My code is mostly working except for one minor issue. While it should only accept ints, it also accepts user input that start with an int, such as 6abc for example. I saw a fix for this here, but it changed the input type to string and added a lot more lines of code. I'm wondering if there's an easier way to fix this:
int ID;
cout << "Student ID: ";
// error check for integer IDs
while( !( cin >> ID )) {
cout << "Must input an integer ID." << endl ;
cin.clear() ;
cin.ignore( 123, '\n' ) ;
}
In a word - no.
But what you can do is instead read a whole word into a std::string first, and then convert that entire word into an int, checking for errors in that conversion, eg:
int ID;
string input;
do
{
cout << "Student ID: ";
if (!(cin >> input))
{
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
}
else
{
size_t pos = 0;
try
{
ID = stoi(input, &pos);
if (pos == input.size())
break;
}
catch (const std::exception &) {}
}
cout << "Must input an integer ID." << endl;
}
while (true);
Live Demo
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'm giving a while statement two conditions but it doesn't appear to be working as I want it to.
cout << "Enter S if you booked a single room or D for a double room and press enter." << endl;
cin >> rtype;
while(rtype !='S' && rtype !='D')
{
cout << "That is not a valid room type, please try again." << endl;
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
cin >> rtype;
}
If I input "Sfbav", it takes the input as valid because the first character is 'S' and ignores that there are other characters there that would make it an invalid input.
How can I change this so that the input has to only be 'S' or 'D' to be treated as correct?
Option 1
Read the token as a string and compare it to "S" and "D".
std::string rtype;
cin >> rtype;
while(rtype != "S" && rtype != "D" )
{
cout << "That is not a valid room type, please try again." << endl;
cin.clear();
cin.ignore(numeric_limits<streamsize>::max(), '\n');
cin >> rtype;
}
Option 2
Read the entire line as a string and compare it to "S" and "D".
std::string rtype;
getline(cin, rtype);
while(rtype != "S" && rtype != "D" )
{
cout << "That is not a valid room type, please try again." << endl;
getline(cin, rtype);
}
You can get the whole line, and check how many characters it has. If it has more than 1, then you know that the user entered Sfbav or something else.
while (true) {
std::string line; // stores the whole input that the user entered
std::getline(std::cin, line); // get the whole input
if (line.size() != 1) // if it does not have exactly 1 character, than it is invalid
std::cout << "That is not a valid room type, please try again.\n";
else { // if it has only 1 character, everything is ok
rtype = line.front();
break;
}
}
if your rtype is char
In this case I prefer to use std::setw( 1 ) to avoid getting more than one char, like:
char rtype;
std::cin >> std::setw( 1 ) >> rtype; // Sabc
std::cout << rtype << '\n'; // S
and:
#include <iomanip>
I have the following code:
qstn:
cout << "Input customer's lastname: ";
getline(cin, lname);
if (lname.find_first_not_of("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ ") != string::npos) {
cout << "You can only input alpha here!\n";
cin.clear();
goto qstn;
} else if (lname.empty()) {
cout << "Please enter your firstname!\n";
cin.clear();
goto qstn;
}
int lnamel = lname.length();
int strl = str.length();
int is = 0;
for (int i = 1; i < strl;) {
i++;
is++;
if (lname[i] == lname[is] && lname[i] == ' ' || lname[0] == ' ') {
cin.clear();
cout << "Please input your lastname properly!\n";
goto qstn;
}
}
// next question here
I'm having a hard time on thinking what will be the proper logic to avoid the
goto statement, since I was college I was using it but someone here said that
it's not good to use it at all cause it might ruin my code.
I tried using the do while loop but it's not smooth as goto.
Please help!
Here's an idiom I like to use:
int i;
if (std::cin >> prompt("enter an integer: ", i))
{
std::cout << "Read user input: " << i << "\n";
} else {
std::cout << "Input failed (too many attempts). Eof? " << std::boolalpha << std::cin.eof() << "\n";
}
Here, prompt is a smart input manipulator, that takes care of handling parse errors or stream failures and retrying.
It's quite generic so actually do many things, but you don't need to indicate all the options. When the manipulator is inserted into the stream it relays to the do_manip member:
template <typename Char, typename CharT>
friend std::basic_istream<Char, CharT>& operator>>(std::basic_istream<Char, CharT>& is, checked_input<T, Prompter>& manip) {
return manip.do_manip(is);
}
The do_manip handles all the logic without any gotos :) :
std::istream& do_manip(std::istream& is) {
auto attempt = [this] { return infinite() || retries_ > 0; };
while (attempt()) {
if (!infinite())
retries_ -= 1;
prompter_(out_);
if (is >> value_) {
if (!run_validators(out_))
is.setstate(is.rdstate() | std::ios::failbit);
else
break;
} else {
out_.get() << format_error_ << "\n";
}
if (attempt()) {
is.clear();
if (flush_on_error_)
is.ignore(1024, '\n');
}
}
return is;
}
You can see that there is a possibility to have validations run before accepting the input.
Here's a somewhat full-blown demo:
Live On Coliru
int main() {
using namespace inputmagic;
int i;
if (std::cin >> prompt("enter an integer: ", i)
.retries(3)
.flush_on_error(false)
.format_error("I couldn't read that (Numbers look like 123)")
.output(std::cerr)
.validate([](int v) { return v > 3 && v < 88; }, "value not in range (3,88)")
.validate([](int v) { return 0 == v % 2; })
.validate([](int v) { return v != 42; }, "The Answer Is Forbidden")
.multiple_diagnostics())
{
std::cout << "Read user input: " << i << "\n";
} else {
std::cout << "Input failed (too many attempts). Eof? " << std::boolalpha << std::cin.eof() << "\n";
}
}
You can see it will only accept valid integers
that are >3 and <88,
that are even
except 42 (forbidden number)
When entering the numbers 21, 42 and 10 on subsequent retries, you get: live
enter an integer: 21
Value not valid
enter an integer: 42
The Answer Is Forbidden
enter an integer: 10
Read user input: 10
However, if you enter 1 all the time you get this: live
enter an integer: 1
value not in range (3,88)
Value not valid
enter an integer: 1
value not in range (3,88)
Value not valid
enter an integer: 1
value not in range (3,88)
Value not valid
Input failed (too many attempts). Eof? false
Or if you read from a single line file: live
enter an integer: value not in range (3,88)
Value not valid
enter an integer: I couldn't read that (Numbers look like 123)
enter an integer: I couldn't read that (Numbers look like 123)
Input failed (too many attempts). Eof? true
Use a function:
bool getLastName(string & lname,
string & str)
{
cout << "Input customer's lastname: ";
getline(cin, lname);
if (lname.find_first_not_of("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ ")
!= string::npos)
{
cout << "You can only input alpha here!\n";
cin.clear();
return false;
}
else if (lname.empty())
{
cout << "Please enter your firstname!\n";
cin.clear();
return false;
}
int lnamel = lname.length();
int strl = str.length();
int is = 0;
for (int i = 1; i < strl;)
{
i++;
is++;
if (lname[i] == lname[is] && lname[i] == ' ' || lname[0] == ' ')
{
cin.clear();
cout << "Please input your lastname properly!\n";
return false;
}
}
return true;
}
All I've done here is replace the gotos with return false. If the program makes it to the end of the function, return true. Make the function call in a while loop:
while (!getLastName(lname, str))
{
// do nothing
}
Not only does this de-spaghettify the code, but it breaks it up into nice, small, easy to manage pieces. This is called procedural programming.
A While loop looks like your best bet. You can redo the loop with the continue keyword.
int incorrect = 0;
while(!incorrect) {
cout<<"Input customer's lastname: ";
getline(cin,lname);
if(lname.find_first_not_of("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ ")!=string::npos)
{
cout<<"You can only input alpha here!\n";
cin.clear();
continue;
}
else if(lname.empty())
{
cout<<"Please enter your firstname!\n";
cin.clear();
continue;
}
int lnamel=lname.length();
int strl=str.length();
int is=0;
for(int i=1; i<strl;)
{
i++;
is++;
if(lname[i]==lname[is]&&lname[i]==' '||lname[0]==' ')
{
cin.clear();
cout<<"Please input your lastname properly!\n";
continue;
}
incorrect = 1;
}
You need to use some do while loops
for instance this:
qstn:
cout<<"Input customer's lastname: ";
getline(cin,lname);
if(lname.find_first_not_of("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ ")!=string::npos)
{
cout<<"You can only input alpha here!\n";
cin.clear();
goto qstn;
}
else if(lname.empty())
{
cout<<"Please enter your firstname!\n";
cin.clear();
goto qstn;
}
could be re-written as this:
int flag;
do{
flag = 1;
cout<<"Input customer's lastname: ";
getline(cin,lname);
if(lname.find_first_not_of( "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ ")!=string::npos)
{
flag = 0;
cout<<"You can only input alpha here!\n";
}
else if(lname.empty())
{
flag = 0;
cout<<"Please enter your firstname!\n";
}
cin.clear();
} while( flag !=1 );
feel free to use a boolean type flag, it doesn't really matter
It seems to me that your code suffers from lack of clarity of purpose.
You apparently don't want the string that's entered to include a leading space, nor multiple consecutive spaces. Other than that, only alphabetic characters should be accepted.
If the user does enter multiple consecutive spaces, I'd probably just ignore all but the first. I'd probably write the code something like this:
#include <string>
#include <iostream>
#include <algorithm>
#include <cctype>
#include <sstream>
bool non_alpha(std::string const &s) {
return !std::all_of(s.begin(), s.end(), [](unsigned char c) { return std::isalpha(c) || std::isspace(c); });
}
std::string get_name(std::string const &prompt) {
std::string result;
std::string line;
do {
std::cout << prompt;
std::getline(std::cin, line);
} while (non_alpha(line));
std::istringstream words(line);
std::string word;
while (words >> word)
result += word + ' ';
return result;
}
int main() {
auto res = get_name("Please enter last name, alpha-only\n");
if (res.empty())
std::cout << "Oh well, maybe some other time";
else
std::cout << "Thanks Mr(s). " << res << "\n";
}
I'd be tempted to consider doing roughly the same for non-alphabetic characters--rather than asking the user to re-enter from the beginning, assume it's a mistake and just ignore it.
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;
}
}