While loop carries on forever (C++) - c++

I'm still quite new to c++ and just experimenting with the language.
I have recently created a 'tictactoe' game.
I have created this simple looking function to get the users board position (from 1 to 9). It seems to work fine but I found a weird bug so to call it;
Whenever I enter a 12 digit number or higher the loop just carries on forever printing 'Invalid position!' and on to the next line 'Choose your position 1-9: '.
I am using visual studio to write code. Is it something with the code or is it perfectly fine? I'm eager to find out to learn from it.
Here's the function:
int getUserPosition()
{
int position;
while (true)
{
cout << " Choose your position 1-9: " << endl;
cin >> position;
if (position < 1 or position > 9)
{
cout << "Invalid position!" << endl;
}
else if (board[position - 1] == 'X')
{
cout << "This position has been taken!" << endl;
}
else
{
return position;
break;
}
}
}

I finally understand the actual behavior here. This is not invoking undefined behavior but rather defined behavior you don't want.
cin >> position;
This tried to read your 12 digit number, which didn't fit into an int, so the read failed. Because it failed on a format error, the 12 digit number remained in the buffer, so the next pass around the loop tried to read it again.
Always use cin::getline() when you intend to read keyboard input. The error/cleanup logic in cin is not designed to resync keyboard input, but rather to resync input piped from a generator that can't read your error messages. In the future, when you try to use cin with a pipe, the best solution is to check cin::fail() and bail the program if it ever gets set.

Related

C++ - Why does my program close after calling a function again?

I'm new to C++ and can't figure out what's going on here...
Basically I have a function with a bunch of Log("...") statements (text-based adventure)
and then I want to basically create a try-catch block to see if the user tries to print anything that isn't 1 or 2 as those are the only 2 choices. I noticed when you input strings into the console it converts it to 0 so basically my code is-
void myfunciton()
{
top:
Log("Choose...");
Log("(1) ...");
Log("(2) ...");
std::cout << std::string(11, '\n') << std:endl;
std::cout << "Enter a number: ";
std::cin >> userchoice; // Userchoice is a global variable defined above and set to 100.
if (userchoice == 0) {
userchoice = 100;
goto top;
}
Instead of the line goto top; I also tried just calling myFunction(); but it gives me the same outcome... The code runs again and I see my story, but I can't enter another number. It prints out the line "Enter a number: " inside my console, but than immediately prints "exited with code 0 press any key to close..."
I just don't understand why it isn't letting me input another number again. Any help would be appreciated :)
Because the numerical extractor leaves non-digits behind in the input buffer.
Try this:
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
and avoid using goto-s.
Also you can use:while (std::cin >> x) (x is for your input) for better flow of code.

Setting up a loop waiting for user input (C++)

I'm trying to get a simple tic-tac-toe program to function in the console as a way to make sure I understand loops and arrays.
It compiles, and runs as expected, with the exception that if a user inputs something that isn't a number the program races through the first if statement infinitely without a chance to add a new input. I really can't see how to fix this.
I think the issue is that chosenSquare is an integer as it needs to be compared to values, but cin can take anything in. Expected behaviour would be to check if the input is an integer between 0 and 8 (the 9 spaces on the board), and if not return a message and repeat, waiting for a new input.
Is there a simple fix for this? I'm trying to avoid specialist packages and namespaces for now while I grok the basics. I've looked at similar problems but don't follow them.
Thanks.
Code snippet:
// Input loop
bool valid = false;
while (valid != true)
{
int chosenSquare = 0;
cout << "Player " << currentPlayer << ", enter a number between 1 and 9:" << endl;
cin >> chosenSquare;
chosenSquare--; // For array indexing
if ((chosenSquare < 0) || (chosenSquare > 8)) // <--- PROBLEM IS THIS LOOP
{
cout << "Invalid input. Try again." << endl;
continue;
}
else if ((board[chosenSquare] == currentPlayer) || (board[chosenSquare] == lastPlayer))
{
cout << "Square not availible. Try again." << endl;
continue;
}
else
{
board[chosenSquare] = currentPlayer;
valid = true;
break;
}
}
There are a couple of things culminating causing this.
The first is that when an alpha character is put into the console, the error bit is set, and 0 is written to the variable you're writing to:
The behavior you want to observe changed in 2011. Until then:
If extraction fails (e.g. if a letter was entered where a digit is expected), value is left unmodified and failbit is set.
But since C++11:
If extraction fails, zero is written to value and failbit is set. [...]
(From cppr.)
That means chosenSquare is 0 after the read, so chosenSquare-- makes it -1. -1, as you know, is less than 0, so the first if-statement is true.
As to why it stays this way infinitely, you need to clear the fail-bit.
So I changed the first loop after a bit of reading and experiementation:
if (cin.fail()) // <--- PROBLEM IS THIS LOOP
{
cout << "Invalid input. Try again." << endl;
cin.clear();
cin.ignore();
continue;
}
This works, but I can't figure out what it's actually doing.
Could anyone elaborate?

Understanding why unwanted iterations of a cout statement occur [duplicate]

This question already has answers here:
Why does std::getline() skip input after a formatted extraction?
(5 answers)
Closed 3 years ago.
I am working with some C++ code. I have a while-loop set up to allow me to run through some code x-number of times. The while loop terminates once the user indicates that they do not want to run through the code again.
#include <iostream>
#include <string>
using namespace std;
char request;
int main() {
while (request != 'N')
{
string getCode = "";
while (getCode.length() != 3)
{
cout << "Please enter your container's region code (A or B followed by two-number identification)" << endl;
getline(cin, getCode);
if (getCode.length() != 3)
{
cout << "Error" << endl;
}
}
//clear the screen
system("cls");
//get letter
if (getCode.at(0) == 'A' || getCode.at(0) == 'B')
{
if ((getCode.at(1) >= '0' && getCode.at(1) <= '9') && (getCode.at(2) >= '0' && getCode.at(2) <= '9'))
{
if (getCode.at(0) == 'A')
{
cout << "The shipping charge is $25" << endl;
}
else if (getCode.at(0) == 'B')
{
cout << "The shipping charge is $30" << endl;
}
}
else
{
cout << "Error" << endl;
}
}
else
{
cout << "Error...Please enter the code as A or B followed by two numbers" << endl;
}
//Again?
cout << "Would you like to enter in another shipping identification number?" << endl;
cin >> request;
}
cout << "Thank you" << endl;
//End Program
system("pause");
return 0;
}
When I indicated that yes (entering 'Y' to the 'Would you like to enter in another shipping identification number question') I would like to run through the code again, the program outputs an unwanted 'Please enter your container's region code (A or B followed by two-number identification' and 'error' statement. Also please note, the code is inside 'int main()' and that I have properly formatted my 'include' statements.
Your question is to understand why this is happening, so here's the explanation. The code you wrote states thusly:
string getCode = "";
while (getCode.length() != 3)
{
cout << "Please enter your container's region code...
As you see, getCode is always initialized to an empty string. Immediately afterwards, if its length is not 3, this question is outputted.
You need to understand that your computer will always do exactly what you tell it to do. Your computer will not do what you want it to do, but only what you tell it to do. The above is what you told your computer to do, and your computer will always obediently follow its strict instructions, every time it runs this code. That's pretty much the explanation, and there's nothing more to understand.
This section of code is inside another loop, and you indicated that you do not wish the prompt to appear on second and subsequent iteration of the loop, only on the initial one.
However, there's nothing in your instructions to your computer, above, that specify this. You didn't tell your computer that this is what it should do, so why do you expect your computer to do that, entirely on its own? Every time your computer executes these statements shown above, this is exactly what will happen. Nothing more, nothing less. Whether it's the first time inside the outer while loop, or on each subsequent time the while loop iterates, it doesn't matter. The code always does exactly the same thing: getCode gets created and set to an empty string, and because its length is not 3, the inner while loop runs, prints the prompt and calls std::getline to read a line of text from std::cin. At the end of your while loop, if your instructions to your computer indicate that it should run the code in the while loop again, from the beginning (because that's what the while loop does), then the above instructions get executed.
If you now understand why your computer does this (because that's what you told it to do), then you should easily figure out what to tell your computer so it doesn't do this. If you want your computer to print the prompt only the first time it executes the while loop, then this is exactly what you need to tell your computer: set a flag before the while loop, print the prompt only if the flag is set (with all other existing logic remaining the same), and then clear this flag afterwards, so the next time the while loop runs, your computer will do exactly what you told it to do, and not print the prompt.
when I indicate 'Y' to the prompt 'Would you like to enter in another shipping identification number?', it outputs the following: 'Please enter your container's region code (A or B followed by two-number identification)' 'error' 'Please enter your container's region code (A or B followed by two-number identification' . When I input 'Y' I only want it to output 'Please enter your container's region code (A or B followed by two-number identification)'...I only want it to output once
Now that I understand your question, what's happening is an newline (\n) is getting added to the std::cin buffer at these lines right here:
//Again?
cout << "Would you like to enter in another shipping identification number?" << endl;
cin >> request;
This makes even more sense especially when combined with your other comment:
Before int main() there should be a 'char request;
So request a single char. That means when you type something like this:
Y
The newline is added to std::cin as well. That can't be stored in a single char, and the >> may not remove it either. That means it's just sitting here.
What this does is when you get to your if statement at the beginning of the loop again:
while (request != 'N')
{
string getCode = "";
while (getCode.length() != 3)
{
cout << "Please enter your container's region code (A or B followed by two-number identification)" << endl;
getline(cin, getCode);
if (getCode.length() != 3)
{
cout << "Error" << endl;
}
}
getline() sees the newline you added previously and instantly returns an empty string. Empty strings have a length of 0, so it fails your if statement, which prints the error.
The solution is simple, just tell std::cin to ignore the newline:
//Again?
cout << "Would you like to enter in another shipping identification number?" << endl;
cin >> request;
cin.ignore(1, '\n');

Why is this code exiting at this point?

I'm new to C++. I stumbled upon one tutorial problem, and I thought I'd use the few things I have learnt to solve it. I have written the code to an extent but the code exits at a point, and I really can't figure out why. I do not want to go into details about the tutorial question because I actually wish to continue with it based on how I understood it from the start, and I know prospective answerers might want to change that. The code is explanatory, I have just written few lines.
Here comes the code:
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
double average_each_student() {
cout << "\n>Enter your scores seperated by spaces and terminated with a period\n"
<< ">Note: scores after total number of six will be truncated.\n"
<< endl;
double sum = 0, average = 0;
int user_input, counter = 0;
const double no_of_exams = 6;
while(cin >> user_input) {
++counter;
if(counter < 5) sum += 0.15 * user_input;
else if(counter > 4 && counter < 7) sum += 0.20 * user_input;
}
return sum / no_of_exams;
}
int reg_number() {
cout << "Enter your registration number: " << endl;
int reg_numb;
cin >> reg_numb;
return reg_numb;
}
int main() {
vector<int> reg_num_list;
vector<double> student_average;
reg_num_list.push_back(reg_number());
student_average.push_back(average_each_student());
string answer;
cout << "\n\nIs that all??" << endl;
//everything ends at this point.
//process returns 0
cin >> answer;
cout << answer;
}
The code exits at cout << "\n\nIs that all??" << endl;. The rest part after that is not what I intend doing, but I'm just using that part to understand what's happening around there.
PS: I know there are other ways to improve the whole thing, but I'm writing the code based on my present knowledge and I wish to maintain the idea I'm currently implementing. I would appreciate if that doesn't change for now. I only need to know what I'm not doing right that is making the code end at that point.
The loop inside average_each_student() runs until further input for data fails and std::cin gets into failure state (i.e., it gets std::ios_base::failbit set).
As a result, input in main() immediately fails and the output of what was input just prints the unchanged string. That is, your perception of the program existing prior to the input is actually wrong: it just doesn't wait for input on a stream in fail state. Since your output doesn't add anything recognizable the output appears to do nothing although it actually prints an empty string. You can easily verify this claim by adding something, e.g.
std::cout << "read '" << answer << "'\n";
Whether it is possible to recover from the fail state on the input stream depends on how it failed. If you enter number until you indicate stream termination (using Ctrl-D or Ctrl-Z on the terminal depending on what kind of system you are using), there isn't any way to recover. If you terminate the input entering a non-number, you can use
std::cin.clear();
To clear the stream's failure stated. You might want to ignore entered characters using
std::cin.ignore(); // ignore the next character
or
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
// ignore everything up to the end of the line
use cin.clear(); before cin >> answer; That will fix the problem. But you are not controlling the input. it just runs out to cin..

Stopping the user from entering letters? C++

Hi i'm newish to C++ but i have a little problem which is i have to stop the user entering letters in a number section. I have made an attempt which works but its dodgy, because it will allow the user to continue then will tell them they have got something wrong and to restart the application. How do i validate it to bring up an error message telling them thats not a number and let them re enter a number?
Here is the code:
double Rheight;
do
{
cout << "Enter height of the room. " << endl;
SetConsoleTextAttribute(GetStdHandle(STD_OUTPUT_HANDLE), 4);
cout << "WARNING: If you enter a letter the program will exit." << endl;
SetConsoleTextAttribute(GetStdHandle(STD_OUTPUT_HANDLE), 7);
cin >> Rheight;
}
while (Rheight > 20 || Rheight == 0);
Ask if you need to see more code.
There are basically two components to the answer:
Detecting that the input failed.
Cleaning up after a failed input.
The first part is rather trivial: you should always test after input that the stream is in a good state before using the input. For example:
if (std::cin >> value) {
// use value
}
else {
// deal with the input error
}
How to deal with the input error depends on your needs. When reading a file you'd probably abort reading the entire file. When reading from standard input you can ignore just the next character, the entire line, etc. Most like you'd want to ignore the entire line. Before doing so you'll need to put the stream back into a good state:
std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
The first line clears the stream's error flags and the second line is a magic incantation ignoring as many characters as necessary until a newline got ignored.
To check if the input was valid you can use
if(!(cin >> Rheight))
{
cout << "Please input a valid number!" << endl;
continue;
}