Validating input with cin - c++

#include<iostream>
#include<limits>
int main()
{
int m;
std::cin >> m;
while (std::cin.fail() || m <= 0) {
std::cin.clear();
std::cin >> m;
}
return 0;
}
The above code is supposed to prompt for an input until a positive integer is entered. But even after entering a positive integer it keeps prompting for an input.
For example, lets say I have the input as
abc
c4
-3
1
2
.
.
So in principle the moment I enter 1, the execution should be transferred out of the loop. But it keeps prompting for an input and looks like an infinite loop.
After looking around for a while, I modified it as
cin >> m;
while (cin.fail() || m <= 0) {
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(),'\n');
cin >> m;
}
As desired, entering a positive number terminates the loop.
I didn't get it. Why the new line character is stuck in the input buffer?
Suppose I have this
int a , b;
cin >> a;
cin >> b;
When my input is
1
2
It correctly sets a to 1 and b to 2, instead of setting b to \n.
Why is the behaviour different in the two cases?

It's not that there's a stray newline, but that the entire input is still in the stream.
cin >> m fails immediately if it can't extract an integer.
It does not read the next "word" and see if it's an integer, and then stop after that "word" if it wasn't - the "read pointer" is left at the point of initial failure.
The ignore skips ahead to the end of the line and ignores everything up to that point.
To illustrate the difference, this program tries to read the input first as int and if that fails, reads the same input again as a std::string for use in the error message:
int main() {
int n = 0;
string s;
while (!(cin >> n))
{
cin.clear();
cin >> s;
cout << "That wasn't an int, it was " << s << endl;
}
cout << "This is an int: " << n << endl;
}
The solution you found with ignore is pretty much the "standard" solution.

The newline will be eaten up when extraction succeeds (whether the extracted integer is positive or not).
It won't be eaten up when extraction fails.

Related

While condition: How to allow comparison between non-integer/double user input and integer limits

Objective: Run a while loop that will repeat until the user inputs an integer between 1-3 (inclusive). So if an integer that is smaller than 1 or bigger than 3 is inputted or any character or string is inputted, the loop repeats.
My noob knowledge: I know how to compare user input when it is an integer, but when it is a char or string datatype input all I get is an infinite loop. Plus I declared the inputtable variable as an integer, so not sure how to go on about this.
I've done some Google searches but can't seem to find a question similar enough to mine.
Advice is highly appreciated :)
Code below:
int Decision1 = 4;
while ( Decision1 < 1 || Decision > 3)
{
std::cout << "Enter answer here: ";
std::cin >> Decision1;
std::cout << "\n\n";
}
int Decision1;
while(std::cin >> Decision1) {
if(Decision1 >= 1 && Decision1 <= 3) break;
// todo
}
You can use ASCII codes for checking number or character.
#include <iostream>
int main() {
char Decision1 = '0';
while ( (Decision1 < 49 && Decision1 >=47) || (Decision1 >51 && Decision1 <=57 ))
{
std::cout << "Enter answer here: ";
std::cin >> Decision1;
std::cout << Decision1;
std::cout << "\n";
}
}
I hope helpful for you.
Keep in mind: when you read input from std::cin you're reading text that's typed at the console. The stream extractor tries to convert that text to the target type. That is, in
int i;
std::cin >> i;
the >> operator looks for digit characters and figures out what value those characters represent. If the characters can't be converted to an integer value the extraction fails. Streams have a conversion to bool that tells you whether the stream is in a good state, or whether an attempted operation failed. So:
int i;
if (std::cin >> i)
std::cout << "got a value!\n";
To write a loop that prompts for valid input you need to reverse the test:
int i;
while (!(std::cin >> i) {
std::cout << "bad input\n":
// now clear the input stream, so it's back in a good state
}
To also check that the value is in the required range, just do it:
int i;
while (!(std::cin >> i) || i < 1 || i > 3) {
std::cout << "bad input\n":
// now clear the input stream, so it's back in a good state
}
To clear the input stream, you have to reset its internal flags. You do that with std::cin.clear();. And, depending on what you think was typed in, you probably want to get rid of any additional characters that the user typed. You do that with std::cin::ignore(std::numeric_limits<std::streamsize>::max(), '\n'). That tells the stream to discard character until it reaches a newline (press ENTER), or until it has read a huge number of characters.
Putting it all together:
int i;
while (!(std::cin >> i) || i < 1 || i > 3) {
std::cout << "bad input\n":
std::cin.clear();
std::cin::ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
Yes, this clears the stream state and flushes the input when the input was a valid integer but out of range; that's harmless. It's a waste of time, but the speed of keyboard input is limited by how fast the user types, which is much slower than anything that the code in this loop does.

C++ How do I prevent the program from endlessly repeating input when a char is received instead of an int? [duplicate]

This question already has answers here:
How do I prevent a runaway input loop when I request a number but the user enters a non-number?
(6 answers)
Closed 4 years ago.
In the below code, if anything that is not a number is entered, the program seems to continuously enter that input automatically. It translates the input to "0" since n is an int, but it does not act the same way if "0" is actually entered.
I have looked up and down and there does not seem to be a reliable way to say something like
if (n != int){cout << "invalid";}
I guess my last resort would be to just allow 0 to be a valid input, but I was hoping there's another way.
int n;
cout << " \nPlease enter a number 1 to 99,999: ";
cin >> n;
while (n < 1 || n> 99999)
{
cout << "\nThat is an invalid entry!"
<< "\nPlease enter a number 1 to 99,999: ";
cin >> n;
}
When you try to extract a value from an std::istream like std::cin and extraction fails, all data remains in the stream for the next input operation. If the same extraction is then tried again, it will again get garbage and fail. And so on.
To repeat trying to extract an int from a stream if extraction fails, you should remove all garbage before you try again:
#inlcude <limits>
#include <iostream>
// ...
int n;
while(!(std::cin >> n)) { // extraction failed
std::cin.clear(); // clear error flags
// ignore everything left in the stream up to a newline:
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
Combining that with limiting the value to a number between 1 and 99999:
#include <limits>
#include <iostream>
int main()
{
int n;
std::cout << "Please enter a number 1 to 99999: ";
while (!(std::cin >> n) || n < 1 || 99999 < n) {
std::cerr << "Input Error!\n\n";
std::cin.clear(); // clear error flags
// ignore everything left in the stream up to a newline:
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
}

How to reject char inputs in cin and define minimum and maximum int values?

I have a program which has the ability to reject user input if a char is entered instead of an int, and this works almost perfectly - anything entered that isn't a number is being rejected.
However, all of these cins need to accept any value between a minimum and a maximum, but I can't get it to work. The code below shows my efforts so far, but there's a slight bug. If a char is entered, followed by an int that is out of range, and another char is entered (I like to test rigorously - I mean, who knows what could happen if an actual end user came across the problem) the program throws the final value of mortgageTerm out as 0.
Could anyone tell me where I'm going wrong and give me any pointers to help me fix it? Thanks in advance to anyone who's able to help me solve my problem!
int mortgageTerm;
string line;
cout << "Mortgage term (1 - 40 years) : ";
while (!(cin >> mortgageTerm))
{
cout << "That's not a valid choice! Try again : ";
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
}
while (getline(cin, line))
{
stringstream linestream;
if (!linestream >> mortgageTerm)
{
cout << "Input was not a number! Try again : ";
cin >> mortgageTerm;
continue;
}
if ((mortgageTerm <= 0 || mortgageTerm > 40))
{
cout << "Input out of range. Try again : ";
cin >> mortgageTerm;
continue;
}
char errorTest;
if (linestream >> errorTest)
{
cout << "Invalid input. Try again : ";
cin >> mortgageTerm;
continue;
}
break;
}
cout << mortgageTerm;
You're almost there. Your first issue is your first while loop is not needed at all. Then we just need to tweak the second loop to make sure that all the input read was used in the value you get. We can also simplify it by using a single error statement, Making those changes gives you
int mortgageTerm;
string line;
cout << "Mortgage term (1 - 40 years) : ";
while (getline(cin, line)) // consume all input given
{
stringstream linestream(line); // you have to construct the stream from the string here
linestream >> mortgageTerm; // try and read the data
if (!linestream.eof() || mortgageTerm <= 0 || mortgageTerm > 40)
{
// either there is input left in linestream or the value is not in range
cout << "Invalid input. Try again : ";
}
}
Just check for the minimum and maximum in the same condition where you check if it was able to be converted into an int, using ||, in a condition the expressions are checked left to right in order, so the first did its work already when you evaluate the second and mortageTerm will have the value.
Edited to address comments.
int mortgageTerm;
cout << "Mortgage term (1 - 40 years) : ";
while (!(cin >> mortgageTerm) ||
mortageTerm < 1 ||
mortgageTerm > 40 )
{
cout << "That's not a valid choice! Try again : ";
cin.clear();
cin.ignore(std::numeric_limits<streamsize>::max(), '\n');
}
// If you are concerned about extra input after the number and want to clear the input stream
// cin.ignore(std::numeric_limits<streamsize>::max(), '\n');

checking C++ string for an int: revised: clearing cin [duplicate]

This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
How to validate numeric input C++
How do you do the following:
while (iNumberOfPlayers <2 || iNumberOfPlayers >5)
{
cout << "Enter number of players (1-4): ";
cin >> iNumberOfPlayers;
cin.clear();
std::string s;
cin >> s;
}
After looking at the loop I'm thrown in, it looks like cin isn't getting reset (if I put in x) cin reads X again as long as I'm in the while loop. Guessing this is a buffer issue, any way to clear it?
I then tried:
while (iNumberOfPlayers <2 || iNumberOfPlayers >5)
{
cout << "Enter number of players (1-4): ";
cin >> iNumberOfPlayers;
cin.clear();
cin.ignore();
}
which works except it reads everything 1 at a time. If I put in "xyz" then the loop goes through 3 times before it stops to ask again.
If the input is not valid, the fail bit is set on the stream. The ! operator used on a stream reads the fail bit (You could also use (cin >> a).fail() or (cin >> a), cin.fail()).
Then you just have to clear the fail bit before trying again.
while (!(cin >> a)) {
// if (cin.eof()) exit(EXIT_FAILURE);
cin.clear();
std::string dummy;
cin >> dummy; // throw away garbage.
cout << "entered value is not a number";
}
Please note that if you're reading from non-interactive input, this would become an infinite loop. So use some variation on the commented error-detection code.
The tricky thing is that you need to consume any invalid input as failure to read doesn't consume the input. The simplest solution to this is to move the call to operator >> into the loop condition and then read up to the \n if it didn't mange to read an int:
#include <iostream>
#include <limits>
int main() {
int a;
while (!(std::cin >> a) || (a < 2 || a > 5)) {
std::cout << "Not an int, or wrong size, try again" << std::endl;
std::cin.clear(); // Reset error and retry
// Eat leftovers:
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
}

while loop to infinity when the input of a cin is a 'dot'

I am having trouble using the cin method to acquire a variable. When the input is a number there is no problem, but when it is a special character like a dot [.],
the whileloop loops into infinity.
What am I doing wrong?
cout << "What is your race" <<endl<<"1.Human\n2.troll\n3.zombie"<<endl;
cin >> *race;
while(*race<1||*race>3)
{
system("cls");
cout << "Wrong choice"<<endl<< "What is your race" <<endl<<"1.Human\n2.troll\n3.zombie"<<endl;
cin >> *race;
}
I searched for the answer and i should have to flush the buffer but i don"t get how to do it. I'm rather new with c++. Thanx
Make race an char, then you will be able do to:
while (*race < '1' || *race > '3')
which is probably what you want to achieve.
Explanation:
When you cin >> into an int, it converts given ASCII string to integer value. . doesn't have an integer meaning, so it isn't read into race and failbit is set - further >>s are no-op, until you clear them. However, if you cin >> into char and compare it with other chars (well, their ASCII codes, actually), you will be able to check it without troubles.
This example exactly reproduces your problem:
#include <iostream>
int main()
{
int i = 5;
while (i < 1 || i > 3)
{
std::cin >> i;
}
}
Here's what happens: When operator>> fails to read an integer (e.g. when you type a dot and press enter), whatever you typed stays in the stream, including the newline character.
So in the next iteration of the while loop the next input is already there and since it's not a valid integer, the loop can never break.
You need to make sure that, when operator>> fails, you empty the stream and clear all the error flags that got set.
#include <iostream>
#include <limits>
int main()
{
int i = 5;
while (i < 1 || i > 3)
{
if (!(std::cin >> i))
{
// clear streams internal error flags
std::cin.clear();
// ignore what's left in the stream, up to first newline character
// or the entire content, whichever comes first
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
}
}
There are several problems with your code. The first is that you don't
verify that your input has succeeded; the correct condition for the
while should be:
while ( !cin || (*race < 1 || *race > 3) )
As written, if the input fails (which is what is happening when you
enter a '.', supposing that race has type int*), then *race
contains its previous value, whatever that was.
The second is that if you do get an error from cin, you don't clear
it. Once the stream is in an error state, it stays that way until you
explicitly clear it. If cin has failed, you need to execute:
cin.clear();
somewhere in the loop.
The third is that if cin fails, you don't extract the character which
made it failed, so that after clearing the error status, you need to
extract it. Given the way you've structured your dialog, you probably
want to ignore everything until the end of the line:
cin.ignore( INT_MAX, '\n' );
You may want to do this even if cin didn't fail, either in the loop
(if entered because of the *race < 1 || *race > 3 condition), or in
case of success. Alternatively, you may want to shift to reading lines,
and ensure that the line only contains whitespace after the character
you're interested in.
This last solution is the one I would adopt, since it handles pretty
much all of the above problems. So my code would look something like:
// return -1 on error in input,
// throw exception on (unexpected) end of file
int
getRace( std::istream& source )
{
std::string line;
if ( !std::getline( source, line ) ) {
throw std::ios_base::failure( "Unexpected end of file" );
}
std::istringstream tmp( line );
int results;
return tmp >> results >> std::ws && tmp.get() == EOF
? results
: -1;
}
// ...
int race = -1;
while ( race < 0 ) {
std::cout << "What is your race\n"
"1. Human\n"
"2. Troll\n"
"3. Zombie\n" << std::flush;
race = getRace( std::cout );
if ( race < 0 ) {
std::cout << "Wrong choice" << std::endl;
}
}
Note that by inputting through a line, you avoid any problems with
resetting format errors, skipping erroneous input or resynchronizing in
case of error.
The other solution besides the one accepted is to clear the cin's failbit and ignore the last input like below:
cout << "What is your race" <<endl<<"1.Human\n2.troll\n3.zombie"<<endl;
cin >> *race;
while(*race<1||*race>3)
{
// Clears the state of cin to be in good state
cin.clear();
// Ignores the last input read so that it's not read back again
cin.ignore();
system("cls");
cout << "Wrong choice"<<endl<< "What is your race" <<endl<<"1.Human\n2.troll\n3.zombie"<<endl;
cin >> *race;
}