C ++插入排序混乱

Question

I'm trying to implement insertion sort on my own in C++. I know there are plenty of examples and I compared my solution with existing ones and don't see why mine isn't working. I know there are libraries for this but I want to implement it on my own. I've got 2 different implementations as shown below ( A - one that works, B - one that doesn't work).

Here is A - One that does work. Nothing new here.

    int myArr[5] = {5,4,3,2,1};

    for (int i = 1; i < 5; i++){

        int j = i - 1;
        int key = myArr[i];

        while(myArr[j] > key && j >= 0){
            myArr[j + 1] = myArr[j];
            j = j - 1;
        }        
        myArr[j + 1] = key;

        //Printing array to see what changed:
        for (size_t k = 0; k < 5; ++k){
            cout << myArr[k] << " ";
        }
        cout << endl;
   }

Sample output from A :

    4 5 3 2 1 
    3 4 5 2 1 
    2 3 4 5 1 
    1 2 3 4 5

Here is B , something I came up with. B is very similar to A except for the lines I've pointed out, where I chose to use myArr subscript instead of key:

    int myArr[5] = {5,4,3,2,1};

    for (int i = 1; i < 5; i++){

        int j = i - 1;
        //int key = myArr[i]; //DIFFERENT FROM **A**


        //************** DIFFERENT FROM A **************
        //I didn't use "key", instead I chose to use myArr[i]
        while(myArr[j] > myArr[i] && j >= 0){
            myArr[j + 1] = myArr[j];
            j = j - 1;
        }

        //************** DIFFERENT FROM A **************
        //Same here: I use myArr[i] instead of key        
        myArr[j + 1] = myArr[i];

        //Printing array to see what changed:
        for (size_t k = 0; k < 5; ++k){
            cout << myArr[k] << " ";
        }
        cout << endl;
   }

Sample output from B :

    5 5 3 2 1 
    5 5 5 2 1 
    5 5 5 5 1 
    5 5 5 5 5 

I don't understand, the only thing I changed was not storing the current value in a variable. I know I can easily do that and all will be well but what bothers me is that I don't know why B is incorrect. Any guidance would be appreciated.

Answer 1

This is a good exercise in walking through the code by hand. What's changed? key is temporary storage for the value at myArr[i] . The problem with the seemingly innocuous refactor is the fact that myArr[j + 1] is myArr[i] on the first iteration of the inner loop. Notice:

int j = i - 1;
...
myArr[j + 1] = myArr[j]; // j + 1 === i

which is essentially:

myArr[i] = myArr[j]; // whoops!

Here, we've reassigned myArr[i] to something else instead of copying and storing the value in key . We're losing one element on every outer loop iteration whenever the myArr[i] element isn't already sorted.

Keep the key variable!

Answer 2

I will use i = 1 to show why it is working different from what you expect.

int j = i - 1;

j becomes 0.

myArr[j + 1] = myArr[j];

Here myArr[1] is assigned value of myArr[0]. This is where the problem lies, as

myArr[j + 1] = myArr[i];

assigns value of myArr[0] (modified in previously mentioned line) to myArr[1].

In short, the shifting erases the value to be inserted. That's why you need to copy it to another variable (key in code A).