c ++中的代码是什么意思？

Question

I do not understand what is going on on variable k. For example I tried to put 1, 2, 3, 4, 5 but k shows me 1.

    int a[5];
    for (int i = 0; i < 5; i++) {
        cin >> a[i];
    }

    int k = 0;
    for(int j = 0; j < 5; j++) {
        k += a[j] > a[j+1];
    }
    cout << k;

Answer 1

k shows a value of one because you are accessing out of bounds array a.

When j = 4 , j+1 is 5 and so you are trying to access a[5] which is out of bound. Hence it is incorrectly showing that a[j] > a[j+1] for one value. This is undefined behaviour.

Change your code to:

for(int j = 0; j < 4; j++) {
  k += a[j] > a[j+1];
}

Now k will have a value of 0 if the input series is 1, 2, 3, 4 and 5.

Answer 2

a[j] > a[j+1] produces a Boolean result ( false or true ). In an int context, true and false convert to 1 and 0 respectively.

So, this is roughly equivalent to:

for (int j=0; j<5; j++)
    if (a[j] > a[j+1])
        ++k;

Answer 3

The loop iterates over the array a , comparing a[0] > a[1] , a[1] > a[2] , a[2] > a[3] etc.. When a boolean is added to an integer, it's first converted to 0 (if false) or 1 (if true). So, k ends up being a count of the number of times an element if greater than the following element.