for loop problem, compiles but wrong output

[clickspiker23]

Im trying to output the population increase for a population. the problem is my for loop doesn't increment or add correctly. here is a sample output. i made it easy with no death rate and a population of 100 to easily calculate the increase in my head.

Starting Population: 100

Annual Birth Rate Percentage: 100
Birth as a percentage is: 1

Annual Death Rate Percentage: 0
death as a percentage is: 0

Number of years to calculate: 5
Year 1 population: 100
Year 2 population: 300
Year 3 population: 500
Year 4 population: 700
Year 5 population: 900

here is the for loop, where the problem is. the rest of the program is just asking for user input but i will include it if asked.

double population = startSize;
    for (int i = 1; i <= years; i++)
    {


        cout << "Year " << i << "   population: " << population << endl;

     
        population += (startSize * (1 + annualBirth) * (1 - annualDeath));
        
    }

[m4ster_r0shi]

I'm pretty sure the statement updating your population should be -> population *= (1 + annualBirth - annualDeath);

[clickspiker23]

I'm pretty sure the statement updating your population should be -> population *= (1 + annualBirth - annualDeath);

Im working out of a book and doing the end of chapter exercises by myself and no solutions are provided but it gave the formula to use:

New population = Starting population * (1 + birth rate) * (1 - death rate)

i also changed the plus sign i had to the times sign and it keeps saying that

error: expected initializer before '*=' token

i think its because the compiler is wanting a pointer or something

[clickspiker23]

I'm pretty sure the statement updating your population should be -> population *= (1 + annualBirth - annualDeath);

you're right i just changed it to population = population * ( 1 + annualBirth - annualDeath) and it works.

thanks.

although i am curious as to why it would give that pointer error when i tried *=

[m4ster_r0shi]

error: expected initializer before '*=' token

Hmmm... I don't know about that. This here works fine for me:

#include <iostream>

using namespace std;

int main()
{
    double population = 100;

    double annualBirth = 0.75;
    double annualDeath = 0.25;

    for (int i = 1; i <= 10; i++)
    {
        cout << "Year " << i << "   population: " << population << endl;

        population *= (1 + annualBirth - annualDeath);
    }

    return 0;
}

New population = Starting population * (1 + birth rate) * (1 - death rate)

I see. Well, in C++ you could write this as -> population *= (1 + annualBirth ) * (1 - annualDeath); However, note that it's quite different than the one I suggested above.

Mine works like this:

temp1 = old_population * annualBirth;
temp2 = old_population * annualDeath;

new_population = old_population + temp1 - temp2;

Your book's works like this:

temp1 = old_population * annualBirth;
temp2 = (old_population + temp1) * annualDeath;

new_population = old_population + temp1 - temp2;

Pick whichever one you like :P