This code gives me errors when compiled using visual studio 2012, but with codeblocks it is okay

Question

#include <iostream>
#include <string>
#include <fstream>
#include <sstream>
#include <vector>
#include <iterator>

#define ADDR "c:\\Users\\Library2\\Desktop\\Books record\\"

using namespace std;

int main()
{
   ifstream fin(ADDR "reportcard.csv", ios::binary);
   string line;
   int rowCount=0;
   int rowIdx=0; 

   while(getline(fin,line)){
       rowCount++;
   }

   vector<string> data[**rowCount**];//this rowCount gave me "expression must have a constant value"

   fin.clear(); 
   fin.seekg(fin.beg); 

   while(getline(fin,line)) 
   {
      stringstream ss(line);  
      string value;
      while(getline(ss,value,',')){       
         data[rowIdx].push_back(value);
      }
      rowIdx++;   
   }

   fin.close();

   int colNum;
   string colName = "LAST PERSON";
   static int it;

   for(vector<string>::iterator it = data[0].begin(); it != data[0].end(); ++it)
   {
       if ((*it)== colName)
       {
           colNum = distance(data[0].begin(),it);//distance() gave me "no instances of function templates matches argument"
           break;
       }
   }
   cout << data[1][colNum] << "\t";

   return 0;
}

im trying to figure out why it gave me the expression must have a constant value.
im trying to find another function that works the same as distance() that can be used in visual studio 2012.

note: this code is for finding and getting the value from the first cell under a column called "LAST PERSON". This code is already okay when using codeblocks. but i need to use visual studio.

i did put the error message that visual studio gave me as a comment on that line. the error was for line; vector data[**rowCount**];//this rowCount gave me "expression must have a constant value" then, colNum = distance(data[0].begin(),it);//distance() gave me "no instances of function templates matches argument" — SectumSempra, Dec 15 '19 at 23:38
Does this answer your question? [Why aren't variable-length arrays part of the C++ standard?](https://stackoverflow.com/questions/1887097/why-arent-variable-length-arrays-part-of-the-c-standard) — alter_igel, Dec 15 '19 at 23:44
wait. let me read and check that one out. btw, thanks for ur feedback guys — SectumSempra, Dec 15 '19 at 23:46

Vlad from Moscow · Accepted Answer · 2019-12-15T23:42:25.507

5

This

vector<string> data[rowCount];

is a declaration of a variable length array.

Variable length arrays is not a standard C++ feature. Some compilers have their own language extensions that allow to use variable length arrays. Other compilers do not have such a language extension.

Instead of the array you could use a vector of vectors as for example

std::vector<std::vector<std::string>> data;

Pay attention to that as the file is opened in the binaru mode

ifstream fin(ADDR "reportcard.csv", ios::binary);

then using the function std::getline is not correct in general case.

while(getline(fin,line)){
    rowCount++;
}

edited Dec 15 '19 at 23:42

answered Dec 15 '19 at 23:35

Vlad from Moscow

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`std::vector> data(rowCount);` looks like it would be the correct declaration, given the rest of the code. – Ken Y-N Dec 15 '19 at 23:41
thank you both of you. actually i havent fully understand yet what does vector> is. but i just tried and applied it. and yes. both of the errors gone. thank you so much guys. now i need to read something about this to understand it. i will start to google about it. – SectumSempra Dec 16 '19 at 00:38

score 0 · Answer 2 · answered Dec 15 '19 at 23:48

When the compiler sees this:

vector<string> data[rowCount];

it starts from data and looks to the right. It sees '[' and interprets data to be a C-style array. Then it sees rowCount and checks if it is a compile-time constant. If not, this code is against the standard. Still, some compilers allow this as a language extension.

All in all, you've defined a C-style array of rowCount elements of type vector<string>. You must have mistaken [] for () or {}. Most likely you wanted to write

std::vector<std::string> data(rowCount); // a vector of rowCount strings

This code gives me errors when compiled using visual studio 2012, but with codeblocks it is okay

2 Answers2

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