c# foreach 循环无法在数组中添加文字

Question

Firstly I know that I can't use a foreach loop in C# to add values in let's say an array... But why? Why for example I can't do this

int[] numbers = { 1, 4, 3, 5, 7, 9 };
foreach (int item in numbers)
{
    numbers[item] = 2;
}

Does it have something to do with the actual realisation of the foreach loop in the back-end? And how does the foreach loop exactly work? I know that it goes through the whole collection(array) but how exactly?

Answer 1

You are passing in the value of an item ( your variable, item , will be the value of the array at each position in sequence, not the index ) in the array as the index. The index used there is meant to be the position of the item you are attempting to access, not the value. So each iteration of the loop you are calling:

• numbers[1]
• numbers[4]
• numbers[3]
• numbers[5]
• numbers[7]
• numbers[9]

The array has 6 numbers, so when you get to numbers[7] , you are asking for a value that is not there, hence the exception.

A better method of doing what you are trying to do would be:

for(int i = 0; i < numbers.Length; i++)
{
    numbers[i] = 2;
}

On each iteration of this loop you would be accessing:

• numbers[0]
• numbers[1]
• numbers[2]
• numbers[3]
• numbers[4]
• numbers[5]

Answer 2

You need to step through your code in a debugger .

A for statement is more like a while statement, not like a foreach .

The line int[] numbers = { 1, 4, 3, 5, 7, 9 }; create this:

numbers[0] = 1;
numbers[1] = 4;
numbers[2] = 3;
numbers[3] = 5;
numbers[4] = 7;
numbers[5] = 9;

Your foreach statement does this:

numbers[1] = 2;
numbers[4] = 2;
numbers[3] = 2;
numbers[5] = 2;
numbers[7] = 2; <- this line overflows your array!
numbers[9] = 2; <- and so would this.

You have to learn the difference between an array index and an array value.

Answer 3

I'm looking at this:

numbers[item] = 2;

In this expression, you're using the item variable like an index , as if it had the values 1 , 2 , 3 , 4 , etc. That's not how the foreach iteration variable works for C# . The only language I know that does it this way is Javascript.

Remember that foreach and for are not the same thing. Just about every other language, including C#, gives you the actual array values in the item variable of a foreach loop: 1 , 4 , 3 , 5 etc. Now, these are integers, so you could try to use them as indexes. You can run the loop for a while like that... until you get to the value 7 . At this point, your array only has six values. You're trying to do this:

numbers[7] = 2;

for an array where the largest valid index you can use is 5 .

This is true even taking your modification of the array into account. Let's look at the array after each iteration through the loop:

{ 1, 4, 3, 5, 7, 9 }  //initial state
{ 1, 2, 3, 5, 7, 9 }  // after 1st iteration (index 0). Value at index 0 is 1, so item as index 1 is set to 2
{ 1, 2, 2, 5, 7, 9 }  // after 2nd iteration (index 1). Value at index 1 is now 2, so item at index 2 is set to 2
{ 1, 2, 2, 5, 7, 9 }  // after 3rd iteration (index 2). Value at index 2 is now 2, so item at index 2 is set to 2
{ 1, 2, 2, 5, 7, 2 }  // after 4th iteration (index 3). Value at index 3 is 5, so item at index 5 is set to 2
// The 5th iteration (index 4). Value at index 4 is 7, which is beyond the end of the array

For the why of this... it sounds like you're used to a more dynamic language. Some these other languages, like php or Javascript, don't have real arrays at all in the pure computer science sense. Instead, they have collection types they'll call an array, but when you get down to it are really something different.

C# has real arrays, and real arrays have a fixed size. If what you really want is a collection , C# has collections, too. You can use List<T> objects, for example, to get an array-like collection you can append to easily.

For the other languages, the results vary depending on what you're talking about, but for the most permissive the result of your 5th iteration is something like this:

{ 1, 2, 2, 5, 7, 2,  ,2 } 

Note the missing value at index 6. That kind of things leads to mistakes that slip through your tests and don't show up until run-time. You also need to start wondering just how densely or sparsely the array will be filled, because the best strategy for handling these arrays can vary wildly depending on your answer... everything from just a big backing array with empty nodes that the programmer has to know about all the way to Hashtables and Dictionaries. And, by the way, C# again has these options available to you.

Answer 4

You need to create counter, in other case you trying to access item outside of array

int[] numbers = new int[]{ 1, 4, 3, 5, 7, 9 };
int i = 0;
foreach (int item in numbers)
{
    numbers[i] = 2;
    i++;
}

// Print the items of the array
foreach (int item in numbers)
{
    Console.WriteLine(item);
}