C++ Variable Checking - c++

Question

Let's say I want the user to input an integer but he enters a double or a character value, how do I check that the the user has input the correct type.
string line;
getline(cin, line);
// create stringstream object called lineStream for parsing:
stringstream lineStream(line);
string rname;
double res;
int node1,node2;
lineStream >> rname >> res >> node1 >> node2;
How do I check for valid input type?

Answer 1

You check the stream is OK:
if (lineStream >> rname >> res >> node1 >> node2)
{
// all reads worked.
}
You may want to check for garbage on the end.
if (lineStream >> rname >> res >> node1 >> node2)
{
char x;
if (lineStream >> x)
{
// If you can read one more character there is junk on the end.
// This is probably an error. So in this situation you
// need to do somethings to correct for this.
exit(1);
}
// all reads worked.
// AND there is no junk on the end of the line.
}
Comment expanded.
From the comments below:
if i input an integer for rname, it still works. for exmaple:
string line; getline(cin, line);
stringstream lineStream(line); // created stringstream object called lineStream for parsing
string rname;
if (lineStream >> rname) { cout << "works"; }
Lets assume there is some properties about rname that allow us to distinguish it from a number. For example: it must be a name. i.e. it must only contain alpha characters.
struct Name
{
std::string value;
friend std::istream& operator>>(std::istream& s, Name& data)
{
// Read a word
s >> data.value;
// Check to make sure that value is only alpha()
if (find_if(data.value.begin(), data.value.end(), [](char c){return !isalpha(c);}) != str.end())
{
// failure is set in the stream.
s.setstate(std::ios::failbit);
}
// return the stream
return s;
}
};
Now you can read a name.
Name rname;
if (lineStream >> rname) { cout << "works"; }
This will fail if you enter an integer for rname.
Stretch Answer
If you have multiple lines of the same information you want to read. Then it is worth wrapping it in a class and defining a input stream operator.
strcut Node
{
Name rname;
double res;
int node1;
int node2;
friend std::istream& operator>>(std::istream& s, Node& data)
{
std::string line;
std::getline(s, line);
std::stringstream linestream(line);
lineStream >> data.rname >> data.res >> data.node1 >> data.node2;
if(!linestream)
{
// Read failed.
s.setstate(std::ios::failbit);
}
return s;
}
};
Now it becomes easy to read the lines in a loop:
Node n;
while(std::cin >> n)
{
// We read another node successfully
}

Answer 2

Since string 123 will also be considered as string, and not an integer, so the better way is to iterate the string to end, until we find any non-digit character. here's how you may do it:
bool is_number(const std::string& s)
{
std::string::const_iterator it = s.begin();
while (it != s.end() && std::isdigit(*it)) ++it;
return !s.empty() && it == s.end();
}

Answer 3

Read node1 and node2 into strings first, then validate with a regular expression.
#include <regex>
...
string node1_s, node2_s;
linestream >> rname >> node1_s >> node2_s
if (regex_match(node1_s, regex("[+-]?[0-9]+") {
/* convert node1_s to int */
} else {
/* node1_s not integer */
}
/* Do same for node2_s */