Taking integers from a string

This is a discussion on Taking integers from a string within the C++ Programming forums, part of the General Programming Boards category; Hello, this is my first question on this site. I have a string with a date in it, like "17/02/1943", ...

  1. #1
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    Taking integers from a string

    Hello, this is my first question on this site. I have a string with a date in it, like "17/02/1943", in the format dd/mm/yyyy. I need to get the date from this string, and put it into my Date struct, which has three int's for Day, Month and Year. How would I go about getting the numbers out, would I get substrings and cast them or something?

  2. #2
    Registered User hk_mp5kpdw's Avatar
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    There are a couple different ways... The first involves using the atoi function:
    Code:
    #include <string>
    #include <iostream>
    #include <cstdlib>
    using namespace std;
    
    int main()
    {
        string str("14/05/1988");
        int day, month, year;
    
        day = atoi( str.substr(0,2).c_str() );
        month = atoi( str.substr(3,2).c_str() );
        year = atoi( str.substr(6,4).c_str() );
    
        cout << "Day: " << day << " Month: " << month << " Year: " << year << endl;
    
        return 0;
    }
    Or you can use string streams:
    Code:
    #include <iostream>
    #include <string>
    #include <sstream>
    using namespace std;
    
    int main()
    {
        string str("14/05/1988");
        int day, month, year;
        istringstream sstr(str);
        char dummy;
    
        sstr >> day >> dummy >> month >> dummy >> year;
    
        cout << "Day: " << day << " Month: " << month << " Year: " << year << endl;
    
        return 0;
    }
    Both ways should output the same thing:
    Code:
    Day: 14 Month: 5 Year: 1988
    "Owners of dogs will have noticed that, if you provide them with food and water and shelter and affection, they will think you are god. Whereas owners of cats are compelled to realize that, if you provide them with food and water and shelter and affection, they draw the conclusion that they are gods."
    -Christopher Hitchens

  3. #3
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    Not too sure about that istringstream thing, and our lecturer looks down on anyone who uses C-style libraries or functions (i.e. cstdlib.h, cstring.h).

    I can get the substrings to just have the number I want ("32"), but can't convert them to int format. <static_cast><int> just gives me heaps of errors.

    Here's my code, Date is a struct:

    Code:
    #include <iostream>
    #include <string>
    #include "Date.h"
    
    using std::cout;
    using std::string;
    
    Date fileInputDate(string entry)
    {
    	Date output;
    	output.Day = static_cast<int>(entry.substr(0,2));
    	output.Month = static_cast<int>(entry.substr(3,2));
    	output.Year = static_cast<int>(entry.substr(6,4));
    	return output;
    }
    This gives me 3 C2440 errors. Is there anyway to do this without using C-style commands or any other libraries?

  4. #4
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    >Is there anyway to do this without using C-style commands or any other libraries?
    You could always do the conversion manually. For small numbers like those in a date, you don't need to worry about the problematic cases that more sophisticated string-to-integer libraries must consider. The conversion is a simple loop.
    Code:
    Date fileInputDate(string entry)
    {
      Date output; // Assumes that output is zero initialized
    
      for (int i = 0; i < 2; i++) {
        output.Day = 10 * output.Day + (entry[i] - '0');
      }
      for (int i = 3; i < 5; i++) {
        output.Month = 10 * output.Month + (entry[i] - '0');
      }
      for (int i = 6; i < 10; i++) {
        output.Year = 10 * output.Year + (entry[i] - '0');
      }
    
      return output;
    }

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