我的range-for循环出现逻辑错误

Question

I'm studying C++ as a beginner (I started 2 months ago) and I have a problem with my simple code. I tried to set the value of each element in this vector to 0 but I can't understand why it doesn't work :

vector<int> numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; 

for (int x : numbers) x = 0; 

I know that I may sound stupid but I am a beginner. If I try to do the same thing with a traditionally for loop it works, why ?

Answer 1

for (int& x : numbers) x = 0; 

为了改变容器元素，必须使用范围循环迭代变量的引用（默认情况下，迭代变量是当前迭代元素的副本 ）

Answer 2

It does not change the values in the array, because in each iteration the value in the array is assigned to to x and you are changing x and not the value in the array.

Basically the range based loop is similar to the following ordinary for loop:

for(int i = 0; i < numbers.length(); i++)
{
     int x = numbers[i];

     //your code
}

.

For more information check out the c++ documentation: http://en.cppreference.com/w/cpp/language/range-for . It states:

range_expression is evaluated to determine the sequence or range to iterate. Each element of the sequence, in turn, is dereferenced and assigned to the variable with the type and name given in range_declaration.

Also you will find there the example "Shmil The Cat" has posted and more examples which will help you understand how the range loop works.

Answer 3

It does not work, you don't have any reference to the elements in the array via your foreach . Think about this, you are retrieving elements in array numbers and making a copy of copy in x which does not have reference. so any changes that x undergoes does not reflect in numbers arrays.

so work how @Shmil The Cat suggested. or traditional way will also do the job like you said.

Answer 4

The items of vector are not assigned the value (zero in this case)because ranged based loops consider container as copy of original until explicitly passed the reference of original container.

Do use

for (int& x : numbers) x = 0; 

as suggested by @Shmil The Cat

Answer 5

To help you understand, here is how you can read the below:

For every element in numbers , copy data into x and set x to 0. // Now this doesn't touch the value in the vector.

for (int x : numbers) 
    x = 0; 

Reading the below loop:
For every element in numbers , make a reference to it, and change the value through the reference to 0. This on the other hand, changes the values in the vector.

for (int &x : numbers)
    x = 0;

Answer 6

如果您只想为所有元素设置特定值，最好使用std::fill

std::fill(numbers.begin(), numbers.end(), 0);