int item;
cin >> item;
That's in my code, but I want the user to be able to type integers or strings. This is basically what I want to do:
if(item.data_type() == string){
//stuff
}
Is this possible?
You can't do exactly that, but with a little more work you can do something similar. The following code works if you have the Boost libraries installed. It can be done without boost, but its tedious to do so.
#include <boost/lexical_cast.hpp>
main() {
std::string val;
std::cout << "Value: " << std::endl;
std::cin >> val;
try {
int i = boost::lexical_cast<int>(val);
std::cout << "It's an integer: " << i << std::endl;
}
catch (boost::bad_lexical_cast &blc) {
std::cout << "It's not an integer" << std::endl;
}
}
no, but you can input string and then convert it to integer, if it is integer.
Try to read this articles:
Use RTTI for Dynamic Type Identification
C++ Programming/RTTI
Run-time type information
What you're doing is not value C++ code. It wouldn't compile!
Your problem is:
That's in my code, but I want the user to be able to type integers or strings
Then do this:
std::string input;
cin >> input;
int intValue;
std::string strValue;
bool isInt=false;
try
{
intValue = boost::lexical_cast<int>(input);
isInt = true;
}
catch(...) { strValue = input; }
if ( isInt)
{
//user input was int, so use intValue;
}
else
{
//user input was string, so use strValue;
}
Related
Currently having issues creating an integer validation loop in my code.
Trying to validate for an integer, however this code fails to work if you input, for example the string '22a' it sets trial_no as 22. Is there any way to check that every charachter input is indeed a string, such that '22a' or '2a2' would be considered erroneous and the loop would continue until a valid integer was input?
int trial_no;
bool valid = false;
while(!valid)
{
valid = true; //assume trial_no will be an integer
cout << "Enter the number of die throws to simulate" << endl;
cin >> trial_no;
if(cin.fail()) // exit loop condition is dependent on trail_no being a valid integer
{
cin.clear(); //corrects stream
cin.ignore(numeric_limits<streamsize>::max(), '\n'); //skips left over stream data (numeric_limit
// is in case user enters more than one letter))
cout << "Please enter an integer value" << endl;
valid = false; //cin not an integer so loop goes round to try again
}
}
Arguably the best way is to read the entire line as a string and utilize the std::stoi function:
#include <iostream>
#include <string>
int main() {
std::cout << "Enter an integer: ";
std::string tempstr;
bool valid = false;
std::getline(std::cin, tempstr);
try {
int result = std::stoi(tempstr);
std::cout << "The result is: " << result;
valid = true;
}
catch (std::invalid_argument) {
std::cout << "Could not convert to integer.";
valid = false;
}
}
As pointed out in the comments, this function can also throw a std::out_of_range exception. This assumes your compiler is C++11 (+) capable. If not, go down the std::stringstream route:
std::string tempstr;
std::getline(std::cin, tempstr);
std::stringstream ss(tempstr);
int result;
bool valid = false;
if (ss >> result) {
valid = true;
}
else {
valid = false;
}
double checkInput() {
double add;
cout << "\n" << endl;
cin >> add;
if (cin.fail()==true)
{
cin.clear();
cin.ignore(INT_MAX, '\n');
cout << "Incorrect input"<<endl;
}
else
{
return add;
}
}
I use this bit of code to filter out character inputs eg "Blarg","bat12cat" and similar inputs where the character/letter come first but when i test with "1gold" ,"0.05cake" etc where number comes first then letters,the program accepts the all numbers up to the first instance of a letter.
My understanding is that it is the cin.ignore() that is causing the issue and is allowing the numbers through.
What would let inputs like"0.05Cats" be ignored/skipped altogether?.
Searching online,people suggest using getline() and stringstream.
Thank you.
When you input something like 1.5dog and then use cin >> some_double; >> is going to extract out a double until it can't read any more. So some_double gets the 1.5 and dog is still in the stream. This is not a failure and as such the failbit is not set. Since it is not set you skip your if statement and return the double value while the rest of the input stays in the stream and will cause you issues the next time you try to read from the stream. My suggestion is to change how you read your inputs. You can take in the input via a std::string and then convert it to the desired type. Then if the conversion fails you you can signal that you had a failure. I would use something like:
bool read_double(std::istream & is, double & number)
{
std::string line;
getline(is, line);
std::size_t pos = 0;
double temp = stod(line, &pos);
if (pos != line.size()) // there is extra content in the streams
return false;
number = temp;
return true;
}
And you can see it working with this Live Example
Usually there is more than one way to do the right thing. Using "c++(11) regex class object" would help you. You can edit regex for your needs (for example to include hex numbers by adding -a,b,c,d,e,f).
#include <iostream>
using namespace std;
#include <string>
using std::string;
#include <regex>
void main()
{
string input;
std::regex reg("[-+]?([0-9]*\.[0-9]+|[0-9]+)");
cout << "Please enter a double type\n";
while (cin >> input)
{
if (std::regex_match(input, reg))
{
std::cout << "Valid input\n";
break;
}
else
{
cout << "Invalid input\n";
cout << "Please enter a double type\n";
}
}
double do_with = std::stod(input,NULL);
cout << "the double is : " << do_with << endl;
cin >> input; // to hold the shell
}
I am trying to represent a variable in the form of a string to a integer, I have done so using;
atoi(str.c_str())
The string is originally obtained from a text file and stored into a;
CharArrayPtr cmemblock;
Which is then represented as a string;
string str;
for(int i = 0; i < numberofvalues; i++)
{
str = cmemblock[i];
int number = atoi(str.c_str());
cout << number;
}
If I was to change the 'cout' to print str;
str = cmemblock[i];
int number = atoi(str.c_str());
cout << str;
The number show correctly as stored in the text file
However, I require the output to be an integer so that I could represent it in a loop to search for a value stored in a array. So this is where 'number' comes into play, which is the reason why I am asking for your help, when;
cout << number;
Whenever a new line is read it is represented as '0' how would I go about removing this? If your require my full code it is in several different .cpp files and to prevent anyone copying my work I can only email it you, im sure you have already guessed it is part of a University Assignment.
Using Member Adosi code I came up with this;
std::string str;
for(int i = 0; i < numberofvalues; i++)
{
str = cmemblock[i];
std::stol(str);
int number = std::stoi(str);
cout << number;
}
I get an error R6010. Have I done this wrong?
std::stoi(str)
Use this instead of atoi
C++11 has this and a few other functions such as std::stol() for longs, std::stof() for floats, etc.
http://en.cppreference.com/w/cpp/string/basic_string/stol
If you dont have C++11 for std::stoi but do have boost you could use lexical cast
#include <boost/lexical_cast.hpp>
int main()
{
std::string s = "100";
try
{
int n = boost::lexical_cast<int>(s);
std::cout << "n = " << n << std::endl;
}
catch (boost::bad_lexical_cast)
{
std::cout << "conversion failed" << std::endl;
}
}
This ensures a valid conversion can take place and throws an exception if it cannot
Regarding your Edit - This requires a C++11 Compiler to work
std::string str;
for(int i = 0; i < numberofvalues; i++)
{
str = cmemblock[i];
//std::stol(str); -> This line is unneeded as it converts string to a long
// Ideally you should check that str is valid here etc. before changing it to an int
int number = std::stoi(str);
cout << number;
}
Another option is to use std::stringstream:
#include <sstream>
#include <string>
int string_to_int(const std::string &string) {
std::stringstream s(string);
s >> number;
if (!s.good()) {
throw std::exception();
}
return s;
}
int main(int argc, const char* argv[]) {
int number = string_to_int(argv[1]);
return 0;
}
This doesn't require any external libraries or C++11, and should be compatible with any C++ compiler out there.
Edit
Fleshed out the example to show how you could write your own string_to_int function to simplify the use of std::stringstream.
I am trying to write a short line that gets a string using getline and checks it for an int using stringstream. I am having trouble with how to check if the part of the string being checked is an int. I've looked up how to do this, but most seem to throw exceptions - I need it to keep going until it hits an int.
Later I will adjust to account for a string that doesn't contain any ints, but for now any ideas on how to get past this part?
(For now, I'm just inputting a test string rather than use getline each time.)
int main() {
std::stringstream ss;
std::string input = "a b c 4 e";
ss.str("");
ss.clear();
ss << input;
int found;
std::string temp = "";
while(!ss.eof()) {
ss >> temp;
// if temp not an int
ss >> temp; // keep iterating
} else {
found = std::stoi(temp); // convert to int
}
}
std::cout << found << std::endl;
return 0;
}
You could make of the validity of stringstream to int conversion:
int main() {
std::stringstream ss;
std::string input = "a b c 4 e";
ss << input;
int found;
std::string temp;
while(std::getline(ss, temp,' ')) {
if(std::stringstream(temp)>>found)
{
std::cout<<found<<std::endl;
}
}
return 0;
}
While your question states that you wish to
get a string using getline and checks it for an int
using stringstream, it's worth noting that you don't need stringstream at all. You only use stringstreams when you want to do parsing and rudimentary string conversions.
A better idea would be to use functions defined by std::string to find if the string contains numbers as follows:
#include <iostream>
#include <string>
int main() {
std::string input = "a b c 4 e 9879";//I added some more extra characters to prove my point.
std::string numbers = "0123456789";
std::size_t found = input.find_first_of(numbers.c_str());
while (found != std::string::npos) {
std::cout << found << std::endl;
found = input.find_first_of(numbers.c_str(), found+1);
}
return 0;
}
And then perform the conversions.
Why use this? Think about happens if you use a stringstream object on something like the following:
"abcdef123ghij"
which will simply be parsed and stored as a regular string.
Exceptions should not scary you.
int foundVal;
found = false;
while(!found || !ss.eof()) {
try
{
foundVal = std::stoi(temp); //try to convert
found = true;
}
catch(std::exception& e)
{
ss >> temp; // keep iterating
}
}
if(found)
std::cout << foundVal << std::endl;
else
std::cout << "No integers found" << std::endl;
I'm working on a small program for school to compute a factorial of an inputted number. I've got a good amount of experience in Java, but this is my first go at C++.
My problem: I need to be able to have a single input from the user, which is either an integer or the character 'q', which signifies the application needs to quit.
Here is my current attempt:
#include <stdio.h>
#include <iostream>
#include "Header.h"
using namespace std;
int x=0;
char y;
int main(int argc, char *argv[])
{
printf("Hello, please enter a number to compute a factorial (or 'q' to quit): ");
cin >> y;
x= (y-'0');
if(y=='q')
{ printf("Thanks for playing!\n");
exit(1);
}
long result= print_Factorial(x);
cout << x << "!= " << result << "\n";
return result;
}
This casting does not work, however. If I enter a double digit number, such as 12, it only converts to x the first digit of the two and computes that factorial. I'm sure this is simple, what am I missing?
Explicit answer or a lead to where I can learn more about this problem, anything is appreciated.
There are functions you can use to try and covert a string into a number, and which you can check if the conversion was a success or not. The std::strtol function is one of them:
std::string input;
std::cin >> input;
char* endptr = nullptr;
const char *input_ptr = input.c_str();
long value = std::strtol(input_ptr, &endptr, 10);
if (endptr == input_ptr)
{
// Input was not a valid number
}
else if (*endptr != '\0')
{
// Input starts with a valid number, but ends with some extra characters
// (for example "123abc")
// `value` is set to the numeric part of the string
}
else
{
// Input was a valid number
}
If you don't mind exceptions, then you can use e.g. std::stoi instead:
std::string input;
std::cin >> input;
int value = 0;
try
{
size_t endpos = 0;
value = std::stoi(input, &endpos);
if (endpos != input.length())
{
// Input starts with a valid number, but ends with some extra characters
// (for example "123abc")
// `value` is set to the numeric part of the string
}
else
{
// Input is a valid number
}
}
catch (std::invalid_argument&)
{
// Input not a valid number
}
catch (std::out_of_range&)
{
// Input is a valid number, but to big to fit in an `int`
}
The reason you're getting the first digit is because you're using
cin >> y; where y is a char, which holds one character. So you're only getting one character.
What you probably would want to do is get the answer as a string, and once you check that the string is not == "q", then you could convert it to an int.
#include <iostream>
#include <sstream>
int main() {
std::string in;
std::cout << "Please enter a digit: ";
while(std::cin >> in) {
std::cout << "Input: " << in << std::endl;
if(in.size() == 1) {
if(in[0] == 'q' || in[0] == 'Q') {
std::cout << "Quit" << std::endl;
return 0;
}
}
std::istringstream parse(in);
int value;
if(parse >> value) {
if(parse.eof()) {
std::cout << "Success" << std::endl;
return 0;
}
}
std::cout << "Please try again: ";
}
std::cerr << "This should not happen <control + d>" << std::endl;
return 1;
}
Your user can enter whatever line of text, you you have to read a "line of text" to validate.
#include <iostream>
#include <string>
#include <stdexcept>
int main()
{
std::string text;
std::getline(std::cin,text);
if(text.size()==1 && text[0]=='q')
{
std::cout << "quit command";
return 0;
}
try
{
int i = std::stoi(text); //may throw if text is not convertible
/* whatever elaboration and output */
return 0;
}
catch(const std::exception& e)
{
std::cout << "bad input: " << text << '\n';
std::cout << "caused excpetion: " << e.what() << std::endl;
}
return 3; //means "excpetion thorown"
}