How to make conversion function work? - c++

Question

I have a code that I'm trying to learn how to parse in C++. I understood everything I did, but I don't understand how to work with the likes of atoi(), atof (), strtod (). I know what it's supposed to do, but I don't understand why the compiler doesn't like it. My focus on the error is "scores[line_count] = strtod (score);"
#include <iostream>
#include <fstream>
#include <string>
#include <stdlib.h>
#include <iomanip>
using namespace std;
int readScores(string inputFile, string name[], float scores[], int array_size)
{
//delcare variables
ifstream infile;
int line_count = 0;
string line;
string named;
float score;
char character;
int word_index;
string names[array_size];
// open input file
infile.open(inputFile);
//Check if file opens succesfully.
if(infile.fail())
{
cout << "File cannot open!!" << endl;
return -1;
}
while(getline(infile, line))
{
cout << line << endl;
// PARSING GOES HERE
word_index = 0;
for(int i=0; i < (int)line.length(); i++)
{
character = line[i];
if (character == ',')
{
names[line_count] = named;
named = "";
word_index++;
}
else
{
if(word_index == 0)
{
named += character;
}
else if (word_index == 1)
{
score += character;
cout << character << " " << endl;
}
}
}
scores[line_count] = strtod (score);
line_count++;
}
//close file
infile.close();
//return line count
return line_count;
cout << line_count << endl;
}
int main(void)
{
int array_size = 50;
string inputFile = "Test.txt";
string name [array_size];
float scores [array_size];
readScores(inputFile, name, scores, array_size);
}

Answer 1

The function strtod() takes the form
double strtod (const char* str, char** endptr);
But you only give it the string.
As you can see it takes two parameters, the string you wish to convert to a double, and an "endptr". The endptr is described here as a
Reference to an already allocated object of type char*, whose value is set by > the function to the next character in str after the numerical value.
This parameter can also be a null pointer, in which case it is not used.
So you need to declare a char pointer to save the next character after the decimal, even if there wont be one. This allows you to pull multiple doubles from a single string, much like a tokenizer.
char * endPtr;
scores[line_count] = strtod(score, &endPtr);
Edit
As Alex Lop pointed out, you aren't even passing a string to strtod, you're passing a float. It appears you would like to cast the float to a double?

Answer 2

Of course the compiler doesn't like it. Please read the description of strtod.
double strtod (const char* str, char** endptr);
Convert string to
double.
Parses the C-string str interpreting its content as a floating
point number (according to the current locale) and returns its value
as a double. If endptr is not a null pointer, the function also sets
the value of endptr to point to the first character after the number.
The function first discards as many whitespace characters (as in
isspace) as necessary until the first non-whitespace character is
found. Then, starting from this character, takes as many characters as
possible that are valid following a syntax resembling that of floating
point literals (see below), and interprets them as a numerical value.
A pointer to the rest of the string after the last valid character is
stored in the object pointed by endptr.
And in your code you pass to strtod only one parameter which is of type float and store the returned result of double into an array of floats. If you want to move the value of float from one variable to another, you don't need any "convertion" function:
scores[line_count] = score;
NOTE: I didn't really review your code as you asked specifically about scores[line_count] = strtod (score);. But after I looked how you modify score, maybe it should have been string and not float. If so, then it is the another point to fix.