C++ check cin for valid data not working

Question

I am doing an assignment for school and I am using an array filled with the ICAO words alphabet. The user enters a letter and then the program displays what ICAO word goes with the letter provided. I am using an index variable to get the ICAO word from the ICAO array. However I need to check that the user only enters a single letter to go into the char input variable. How can I do this? Below is what I have but is not working correctly. It reads the first letter and spits out the result from the first letter, then closes right away.

int main() string icao[26] = "Alpha", "Bravo", "Charlie", "Delta", "Echo", "Foxtrot", "Golf", "Hotel", "India", "Juliet", "Kilo", "Lima", "Mike", "November", "Oscar", "Papa", "Quebec", "Romeo", "Sierra", "Tango", "Uniform", "Victor", "Whiskey", "X-ray", "Yankee", "Zulu" }; int index; char i; cout << "Enter a letter from A-Z to get the ICAO word for that letter: "; while(!(cin >> i)) { cout << "Please enter a single letter from A-Z: "; cin.clear(); cin.ignore(1000,'\n'); } i = toupper(i); index = int(i)-65; cout << "The ICAO word for " << i << " is " << icao[index] << ".\n"; cin.get(); cin.get(); return 0;

}

I figured it out from a little of each of the answers. The solution is below:

int main() //store all the ICAO words in an array string icao[26] = {"Alpha", "Bravo", "Charlie", "Delta", "Echo", "Foxtrot", "Golf", "Hotel", "India", "Juliet", "Kilo", "Lima", "Mike", "November", "Oscar", "Papa", "Quebec", "Romeo", "Sierra", "Tango", "Uniform", "Victor", "Whiskey", "X-ray", "Yankee", "Zulu" }; int index; string input = ""; cout << "Enter a letter from A-Z to get the ICAO word for that letter: "; // get the input from the user cin >> input; //get the first character the user entered in case the user entered more than one character char input1 = input.at(0); //if the first character is not a letter, tell the user to enter a letter while (!isalpha(input1)) { cout << "Please enter a letter from A-Z: "; cin >> input; input1 = input.at(0); cin.clear(); } //capitalize the input to match the internal integer for the characters input1 = toupper(input1); index = int(input1)-65; cout << "The ICAO word for " << input1 << " is " << icao[index] << ".\n"; cin.get(); cin.get(); return 0;

You need to put some bounds checking in place on the index lest you try to use an index that may be out of bounds. — user195488
– user195488, Commented Apr 18, 2011 at 0:10

Captain Giraffe · Accepted Answer · 2011-04-18 00:03:56Z

Your check for

while( !cin )

checks if the stream has failed. End-Of-File or some other cause. What you want to accomplish is trickier. Maybe you could do a getline( cin, string ) to check if the user only input one character and then hit return.

string input; getline( cin, input ); if ( input.size() == 1 && *input.c_str()>='A' && *input.c_str()<='Z' )

Or something to that effect. Note that the condition states the opposite of what I think your intentions were with your while statement.

Solely checking the state of the stream in the while condition is nearly always wrong. while( !(cin >> i )) is a much better technique. It also mirrors all imperative languages that pretend files are streams.
In this case the checking of failbit|badbit is erroneous. I did not find it in the scope of the question necessary to explain the state of fail|bad|eof. And yes while( !(cin >> i )) would be the way to go if this was the case

Loki Astari · Accepted Answer · 2011-04-17 23:55:23Z

OK.

so std::cin is buffered so you probably need to type: "a<enter>" to make it work.
This works for me:

cin >> i: reads the 'a' character. cin.get(): reads the enter character. cin.get(): Waits for me to hit enter a second time before quitting.

Note: If I type "1<enter>" It works but I get a segmentation fault when trying to access the array.

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