在C ++中解析日期字符串

Question

My son is learning C++ and one of his exercises it to have the user input a date in DD/MM/YYY and then output it to Month day, year

So: 19/02/2013
Output: February 19, 2013.

I am trying to help him understand the various ways but now I have confused myself.

getline() std::string::substr() std::string::find() std::string::find_first_of() std::string::find_last_of()

I can't figure it out with any of these quite right.

my current attempt to parse is:

#include <iostream>
#include <string>

using namespace std;

int main (void)
{
    string date;
    string line;

    cout << "Enter a date in dd/mm/yyyy format: " << endl;
    std::getline (std::cin,date);

    while (getline(date, line))
    {
        string day, month, year;
        istringstream liness( line );
        getline( liness, day, '/' );
        getline( liness, month,  '/' );
        getline( liness, year,   '/' );

        cout << "Date pieces are: " << day << " " << month << " " << year << endl;
    }
}

but I am getting errors like:

`g++ 3_12.cpp -o 3_12`
`3_12.cpp: In function ‘int main()’:`
`3_12.cpp:16: error: cannot convert ‘std::string’ to ‘char**’ for argument ‘1’ to ‘ssize_t getline(char**, size_t*, FILE*)’`
`3_12.cpp:18: error: variable ‘std::istringstream liness’ has initializer but incomplete type`

Answer 1

int day, month, year;
char t;
std::cin >> day >> t >> month >> t >> year;

Answer 2

You've missed the std Regular Expressions Library ! I reckon it's the safest and most effective way of doing this.

Back on topic, I think that since getline is an extern "C" function, you can't overload it using a using namespace std (which should be banned by the way). You should try prepending std to all getline calls.

Answer 3

For std::istringstream , you need:

#include <sstream>

PS Don't use using namespace std; . It's a bad habit, and it will eventually get you into trouble.

Answer 4

The error

3_12.cpp:16: error: cannot convert 'std::string' to 'char**' for argument '1' to 'ssize_t getline(char**, size_t*, FILE*)'

means the compiler couldn't make sense of this line:

while (getline(date, line))

Both date and line are declared as std::string and there's no overload of getline that takes two strings. Compiler guessed that you were trying to call this function that is not part of C++ library (apparently, one of standard library headers includes stdio.h which is where that function is coming from).

The error

3_12.cpp:18: error: variable 'std::istringstream liness' has initializer but incomplete type

means the compiler doesn't know what std::istringstream is. You forgot to include <sstream> .

If you want to extract lines from a string by means of std::getline , you need to put it in a stringstream first, just like you do inside the loop.

Parsing dates isn't that easy. You don't want the user to be able to enter 44/33/-200 and get away with it. How I'd might approach this:

std::istringstream iss(date); // date is the line you got from the user

unsigned int day, month, year;
char c1, c2;

if (!(iss >> day >> c1 >> month >> c2 >> year)) {
    // error
}
if (c1 != '/' || c2 != '/') { /* error */ }
if (month > 12) { /* error  / }

// ... more of that, you get the idea