I just want to ask what's the meaning of p[x] below in this C++ syntax.

Question

int main() {
int p[ ]={5,4,3,21,1};
int s = 0;
for (int x = 0; x < 5; x++){s+=p[x];}
cout << s;
return 0; }

Answer 1

You probably understand that p[ ] is an array that holds five numbers. To access one of the numbers in p, you use an index value, like
p[0] to get 5
p[1] to get 4
p[2] to get 3

The code above replaces the index value with a variable, x. Whatever value is stored in x becomes the index.
If x=0, p[x] means p[0], 5
If x=1, p[x] means p[1], 4
If x=2, p[x] means p[2], 3
If x=3, p[x] means p[3], 21
If x=4, p[x] means p[4], 1

The for() loop starts with x=0, and tests whether x<5. If true, then it executes the statement inside the braces. Afterward it increments x by 1 and tests whether x<5. If x is still less than 5, it repeats the statement. This continues as x goes through values 0, 1, 2, 3, and 4, executing the statement s+=p[x]; each time. Now after the loop exits, s holds the sum of all the values in p[ ].

Answer 2

It's inside a for loop, so you are iterating the items of the array p and accumulating it on s.
s+=p[0] // s=5
s+=p[1] // s=5+4=9
s+=p[2] // s =9+3=12
s+=p[3] // s=12+21=33
s+=p[4] // s=33+1=34

Answer 3

Very thanks for helping me out with it.