Looping if user input invalid - c++

Question

I want to create a program that when a user inputs something that I didn't define, the program prompts him again.
I did it with if statements but it only loops for 1 time and doesn't do it again. I tried loops but whenever the input is false it just breaks the condition and refuses all inputs alike. In c++.
Any help is much appreciated.
#include <iostream>
#include <string>
using namespace std;
void xD(){string x;
do{cout << "Retry\n";
cin >> x;}while(true);}
//declaring a function to make the shop
void shop(){
string x;
float coins = 500;
float bow_cost = 200;
cout << "welcome to the shop\n";
cout << "Bow(bow)costs 150 coins.\n";
cin >> x;
// if u chose bow you get this and get to choose again
if (x == "bow"){
cout << "you bought the bow.\n you now have " <<coins - bow_cost << " coins." << endl; cin >> x;}
/*now the problem that whenever I excute the code and type something other than bow it gives me the cin only once more and then fails even if I type bow in the 2nd attempt*/
//in my desperate 5k attempt, I tried creating a function for it.. no use.
//i want it o keep prompting me for input till i type "bow" and the other block excutes. but it never happens.
else{xD();}
}
int main(){
string name;
string i;
cout << "if you wish to visit the shop type \"shop\"\n";
cin >> i;
if(i == "shop"){shop();}
else{cin >> i;}
return 0;
}

Answer 1

The problem lies on the condition in this loop block
void xD(){
string x;
do{
cout << "Retry\n";
cin >> x;
}while(true);
}
The while(true) condition makes it loops forever regardless of the input. To fix this, you can change the condition:
void xD(){
string x;
do{
cout << "Retry\n";
cin >> x;
}while(x!="bow");
cout << "you bought the bow. and some other messages"<<endl;
}
That should work. However, it is still too complicated for me. This can be simplified into the snippet below:
void shop(){
string x;
float coins = 500;
float bow_cost = 200;
cout << "welcome to the shop\n";
cout << "Bow(bow)costs 150 coins.\n";
cin >> x;
while (x!="bow"){
cout << "Retry\n";
cin>>x;
}
cout << "you bought the bow.\n you now have " <<coins - bow_cost << " coins." << endl; cin >> x;
}

Answer 2

Instead of doing this approach (which is checking the condition only once):
if (x == "bow"){
cout << "you bought the bow.\n you now have " <<coins - bow_cost << "
coins." << endl; cin >> x;
} else{
xD();
}
which is actually a RECURSIVE invocation to the method xD()
you should do a do-while loop,
example:
while (x.compare("bow") != 0)
{
cout << "sorry, wrong input, try again...";
cin >> x;
}
note the use of the compare method instead of the == operator
here more about it in the documentation

Answer 3

You can use return value of cin >> [your input object] here to check status or istream's method fail(). As soon as input stream fails to parse whole or part of streams it fails and stay in state of failure until you clear it. Unparsed input is preserved (so you can try to parse it differently?)m so if you try to >> again to object of same type, you'll get same failure. To ignore N chars of imput, there is method
istream::ignore(streamsize amount, int delim = EOF)
Example:
int getInt()
{
while (1) // Loop until user enters a valid input
{
std::cout << "Enter an int value: ";
long long x; // if we'll use char, cin would assume it is character
// other integral types are fine
std::cin >> x;
// if (! (std::cin >> x))
if (std::cin.fail()) // has a previous extraction failed?
{
// yep, so let's handle the failure, or next >> will try parse same input
std::cout << "Invalid input from user.\n";
std::cin.clear(); // put us back in 'normal' operation mode
std::cin.ignore(std::numeric_limits<std::streamsize>::max(),'\n'); // and remove the bad input
}
// Thechnically you may do only the above part, but then you can't distingusih invalid format from out of range
else if(( x > std::numeric_limits<int>::max()) ||
( x < std::numeric_limits<int>::min()))
{
std::cout << "Invalid value.\n";
}
else // nope, so return our good x
return x;
}
}
For strings parsing is almost always successful but you'll need some mechanism of comparison of string you have and one that is allowed. Try look for use of std::find() and some container that would contain allowed options, e.g. in form of pair<int,string>, and use int index in switch() statement (or use find_if and switch() within the function you give to it).

Answer 4

Consider that if() statement is a one_direction road, it checks the condition and if the condition was satisfied it goes to its bracket and do blah blah blah , if there is any problem with condition compiler passes ifand jump to compile other codes.
Every time that you begin to compile the codes it begins from int main() function. You did the wrong thing in the if and else statements again
Here is the correct code .I did the necessary changes.
#include "stdafx.h"
#include <iostream>
#include <string>
using std::string;
using std::cin;
using std::cout;
#define coins 500 ;
#define bow_cost 200 ;
int shop(string x)
{
//There is no need to allocate extra memory for 500 and 200 while they are constant.``
cout << "welcome to the shop\n";
cout << "Bow(bow)costs 150 coins.\n";
do
{
cout << "Input another :\n";
cin >> x;
if (x == "bow")
{
return (coins - bow_cost); //return to function as integer
}
} while (true);
}
int main()
{
string name, i;
cout << "if you wish to visit the shop type \"shop\"\n";
cin >> i;
if (i == "shop")
{
cout << "Input :\n";
cin >> name;
cout << shop(name) << "you bought the bow.\n you now have " << " coins." << "\n";
}
//argument passed to shop funnction parameters.
system("pause");
return 0;
}