返回列表的问题<int>来自 C# 中的方法和打印值</int>

Question

I have a method like this static List<int> Compare(List<int> a, List<int> b) I would like this function to return [aPoints bPoints] something like [2 1] But, I'm stuck on storing the values after the loop with if statements and putting them in score. I tried this:

static List<int> Compare(List<int> a, List<int> b)
{        
    int aPoints = 0;
    int bPoints = 0;
    List<int> score = new List<int>() { aPoints, bPoints }; 

    for (int i = 0; i < 3; i++)
    {
        if (a[i] > b[i])
        {
            aPoints++;
        }
        else if (a[i] < b[i])
        {
            bPoints++;
        }
    }

    return score;
}

And print them on:

static void Main(string[] args){}

Answer 1

Here's a very simple solution. Just create an empty list, and add aPoints and bPoints to it right before returning to the caller.

static List<int> Compare(List<int> a, List<int> b)
{        
    int aPoints = 0;
    int bPoints = 0;
    List<int> score = new List<int>();

    for (int i = 0; i < 3; i++)
    {
        if (a[i] > b[i])
        {
            aPoints++;
        }
        else if (a[i] < b[i])
        {
            bPoints++;
        }
    }
    score.Add(aPoints);
    score.Add(bPoints);
    return score;

• Or you can just create your list right at the return like this:

static List<int> Compare(List<int> a, List<int> b)
{        
    int aPoints = 0;
    int bPoints = 0;

    for (int i = 0; i < 3; i++)
    {
        if (a[i] > b[i])
        {
            aPoints++;
        }
        else if (a[i] < b[i])
        {
            bPoints++;
        }
    }

    return new List<int>() { aPoints, bPoints};

• Also, you could use a more semantically correct way since your list always has 2 values. Use a tuple :

static (int aPoints, int bPoints) Compare(List<int> a, List<int> b)
{        
    int aPoints = 0;
    int bPoints = 0;

    for (int i = 0; i < 3; i++)
    {
        if (a[i] > b[i])
        {
            aPoints++;
        }
        else if (a[i] < b[i])
        {
            bPoints++;
        }
    }

    return (aPoints, bPoints);
}

Following up on your comment, if you want to print the contents of what the Compare method returns, then, for a List<int> you can do:

List<int> ret = Compare(someList, anotherList);
foreach (int n in ret)
{
    Console.WriteLine(n);
}

• Or you can use a classic for loop like this:

List<int> ret = Compare(someList, anotherList);
for (int i = 0; i < 2; i++)
{
    Console.WriteLine(ret[i]);
}

However, in this loop, it's assumed that your list will always have 2 elements inside. A general approach would be to change the loop definition to for (int i = 0; i < ret.Count; i++) so that it iterates over every available item in the list like with the foreach loop.

Answer 2

Currently you're creating the list before you give aPoints and bPoints useful values. When you add aPoints and bPoints to the list, you're adding the values - so you end up with a list of [0, 0] .

Instead, you should create the list at the end - and you don't really need a variable for it:

static List<int> Compare(List<int> a, List<int> b)
{        
    int aPoints = 0;
    int bPoints = 0;

    for (int i = 0; i < 3; i++)
    {
        if (a[i] > b[i])
        {
            aPoints++;
        }
        else if (a[i] < b[i])
        {
            bPoints++;
        }
    }

    return new List<int>() { aPoints, bPoints };
}

A few things to note:

• Your code always loops 3 times, regardless of how long a and b are. While that may be okay now, it's a bit inflexible... any reason you don't want to check they're the same length, and then iterate over them completely?
• You might want to consider returning a tuple instead:

static (int aPoints, int bPoints) Compare(List<int> a, List<int> b)
{        
    int aPoints = 0;
    int bPoints = 0;

    for (int i = 0; i < 3; i++)
    {
        if (a[i] > b[i])
        {
            aPoints++;
        }
        else if (a[i] < b[i])
        {
            bPoints++;
        }
    }

    return (aPoints, bPoints);
}

After all, the list you'll return always has two elements, and I suspect the caller won't want to treat it as a general-purpose list anyway.

---

Printing the results

If you're still using the list approach for returning the values, your Main method should look something like this:

static void Main()
{
    List<int> a = ... ; // However you want to populate this
    List<int> b = ... ; // However you want to populate this
    List<int> results = Compare(a, b);
    Console.WriteLine($"Score for a: {results[0]}");
    Console.WriteLine($"Score for b: {results[1]}");
}

If you use the tuple approach, you can be clearer:

static void Main()
{
    List<int> a = ... ; // However you want to populate this
    List<int> b = ... ; // However you want to populate this
    var results = Compare(a, b);
    Console.WriteLine($"Score for a: {results.aPoints}");
    Console.WriteLine($"Score for b: {results.bPoints}");
}

Answer 3

Integers are passed by value and not by reference . This means when you make a new List<int>() { aPoints, bPoints }; , the values of aPoints and bPoints , which are both 0, are copied into the list. Changing their values afterwards won't affect the copies stored in the list.

For this specific situation, the easiest thing would be to simply move your line List<int> score = new List<int>() { aPoints, bPoints }; after your loop. Then, the points values will be copied over after they've been set correctly.

Answer 4

aPoints and bPoints are value types, not reference types, so put them in the end:

static List<int> Compare(List<int> a, List<int> b)
{        
    int aPoints = 0;
    int bPoints = 0;

    for (int i = 0; i < 3; i++)
    {
        if (a[i] > b[i])
        {
            aPoints++;
        }
        else if (a[i] < b[i])
        {
            bPoints++;
        }
    }

    return new List<int>() { aPoints, bPoints };
}