Create a new array using loops in C# without built-in methods-CodePudding

I'm studying for my first test in C# (beginner). I have a problem with assingments where I'm supposed to create a new array using loops. For example this task where the task is to write a method that recieves a sentence(string) and a letter(char). The method must then identify at which index positions the letter occurs at in the sentence and then place these positions in a array. For example, we have the short sentence "Hello world!" and the letter 'o' then the array should contain 4 (the index position of the first instance) and 7 (the index position of the second instance).

I'm not allowed to use built-in methods except for .Length, Console.WriteLine..

You can see my code below. It is not working at all. I want it to print out "4, 7, "

static void Main(string[] args)
{
    int[] result = IndexesOfChar("Hello world", 'o');
    for(int i = 0; i<result.Length; i  )
    {
        Console.Write(result[i]   ", ");
    }
}

static int[] IndexesOfChar(string sentence, char letter)
{
    int count = 0;
    int[] newArr = new int[count];

    for(int i =0; i < sentence.Length; i  )
    {
        if(sentence[i] == letter)
        {
            newArr[count] = i;
            count  ;
        }
        
    }
    return newArr;
}

CodePudding user response：

The problem is that you don't know the array Length beforehand. So you have to compute count and only then create the array:

static int[] IndexesOfChar(string sentence, char letter) 
{
    // Required array length computation:
    int count = 0;

    for (int i = 0; i < sentence.Length; i  )
        if (sentence[i] == letter)
            count  ;
 
    // We know count, we are ready to create the array:
    int[] newArr = new int[count];

    // Finally, we fill in the array
    // let do not re-use count, but declare separate index variable
    int index = 0;

    for (int i = 0; i < sentence.Length; i  )
        if (sentence[i] == letter)
            newArr[index  ] = i;

    return newArr;     
}

Your task is not a good example for arrays, usually we put List<T> when we don't know size:

using System.Linq;

...

static int[] IndexesOfChar(string sentence, char letter) {
  List<int> result = new List<int>();

  for (int i = 0; i < sentence.Length;   i)
    if (sentence[i] == letter)
      result.Add(i); // <- unlike array we can just Add a new item

  // create an array from list with a help of Linq
  return result.ToArray();
}

CodePudding user response：

It is not working, because in the method IndexesOfChar, you create an array with a length of count (that is at that point is zero). You can't modify an array's length once you declared it.

If you can't use any built in methods, I suggest you to declare the newArr as a list. This is what you should fill the indexes into, then create an array, and fill the list's values into that array with another for loop.

CodePudding user response：

Unlike type List, you can't change the size of an array, so you can't do newArr[count] = i; because the size of newArr is 0. Instead if you only want to use arrays, you can reassign newArr with its old value the new integer :

static void Main(string[] args)
{
    int[] result = IndexesOfChar("Hello world", 'o');
    for(int i = 0; i<result.Length; i  )
    {
        Console.Write(result[i]   ", ");
    }
}

static int[] IndexesOfChar(string sentence, char letter)
{
    int count = 0;
    int[] newArr = new int[count];

    for(int i =0; i < sentence.Length; i  )
    {
        if(sentence[i] == letter)
        {
            var updateArr = new int[newArr.Length   1];
            for (int j = 0; j < newArr.Length; j  )
            {
                updateArr[j] = newArr[j];
            }
            updateArr[newArr.Length] = i;
            newArr = updateArr;
            count  ;
        }
        
    }
    return newArr;
}