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Checking cin input stream produces an integer
(8 answers)
Closed 4 years ago.
I want to obtain an input from the user and validate it as an integer, however my method seems to fail, when a letter or word the program just ends. Any one have a quick and easy fix for this?
int getUserInput(){
int maxNumber;
// the user input a valid integer, process it
if (cin >> maxNumber)
{
if (maxNumber>1 || maxNumber<=100) {
return maxNumber;
}
} else{
getUserInput();
}
}
If the user does not type in an integer, your code enters an infinite recursive loop that it does not recover from, eventually overflowing the call stack.
Try something more like this instead:
int getUserInput()
{
int number;
cout << "Enter a number between 1 - 100: ";
do
{
// if the user input a valid integer, process it
if (cin >> number)
{
if (number >= 1 && number <= 100)
break;
cout << "Number out of range, try again: ";
}
else
{
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
cout << "Invalid input, try again: ";
}
}
while (true);
return number;
}
You didn't state what range you wanted the input to be in so adjust accordingly.
int getUserInput() {
int maxNumber;
while (!(cin >> maxNumber) || maxNumber <= 1 || 100 < maxNumber) {
if (cin)
cout << "number out of range, try again: ";
else {
cin.clear();
cin.ignore(numeric_limits<int>::max(), '\n');
cout << "Not a number, please try again : ";
}
}
return maxNumber;
}
Related
I found this similar question being asked so many times but I still couldn't find a solution for mine.
In my case, I want to display something when the user enters a number from 1 - 5, give an error when he inputs something wrong like characters, "3g", "3.", "b3" and any float number.
I tried the code below, but it created so many other problems. Like if I enter 3g or 3.5, it'll only take the 3 and ignore the rest so the (!cin) doesn't work at all.
Second, if I input something like a character, the __userChoice will be automatically converted into 0 and the program prints out "Please select a number from 1 to 5." instead of "Invalid input, please input an integer number.\n", which is what I want.
cout << "Please select: ";
cin >> __userChoice;
if (__userChoice > 0 && __userChoice < 5) {
cout << "You select menu item " << __userChoice <<". Processing... Done!\n";
}
else if (__userChoice == 5) {
Finalization(); //call exit
}
else if (__userChoice <= 0 || __userChoice > 5) {
cout << "Please select a number from 1 to 5.\n";
}
else (!cin) {
cout << "Invalid input, please input an integer number.\n";
}
cin.clear();
cin.ignore(10000, '\n');
operator>> is not guaranteed to output a meaningful integer value if a failure occurs, but you are not checking for failure before evaluating __userChoice, and the way your ifs are structured the else (!cin) check will never be reached. But even if operator>> is successful, you are not checking if the user entered more than just an integer.
To do what you are asking for, you should read from std::cin into a std::string first using std::getline(), and then use std::istringstream or std:stoi() (or equivilent) to convert the string to an int with error checking.
For example:
bool strToInt(const std::string &s, int &value)
{
std::istringstream iss(s);
return (iss >> value) && iss.eof();
// Or:
std::size_t pos;
try {
value = std::stoi(input, &pos);
}
catch (const std::exception &) {
return false;
}
return (pos == input.size());
}
...
std::string input;
int userChoice;
std::cout << "Please select: ";
std::getline(std::cin, input);
if (strToInt(input, userChoice))
{
if (userChoice > 0 && userChoice < 5)
{
std::cout << "You selected menu item " << userChoice <<". Processing... Done!\n";
}
else if (userChoice == 5)
{
Finalization(); //call exit
}
else
{
std::cout << "Please select a number from 1 to 5.\n";
}
}
else
{
std::cout << "Invalid input, please input an integer number.\n";
}
I have this snippets of code from my original long program, and as much as it looks simple, it doesn't work correctly! I am brand-new to c++ language, but I know in Java that would be the way to do it (Regardless of the syntax).
Simply put, this should ask the user for an input to answer the following multiplication (5*5), however, it should also check if the user entered a wrong input (not number), keep asking the user again and again... Somehow, it keeps running forever without taking a new input!!
I hope to get, not only an answer, but also a reason for such an error!
int main() {
int userAnswer;
bool isValidAnswer = true;
cout << 5 << " * " << 5 << " = ";
cin >> userAnswer;
cin.ignore();
do {
if (cin.fail()) { //user input is not an integer
cout << "Your answer is not valid! Please enter only a natural number: ";
cin >> userAnswer;
cin.ignore();
} else {
isValidAnswer = false;
}
} while (isValidAnswer);
return 0;
}
Well you need to clear the error state before accepting new input. Call cin.clear() then cin.ignore() before trying to read input again.
I would do something like.
cout << "Enter a number: ";
cin >> number;
while(cin.fail())
{
cin.clear();
cin.ignore(1000, '\n'); //some large number of character will stop at new line
cout << "Bad Number Try Again: ";
cin >> number;
}
First, cin.fail() is not going to adequately check if your answer is a natural number or not with the type set to int (could also be negative).
Second, your boolean isValidAnswer is really checking if it's is an invalid answer.
Third (and most importantly), as another answer suggests, you should put in cin.clear() to clear the failure state, and then followed by cin.ignore(), which will remove the failed string from cin.
Fourth, cin will only check if an int exists somewhere in the string. You'll need to perform your own string comparison to determine if the entire input is a int (see answer below, based on this answer).
Updated:
#include <iostream>
#include <string>
#include <cstdlib>
using namespace std;
bool isNum(string line)
{
char* p;
strtol(line.c_str(), &p, 10);
return *p == 0;
}
int main() {
int userAnswer;
string input;
bool isInvalidAnswer = true;
cout << 5 << " * " << 5 << " = ";
while (isInvalidAnswer) {
if (!(cin >> input) || !isNum(input)) {
cout << "Answer is not a number! Please try again:\n";
cin.clear();
cin.ignore();
}
else {
userAnswer = atoi(input.c_str());
if (userAnswer < 0) { //user input is not an integer
cout << "Answer is not a natural number! Please try again:\n";
} else {
isInvalidAnswer = false;
}
}
}
cout << "Question answered!\n";
return 0;
}
I am just a newbie to programming and I was trying to write a while loop that runs as long as the input (num) is not a integer which doesn't ends with zero. What happen is when I enter a number that ends with zero, the program runs the loop correctly, but when i enter something nonsense such as rofl the program only print The input is not valid. and won't repeat the loop. I have tried to look for solutions but I am still stuck after a hour. Anyone can help me here? Thx so much!
void rev_sum() {
int num;
int a = 1;
while (a < 2) {
cout << "Please input a natural number without zero at the end:\n";
cin >> num;
if (!cin) {
cout << "The input is not valid.\n";
cin.clear();
cin.ignore(INT_MAX);
}
if (num % 10 == 0) {
cout << "The number cannot have zero at the end\n";
} else {
cout << "gj\n";
break;
}
}
}
Try replacing
cin.ignore(INT_MAX);
With
cin.ignore(numeric_limits<streamsize>::max(), '\n');
And change the
if (num % 10 == 0)
To
else if (num % 10 == 0)
Your final code should look like this:
void rev_sum() {
int num;
int a = 1;
while (a < 2) {
cout << "Please input a natural number without zero at the end:\n";
cin >> num;
if (!cin) {
cout << "The input is not valid.\n";
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
}
else if (num % 10 == 0) {
cout << "The number cannot have zero at the end\n";
} else {
cout << "gj\n";
break;
}
}
}
if (num % 10 == 0) {
could be
else if (num % 10 == 0) {
otherwise the other else case might get executed
You set a = 1 at the start of the loop and then never change a which means the only way to leave your while loop is if you hit the break statement. If your loop is not looping then it must be stuck somewhere. I'm unfamiliar with the statements if (!cin) and cin.ignore(...) so those are top suspects to check (or change). The statement cin >> num; completes no matter what they type so you can check to see what 'num' is equal to when you enter "rofl". Then after it fails, you still use num so you are processing this unintentional entry. You can add continue; after cin.ignore(...) to jump back to the top of the while loop and ask the question again. You can also print something after the while loop in order to know when you get out.
With all that said, I would never trust the user to enter acceptable information and I would never trust cin to process it for me. Personally, I would read cin as a string using cin.getline(buffer, buffer_size); And then I would complain to the user if they filled the buffer or gave me something that was not an integer (which you can check with a function like scanf()). Then you can spit back exactly what they gave you and you can be specific about your complaint.
Would it not make more sense to only check that the number ends in 0 if the input was valid....
void rev_sum() {
int num;
int a = 1;
while (a < 2) {
cout << "Please input a natural number without zero at the end:\n";
cin >> num;
if (!cin) {
cout << "The input is not valid.\n";
cin.clear();
cin.ignore(INT_MAX);
} else {
if (num % 10 == 0) {
cout << "The number cannot have zero at the end\n";
} else {
cout << "gj\n";
break;
}
}
}
}
cin.ignore(numeric_limits<streamsize>::max())
I am trying to make a cin where the user can only enter 0 to 1. If the user doesnt enter those numbers then he should get an error saying "Please enter within the range of 0 to 1."
But its not working.
What am i doing wrong?
int alphaval = -1;
do
{
std::cout << "Enter Alpha between [0, 1]: ";
while (!(std::cin >> alphaval)) // while the input is invalid
{
std::cin.clear(); // clear the fail bit
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // ignore the invalid entry
std::cout << "Invalid Entry! Please Enter a valid value: ";
}
}
while (0 > alphaval || 1 < alphaval);
Alpha = alphaval;
Try this:
int alphaval;
cout << "Enter a number between 0 and 1: ";
cin >> alphaval;
while (alphaval < 0 || alphaval > 1)
{
cout << "Invalid entry! Please enter a valid value: ";
cin >> alphaval;
}
If you want to trap empty lines I'd use std::getline and then parse the string to see if the input is valid.
Something like this:
#include <iostream>
#include <sstream>
#include <string>
int main()
{
int alphaval = -1;
for(;;)
{
std::cout << "Enter Alpha between [0, 1]: ";
std::string line;
std::getline(std::cin, line);
if(!line.empty())
{
std::stringstream s(line);
//If an int was parsed, the stream is now empty, and it fits the range break out of the loop.
if(s >> alphaval && s.eof() && (alphaval >= 0 && alphaval <= 1))
{
break;
}
}
std::cout << "Invalid Entry!\n";
}
std::cout << "Alpha = " << alphaval << "\n";
return 0;
}
If you want a different prompt on error then I'd put the initial prompt outside the loop and change the inner prompt to what you prefer.
Week one of C++, starting with Peggy Fisher's Learning C++ on Lynda.com.
This is what I came up with. Love to receive feedback.
int GetIntFromRange(int lower, int upper){
//variable that we'll assign input to
int input;
//clear any previous inputs so that we don't take anything from previous lines
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
//First error catch. If it's not an integer, don't even let it get to bounds control
while(!(cin>>input)) {
cout << "Wrong Input Type. Please try again.\n";
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
}
//Bounds control
while(input < lower || input > upper) {
cout << "Out of Range. Re-enter option: ";
cin.ignore(numeric_limits<streamsize>::max(), '\n');
//Second error catch. If out of range integer was entered, and then a non-integer this second one shall catch it
while(!(cin>>input)) {
cout << "Wrong Input Type. Please try again.\n";
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
}
}
//return the cin input
return input;
}
As the exercise was to order Hamburgers, this is how I ask for the amount:
int main(){
amount=GetIntFromRange(0,20);
}
I'm basically expecting a number as input. The magnitude is negligible now as I know my else if loop works fine. But testing if its a number proves to be a bit trickier. I just want to call the function again and start over if the user enters in something alphanumeric or just plain words. Or pressed enter. Something that is not a number. I tried !cin since I am inputting into int numTemp, but that just results in an infinite loop that spills out "what is the bitrate" countless times. Anyone know what I'm doing wrong? I tried putting cin.clear() and cin.ignore(100, "\n") inside the first if statement but to no avail. Thanks in advance.
bool iTunes::setBitRate()
{
cout << "What is the bitrate? ";
int numTemp;
cin >> numTemp;
if (!cin)
{
cout << "WRONG" << endl;
setBitRate();
}
else if( numTemp < MIN_BITRATE || numTemp > MAX_BITRATE)
{
cout << "Bit Rate out of range" << endl;
setBitRate();
}
else
{
bitRate = numTemp;
}
}
You can just read a string from the user instead of an int, and then check it and prompt for new input if you don't like the string (e.g. if it doesn't cleanly convert to a number, which you can check with strtol).
If you want to check whether the input is a number or character, you can use isdigit, but you have to pass it a char and then when it is a digit you can convert it to a int with atoi.
When the statement cin >> numTemp fails due to non-numeric input the character causing the failure is NOT removed from the input stream. So the next time the stream extraction operator is called it will see the same non-numeric input as the last time. To avoid this you need to skip the existing input.
One way of doing this is to use getline() to read a complete line of text before trying to converting it to and integer. The folllowing code snippet illustrates this:
#include <cstdlib>
bool getint(istream& in, int & out) {
string line;
getline(in, line);
char* endptr;
out = strtol(line.c_str(), &endptr, 10);
return endptr!=line.c_str();
}
bool iTunes::setBitRate()
{
cout << "What is the bitrate? ";
int numTemp;
if ( !getint(cin, numTemp) && cin )
{
cout << "WRONG" << endl;
setBitRate();
}
else if( numTemp < MIN_BITRATE || numTemp > MAX_BITRATE)
{
cout << "Bit Rate out of range" << endl;
setBitRate();
}
else
{
bitRate = numTemp;
}
}
NOTE: You should also check the status of cin after each read to ensure that some error has not occurred.
i think this will helps
bool iTunes::setBitRate()
{
cout << "What is the bitrate? ";
int numTemp = 0;
cin >> numTemp;
if (!numTemp)
{
cout << "WRONG" << endl;
setBitRate();
}
else if( numTemp < MIN_BITRATE || numTemp > MAX_BITRATE)
{
cout << "Bit Rate out of range" << endl;
setBitRate();
}
else
{
bitRate = numTemp;
}
}