computer-programming/programming/arrays/pt/intro-to-arrays

Hope this helps!

Question 6

I am confused you asked us to sort but I can't drag any numbers.

Accepted Answer

You're not supposed to drag, you're suppose to swap.
"You can swap any pair of cards by clicking on one card, and then the other."

Question 7

I used the strategy of sorting by finding the smallest value each time, excluding the previous small value.

Accepted Answer

What happens if two values in your list have the same value ?
Could you sort [3,1,3,4,3,2,3,5] using that technique ?

Question 8

how does it work

Accepted Answer

by putting them from least to greatest

Question 9

can you sort in any way that is not least to greatest?

Accepted Answer

You can sort in any way you want as long as you can take two elements and compare them. Comparing means taking two element and saying which element is more X than the other.
e.g.
-If I compare elements to say which element is *larger* than the other, I get the standard least to greatest sort.  
-If I compare elements to say which element is *smaller* than the other, I get a greatest to least sort.
-If I compare elements to say which element has *the largest absolute value*, I get elements sorted by least to greatest magnitude (value ignoring the sign).

Course: Computer science theory > Unit 1

Sorting

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