C ++迭代整数数组，其大小未知？

Question

I have the following array:

int* myArray = new int[45];

If I wanted to iterate each element without knowing the actual size of the array, I would need to use a for_each ?

If so, then how would you write the for_each ? I was looking over the following site and reading up on for_each but can't figure out how to put this together.

http://www.cplusplus.com/reference/algorithm/for_each/

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Update: A for_each is not a good choice in this case, due to the fact that the size of the array has to be known. vectors are the proper way to accomplish such task. My reason for using arrays, in this case, was for learning purposes. if this was a serious project I would move to something such as Lists/Vectors.

Answer 1

Note when the question was first posted, the array in question was declared as

int myArray[45];

This answer deals with that particular case.

If you have C++11 support, you can use a range based loop:

for (int& i : myArray) {
  std::cout << i << "\n";
}

C++11 also provides std::begin and std::end , which you can use with a fixed size array to obtain iterators:

std::for_each(std::begin(myArray), std::end(myArray), <func>);

Another option, which works for C++03 and you are dealing with fixed size arrays, is to define a function template:

// taken a fixed size array by reference and loop over it
template <typename T, unsigned int N>
void array_for_each( T (&a)[N]) {

  for (unsigned int i = 0; i < N; ++i) {
    // do something with array elements
    std::cout << a[i] << " ";
  }

}

int main() {
  int a[5];
  array_for_each(a);
}

Answer 2

If you use MSVC, you can use "for each."

for each(int i in arr) {
    cout << i << ' ';
}

NOTE: This only works in the block of code the array is declared in.

If not, you can also use the new range-based for loop in the C++11 standard.

for(int i : arr) {
    cout << i << ' ';
}

If you're intent upon the std::for_each:

for_each(arr,arr + 10,[] (int i) {
        cout << i << ' ';
});

NOTE: This requires knowledge of the size of the array (in this example, 10).

Answer 3

You have described an array of int, not a class that implements a InputIterator , which is what the for_each is designed for, even though you can use it to iterate an array, but you need to know the size of the array to iterate it.

If you want to use for_each you need to use a vector , list , or implement a class that keeps track of the number of elements it contains. IMO it is much easier to just use a vector

If you want to just iterate your current array, assuming it is 0 terminated:

for(int *value = myArray; *value != 0; ++value)
  printf("%d\n", *value);

Or, you can use indexes:

for(int index = 0; myArray[index] != 0; ++index)
  printf("%d\n", myArray[index]);

IMO the pointer method is cleaner.

This code is still dangerous though, you should either keep track of the number of records in a seperate variable, or use a vector.

Answer 4

You could use a for_each . In this case, you have allocated space for 45 elements in your array, but since it is NULL, you'd probably get a segfault if you tried to do anything. You either need to hold a value of the array, or use something like sizeof(myArray)/sizeof(myArray[0]) (which has its own problems).

Anyway, for a for_each here, if we actually had 45 elements:

 std::for_each(myArray, myArray + 45, <func>);

Anyway, this is part of the reason to use vectors: .begin() and .end() reduces errors with using incorrect indexing.