Program outputs incorrect numbers from array - c++

Question

Jeopardy point adding code.
The program is supposed to get the number of players with a dynamic array.
From there, you can enter players names and it will insert the names into a two-dimensional array.
Then you can choose to call on a player to start adding points to.
After looping a certain amount of times and pressing 0, the while loop will cease to run and will skip down to the for loop outputting players name and then points.
Problem: If I input "1" for playerNumber, and I start adding points to index [0][1] and outputs numbers quite different from the original numbers I put in. If there are more than 2 players, 2 of the 3 players have random numbers while one remains an accurate point count.
#include <iostream>
#include <string>
using namespace std;
int main(void)
{
//GLOBAL SCOPE, all loops and other boxes can use these
//Declaration variables w cout & cin.
int playerNumber;
string playerNames;
bool flag = true;
cout << "How many players are there: ";
cin >> playerNumber;
cout << endl;
//Array Declaration.
string playerList[playerNumber][2]; //Dynamic array. changes during program runtime.
int points[playerNumber]; //Dynamic array. Changes during runtime
//GLOBAL SCOPE, all loops and other boxes can use these
//Assigning values to arrays now.
cout << "Enter the players names: " << endl;
//Assigns player name to each row.
for(int i = 0; i < playerNumber; i++){
cin >> playerNames;
playerList[i][0] = playerNames; //Assigns players name to the array
cout << "Player " << i + 1 << ": " << playerList[i][0] << endl;
}
while(flag){
//LOCAL VARIABLES
int choice = 0; //Always reverts back to zero to prevent addition error.
int pointsValue = 0; //Always reverts back to zero to prevent addition error.
//LOCAL VARIALES
cout << "Press 0 to end game, if not, enter player number: " << endl;
cin >> choice;
if(choice == 0){ //Exit out of the while loop
flag = false;
}
else if(cin.fail()){
cout << "Not a number. Try again." << endl;
cin.clear();
cin.ignore(256, '\n');
}
else if(choice < 0 || choice > playerNumber){
cout << "Choice is less than 0 or greater than player count. Try again." << endl;
}
else{
cout << "Enter points: " << endl;
cin >> pointsValue;
cout << endl;
points[choice - 1] += pointsValue; //Assigns points to points array
playerList[choice - 1][1] = (to_string(points[choice - 1])); //Assigns points to playerNumber.
}
}
cout << endl;
cout << "END OF JEOPARDY. HERE ARE THE POINTS!!!" << endl;
cout << endl;
//Current points for each player
//Shows their name and points
for(int i = 0; i < playerNumber; i++){ //Loops so that player name and points are displayed
string playerName = playerList[i][0];
string totalPoints = playerList[i][1];
cout << playerName << " points: " << totalPoints << endl;
}
return 0;
}

Answer 1

Technically, this is illegal:
string playerList[playerNumber][2];
int points[playerNumber];
The size of the array must be known at compile time (not runtime). Though some compilers allow this as an extension to the language, it is not part of the standard language.
Better choice would have been to use std::vector.
Lets assume your compiler allows this:
The next issue is that you don't initialize the values in the array.
int points[playerNumber]; // This is an array of random numbers.
// Sure if you are doing a debug build the
// compiler may be nice and just set all the
// values to zero as speed is not important
// during debugging. But on a release build
// these could be any value.
Also if we want to be technical its actually undefined behavior to read from the array unless you first write a value. Though usally this is not going to cause the program to crash on most architectures and unfortuantely the code just runs like nothing bad is happening.
You can normall initialize the array like this:
int points[10] = {0}; // but your variable size stuff
// will stop working for that.
-------
// so you will have to manually initialize the members
int points[playerNumber];
for(int loop = 0; loop < playerNumber; ++loop) {
points[loop] = 0;
}
Or if you upgrade to vector:
std::vector<int> points(playerNumber,0); // Size and initial value.
// Though you don't need the
// initial value as vector will
// zero init members.
The last issue I will mention is that you are not consistent on checking if the read worked.
cin >> pointsValue;
What do you think happens to pointsValue if the read fails (ie. the user puts in an illegal value). The proper way to check the user input is to put the read inside an if statement.
if ( std::cin >> pointValue) {
// The read worked `pointsValue` has valid user input
}
else {
// The read faild.
// We don't know what the user input should be
}
The other thing to think about is that user input is line based. They add values hit return. So it is a good tactic to exploit that and read user input by the line. Once you have the line parse that and validate input. That way the standard input stream does not go into a bad state.
std::string line;
if (std::get(std::cin, line)) {
// We have a line of user input.
std::stringstream lineStream(std::move(line));
line.clear();
int choice;
if (lineStream >> choice) {
// We have a valid choice from the user.
// or do we. If the user entered `2x` is that valid input?
// because `choice` is 2; but there is still `x` on the
// input stream.
// That's a choice for you as the developer to make.
// if you don't care, then you have valid input. If you do
// care then you need to check there is no bad data on
// the line.
}
else {
// invalid choice
}
}
else {
// The user input stream just ended.
// This could mean the user entered the end-of-stream character
// or if the user had connected some other stream to the
// standard input on the command line and there is no more
// data to read.
}
I would note that this will probably never work for invalid input.
if(choice == 0){ //Exit out of the while loop
flag = false;
}
else if(cin.fail()){
// If you failed to read data from the std::cin
// the the value of `choice` is probably zero
// (if I remember my standard correctly) so the
// first branch of the if tree will be entered
// and you will never enter this branch.
cout << "Not a number. Try again." << endl;
cin.clear();
cin.ignore(256, '\n');
}