Infinite outputs of an if-else/while loop?

Put your std::cin into the loop. You also need to set z at first and add it by 1 every time the loop runs.

int main() {
  srand (time(NULL));
  int x , y, z;
  x = rand() % 1000 + 1;
  z = 0;

  do{
 std::cin>>y;
    if (y > x){
      std::cout<<"higher"<< std::endl;

    } else if (y < x) {
      std::cout<<"lower"<< std::endl;

    } else if (y == x) {
      std::cout<<"correct"<< std::endl;
    }
  } while (y != x && z++ < 9);
  return 0;
}

1- the variable z is not initialized

2-you are supposed to read the input everytime you loop i guess ( to read the guess )? like this

do{
   std::cin>> y ;
   // the rest of your loop
while(your condition);

Note : you should initialize random seed before you use your random function :

srand (time(NULL));

You have undefined behavior:

z++;

z is used before initialization.

1. Just take input inside do-while loop.
2. Another issue initializes Z with zero. It is default initialized and can have any value.
3. Suggestion initialize random seed:

srand (time(NULL)); // it will give 'x' every time a different value.
int  z=0; // Dont forget to do this thing.

do{
    cout<<"Enter number - "; // This thing your are missing.
    std::cin>>y;
    if (y > x){
      std::cout<<"higher"<< std::endl;
      z++;
    }
    else if (y < x) {
      std::cout<<"lower"<< std::endl;
      z++;
    }
    else if (y == x) {
      std::cout<<"correct"<< std::endl;
    }
  } while (y != x && z < 11);
  return 0;

You're doing wrong a few things:

• As you must use flush operation explicitly, you must use std::cout << std::flush; somewhere.
• You need to get input from the user inside do-while loop body.
• You need to initialize random number generator first before using it.
• You need to initialize z variable to 0, because you've undefined behaviour - you're incrementing the variable which has undefined value (not initialized earlier).

Here is your code with these fixes:

#include <iostream>
#include <ctime>

int main() {
    int x, y, z = 0;
    srand(time(NULL));
    x = rand() % 1000 + 1;

    std::cout << x << std::endl;

    do {
        std::cin >> y;
        if(y > x) {
            std::cout << "higher" << std::endl;
            z++;
        } else if(y < x) {
            std::cout << "lower" << std::endl;
            z++;
        } else {
            std::cout << "correct" << std::endl;
        }
        std::cout << std::flush;
    } while(y != x && z <= 10);
    return 0;
}

You said you don't know what is a flush operation. This is the operation of empting output buffer (used by std::cout object internally). Normally you don't need to explicitly flush output buffer, but if you need so...