C++: How to take user input and put it into Tic Tac Toe board

Question

I'm currently working on a tic tac toe game. I need to make a Tic Tac Toe board which updates(shows the "X" and the "O".) every time the player or the computer makes a move. So far, I've managed to make the board but I don't know how to efficiently turn the user's input into "X"s. The first thing I did was spam something like this:

if(playerChoice == 1)
block[0][0] = "X";
if(playerChoice == 2)
block[0][1] = "X";
if(playerChoice == 3)
block[0][2] = "X";
if(playerChoice == 4)
block[1][0] = "X";
if(playerChoice == 5)
block[1][1] = "X";
if(playerChoice == 6)
block[1][2] = "X";
if(playerChoice == 7)
block[2][0] = "X";
if(playerChoice == 8)
block[2][1] = "X";
if(playerChoice == 9)
block[2][2] = "X";

Although It works, it is probably the worst format possible. (Right now it effectively puts the player's choice into the correct squares.)

Here's the full code(It's not completed and might not even work.):

#include <iostream>
#include <ctime>
#include <cstdio>
#include <cstdlib>

using namespace std;

int main()
{
//random starting turn chooser
int turnFirst; //variable to decide whoever goes first
int computerRandomPick; //variable to decide which grid the computer will place "O"
srand(time(0));
turnFirst = rand()% (2 - 1 + 1)+1;//generates starting person.
computerRandomPick = rand()% (9 - 1 + 1)+1;//computer first random pick

//board arrays
string block[3][3];
block [0][0] = {" "};
block [0][1] = {" "};
block [0][2] = {" "};
block [1][0] = {" "};
block [1][1] = {" "};
block [1][2] = {" "};
block [2][0] = {" "};
block [2][1] = {" "};
block [2][2] = {" "};

//player interaction
int playerChoice;

//BEGIN OF PROGRAM
cout << "Welcome to Tic Tac Toe!" <<endl<< endl;


    if(turnFirst == 1)//player first
    {
            cout << "Please choose a grid to place (X): "<<endl<<endl;
                        cout << "       1  |  2  |  3"<< endl;
                        cout << "     -----+-----+-----" << endl;
                        cout << "       4  |  5  |  6" << endl;
                        cout << "     -----+-----+-----" << endl;
                        cout << "       7  |  8  |  9" <<  endl;
                        cout << "     -----+-----+-----" << endl<<endl;
                        cin >> playerChoice;
                        system("CLS");
    }
                if(playerChoice == 1)
                    block[0][0] = "X";
                if(playerChoice == 2)
                    block[0][1] = "X";
                if(playerChoice == 3)
                    block[0][2] = "X";
                if(playerChoice == 4)
                    block[1][0] = "X";
                if(playerChoice == 5)
                    block[1][1] = "X";
                if(playerChoice == 6)
                    block[1][2] = "X";
                if(playerChoice == 7)
                    block[2][0] = "X";
                if(playerChoice == 8)
                    block[2][1] = "X";
                if(playerChoice == 9)
                    block[2][2] = "X";



    if(turnFirst == 2)//computer first
    {
            system("CLS");
            cout << "The computer picked: " <<endl<<endl;
    }
                        if(computerRandomPick == 1)
            block[0][0] = "O";
            if(computerRandomPick == 2)
            block[0][1] = "O";
            if(computerRandomPick == 3)
            block[0][2] = "O";
            if(computerRandomPick == 4)
            block[1][0] = "O";
            if(computerRandomPick == 5)
            block[1][1] = "O";
            if(computerRandomPick == 6)
            block[1][2] = "O";
            if(computerRandomPick == 7)
            block[2][0] = "O";
            if(computerRandomPick == 8)
            block[2][1] = "O";
            if(computerRandomPick == 9)
            block[2][2] = "O";



//Game Board.


                        cout << "       " << block[0][0] << "  |  " << block [0][1] << "  |  " << block [0][2] << endl;
                        cout << "     -----+-----+-----" << endl;
                        cout << "       " << block [1][0] << "  |  " << block [1][1] << "  |  " << block [1][2] << endl;
                        cout << "     -----+-----+-----" << endl;
                        cout << "       " << block [2][0] << "  |  " << block [2][1] << "  |  " << block [2][2] << endl;
                        cout << "     -----+-----+-----" << endl;
    return 0;
}

Answer 1

Presuming block[3][3] declared as:

char block[3][3];

playerChoice could be 1-9. So, you may optimize as:

playerChoice --; // to make it 0-8
block[playerChoice / 3][playerChoice %3] = 'X'; // instead of "X"