C++ detecting if input is Int or String

Question

I'm a C++ newbie who came from Java, so I need some guidance on some really basic issues I'm stumbling upon as I go.

I'm reading lines from a file, and each line consists of 6 strings/ints, which will be sent as parameters to a temporary variable.

Example:

Local1,Local2,ABC,200,300,asphalt

However, there are two subtypes of variable. One has a string as the last parameter (like 'asphalt' in the example above). The other one has an int instead. I have a method that reads each parameter and sends it to a variable, but how do I detect if the last bit of string is an integer or a string beforehand, so I know if I should send it to a Type1 variable or a Type2 one?

Many thanks!

My bad, I forgot to change it from another question I had before I solved it, it is now fixed — Jaqualembo
– Jaqualembo, Commented Oct 27, 2013 at 19:04
You really need basic language implementation education- lexing/parsing. — Puppy
– Puppy, Commented Oct 27, 2013 at 19:13

jrd1 · Accepted Answer · 2013-10-28 01:24:00Z

Since you want to determine the type of the last column, then this ought to work:

#include <iostream>
#include <string>
#include <cstdlib>
#include <vector>
#include <sstream>
#include <cctype>
#include <algorithm>

enum Types {
    NONE,
    STRING,
    INTEGER,
    DOUBLE
};

struct Found {
    std::string string_val;
    int integer_val;
    double double_val;
    enum Types type;
};

//copied verbatim from:
//http://stackoverflow.com/a/2845275/866930
inline bool isInteger(const std::string &s) {
   if(s.empty() || ((!std::isdigit(s[0])) && (s[0] != '-') && (s[0] != '+'))) return false;

   char * p ;
   std::strtol(s.c_str(), &p, 10);

   return (*p == 0);
}

//modified slightly for decimals:
inline bool isDouble(const std::string &s) {
   if(s.empty() || ((!std::isdigit(s[0])) && (s[0] != '-') && (s[0] != '+'))) return false ;

   char * p ;
   std::strtod(s.c_str(), &p) ;

   return (*p == 0);
}

bool isNotAlpha(char c) {
    return !(std::isalpha(c));
}

//note: this searches for strings containing only characters from the alphabet
//however, you can modify that behavior yourself.
bool isString (const std::string &s) {
    std::string::const_iterator it = std::find_if(s.begin(), s.end(), isNotAlpha);
    return (it == s.end()) ? true : false;
}

void determine_last_column (const std::string& str, Found& found) {
    //reset found:
    found.integer_val = 0;
    found.double_val = 0;
    found.string_val = "";
    found.type = NONE;

    std::string temp;
    std::istringstream iss(str);

    int column = 0;
    char *p;

    while(std::getline(iss, temp, ',')) {
        if (column == 5) {
            //now check to see if the column is an integer or not:
            if (isInteger(temp)) {
                found.integer_val = static_cast<int>(std::strtol(temp.c_str(), &p, 10));
                found.type = INTEGER;
            }
            else if (isDouble(temp)) {
                found.double_val = static_cast<double>(std::strtod(temp.c_str(), &p));
                found.type = DOUBLE;
            }
            else if (isString(temp)) {
                found.string_val = temp;
                found.type = STRING;
            }
        }
        ++column;
    }

    if (found.type == INTEGER) {
        std::cout << "An integer was found: " << found.integer_val << std::endl;
    }
    else if(found.type == DOUBLE) {
        std::cout << "A double was found: " << found.double_val << std::endl;
    }
    else if(found.type == STRING) {
        std::cout << "A string was found: " << found.string_val << std::endl;
    }
    else {
        std::cout << "A valid type was not found! Something went wrong..." << std::endl;
    }
}

int main() {
    std::string line_t1 = "Local1,Local2,ABC,200,300,asphalt";
    std::string line_t2 = "Local1,Local2,ABC,200,300,-7000.3";

    Found found;

    determine_last_column(line_t1, found);
    determine_last_column(line_t2, found);

    return 0;
}

This outputs and correctly assigns the appropriate value:

A string was found: asphalt
An integer was found: -7000.3

This version works on int, double, string; does not require boost; and, is plain vanilla C++98.

REFERENCES:

UPDATE:

This version now supports both positive and negative numbers that are integers or doubles, in addition to strings.

Thank you very much for the excellent input, I hope this does work out, which I believe it should. One more question, would this work with a double instead of an integer? Mainly in regard to the 'isdigit'.
I managed to get it working by adapting some of the code you provided. Many thanks once again

score 1 · Accepted Answer · 2013-10-27 19:14:01Z

1

First, create an array that can store both strings and integers:

std::vector<boost::variant<std::string, int>> items;

Second, split the input string on commas:

std::vector<std::string> strings;
boost::split(strings, input, boost::is_any_of(","));

Last, parse each token and insert it into the array:

for (auto&& string : strings) {
    try {
        items.push_back(boost::lexical_cast<int>(string));
    } catch(boost::bad_lexical_cast const&) {
        items.push_back(std::move(string));
    }
}

edited Oct 27, 2013 at 19:14

answered Oct 27, 2013 at 19:01

user1804599

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