C++ Beginner Logic Error - Returning 0 - c++

Question

I don't understand why my code is not calculating the birthrate and the deathrate. I keep on getting 0 for both. I included the static_cast<double> to ensure this wouldn't happen. Any feedback / help?
#include <iostream>
#include <string>
using namespace std;
double calculateBirthRate();
double calculateDeathRate();
class PopInfo
{
private:
string cityName;
long totalCityPopulation;
int numberOfBirths;
int numberOfDeaths;
double birthrate;
double deathrate;
int bir;
int dea;
long citpop;
public:
PopInfo()
{
cityName = "";
totalCityPopulation = numberOfBirths = numberOfDeaths = 0;
}
long getPopulation()
{
return totalCityPopulation;
}
int getBirths()
{
return birthrate;
}
int getDeaths()
{
return deathrate;
}
string getCity()
{
return cityName;
}
void setCityName(string nameOfCity)
{
cityName = nameOfCity;
}
void setTotalCityPopulation(long populationOfCity)
{
totalCityPopulation = populationOfCity;
}
void setNumberOfBirths(int birthNumbers)
{
numberOfBirths = birthNumbers;
}
void setNumberOfDeaths(int deathNumbers)
{
numberOfDeaths = deathNumbers;
}
void calculateBirthRate(PopInfo);
void calculateDeathRate(PopInfo);
};
int main()
{
PopInfo newCity;
string cit;
long citpop;
int bir;
int dea;
cout << "What is the city name?: " << endl;
cin >> cit;
cout << "What is the total city population?: " << endl;
cin >> citpop;
while (citpop < 1)
{
cout << "Please enter a valid total city population: " << endl;
cin >> citpop;
}
cout << "What are the number of births?: " << endl;
cin >> bir;
while (bir < 0)
{
cout << "Please enter a valid number of births: " << endl;
cin >> bir;
}
cout << "What are the number of deaths?: " << endl;
cin >> dea;
while (dea < 0)
{
cout << "Please enter a vaild number of deaths: " << endl;
cin >> dea;
}
newCity.setCityName(cit);
newCity.setTotalCityPopulation(citpop);
newCity.setNumberOfBirths(bir);
newCity.setNumberOfDeaths(dea);
cout << endl;
cout << "The city name is " << newCity.getCity() << endl;
cout << "The total city population is " << newCity.getPopulation() << endl;
cout << "The birth rate is " << newCity.getBirths() << endl;
cout << "The death rate is " << newCity.getDeaths() << endl;
return 0;
}
void PopInfo::calculateBirthRate(PopInfo newCity)
{
double birthrate = static_cast<double>(newCity.bir) / newCity.citpop;
}
void PopInfo::calculateDeathRate(PopInfo newCity)
{
double deathrate = static_cast<double>(newCity.dea) / newCity.citpop;
}

Answer 1

You accidentally made birthrate and deathrate as local variables. Remove the leading keyword double, to make it:
void PopInfo::calculateBirthRate(PopInfo newCity)
{
birthrate = static_cast<double>(newCity.bir) / newCity.citpop;
}
void PopInfo::calculateDeathRate(PopInfo newCity)
{
deathrate = static_cast<double>(newCity.dea) / newCity.citpop;
}
Even so, it's kind of strange that you're passing newCity by value – did you mean to store the rates back in the same object, as in:
void PopInfo::calculateBirthRate(PopInfo& newCity)
{
newCity.birthrate = static_cast<double>(newCity.bir) / newCity.citpop;
}
void PopInfo::calculateDeathRate(PopInfo& newCity)
{
newCity.deathrate = static_cast<double>(newCity.dea) / newCity.citpop;
}
or did you mean to operate on the object in-place, as in:
void PopInfo::calculateBirthRate()
{
birthrate = static_cast<double>(bir) / citpop;
}
void PopInfo::calculateDeathRate()
{
deathrate = static_cast<double>(dea) / citpop;
}

Answer 2

I don't think you ever call the functions that calculate birth rate and death rate! That is on top of the issues already identified, but I'm pretty sure it matters... Put a cout debug statement in there and see if I'm right...
Another problem: since "rate" is a number between zero and 1, and your function getBirths returns an int, you are going to run into a rounding problem...
Also not sure you ever set dea and bir in the context of the class (you declare them at the main level). So many places where you are inviting problems...
The easiest solution would be to rewrite these two functions:
double getBirths()
{
return (double)numberOfBirths/citypop;
}
double getDeaths()
{
return (double)numberOfDeaths/citypop;
}
But read your code, and ask yourself what the scope of your variables is, where they are set (and if you ever set them...), where they are used, where you perform type conversions.... You can learn a lot from that.
EDIT
I couldn't help myself, and decided to copy your program and debug it. After a few simplifications in the structure I came up with the following (note I moved the two functions calculateBirthRate and calculateDeathRate inside the class definition for consistency; and I used the "internally known" variables totalCityPopulation etc, rather than some of the "alternative" ones you were using... it was getting very confusing. Finally, as I mentioned in the original answer - I made sure the birth and death rates were actually calculated. I have marked changed lines with //*** :
#include <iostream>
#include <string>
using namespace std;
double calculateBirthRate();
double calculateDeathRate();
class PopInfo
{
private:
string cityName;
long totalCityPopulation;
int numberOfBirths;
int numberOfDeaths;
double birthrate;
double deathrate;
int bir;
int dea;
long citpop;
public:
PopInfo()
{
cityName = "";
totalCityPopulation = numberOfBirths = numberOfDeaths = 0;
}
long getPopulation()
{
return totalCityPopulation;
}
double getBirths() //*** was int
{
return birthrate;
}
double getDeaths() //*** was int
{
return deathrate;
}
string getCity()
{
return cityName;
}
void setCityName(string nameOfCity)
{
cityName = nameOfCity;
}
void setTotalCityPopulation(long populationOfCity)
{
totalCityPopulation = populationOfCity;
}
void setNumberOfBirths(int birthNumbers)
{
numberOfBirths = birthNumbers;
}
void setNumberOfDeaths(int deathNumbers)
{
numberOfDeaths = deathNumbers;
}
//*** this function moved into the class definition
void calculateBirthRate()
{
birthrate = (double)numberOfBirths/totalCityPopulation; //*** using different variables
}
//*** this function moved into the class definition
void calculateDeathRate()
{
deathrate = (double)numberOfDeaths / totalCityPopulation; //*** using different variables
}
};
int main()
{
PopInfo newCity;
string cit;
long citpop;
int bir;
int dea;
cout << "What is the city name?: " << endl;
cin >> cit;
cout << "What is the total city population?: " << endl;
cin >> citpop;
while (citpop < 1)
{
cout << "Please enter a valid total city population: " << endl;
cin >> citpop;
}
cout << "What are the number of births?: " << endl;
cin >> bir;
while (bir < 0)
{
cout << "Please enter a valid number of births: " << endl;
cin >> bir;
}
cout << "What are the number of deaths?: " << endl;
cin >> dea;
while (dea < 0)
{
cout << "Please enter a vaild number of deaths: " << endl;
cin >> dea;
}
newCity.setCityName(cit);
newCity.setTotalCityPopulation(citpop);
newCity.setNumberOfBirths(bir);
newCity.setNumberOfDeaths(dea);
newCity.calculateBirthRate(); //*** added, or it's never calculated
newCity.calculateDeathRate(); //*** added, or it's never calculated
cout << endl;
cout << "The city name is " << newCity.getCity() << endl;
cout << "The total city population is " << newCity.getPopulation() << endl;
cout << "The birth rate is " << newCity.getBirths() << endl;
cout << "The death rate is " << newCity.getDeaths() << endl;
return 0;
}
When I run this code, I get the following:
What is the city name?:
Amsterdam
What is the total city population?:
1234567
What are the number of births?:
12345
What are the number of deaths?:
54321
The city name is Amsterdam
The total city population is 1234567
The birth rate is 0.00999946
The death rate is 0.044
The diff between your code and mine is:
33c33
< double getBirths()
---
> int getBirths()
38c38
< double getDeaths()
---
> int getDeaths()
68,71c68,69
< void calculateBirthRate()
< {
< birthrate = (double)numberOfBirths/totalCityPopulation;
< }
---
> void calculateBirthRate(PopInfo);
> void calculateDeathRate(PopInfo);
73,76d70
< void calculateDeathRate()
< {
< deathrate = (double)numberOfDeaths / totalCityPopulation;
< }
117,118d110
< newCity.calculateBirthRate();
< newCity.calculateDeathRate();
129a122,125
> void PopInfo::calculateBirthRate(PopInfo newCity)
> {
> double birthrate = static_cast<double>(newCity.bir) / newCity.citpop;
> }
130a127,130
> void PopInfo::calculateDeathRate(PopInfo newCity)
> {
> double deathrate = static_cast<double>(newCity.dea) / newCity.citpop;
> }

Answer 3

double birthrate = static_cast<double>(newCity.bir) / newCity.citpop;
...
double deathrate = static_cast<double>(newCity.dea) / newCity.citpop;
Here you are driving two new variable names birthrate and death rate. You're not writing the values two the class data members. Writing the type before the names overwrites it. To change, simply remove it.
birthrate = static_cast<double>(newCity.bir) / newCity.citpop;
...
deathrate = static_cast<double>(newCity.dea) / newCity.citpop;