# A First C++ Project¶

Cyganek section 2.5

## Compound interest¶

We want to write a program that calculates compound interest. The general expression for this is:

$A = P \left ( 1 + \frac{r}{n} \right )^{nt}$

where

• $$P$$ is the principal

• $$r$$ is the interest rate on an annual basis (in units of 1 / year)

• $$n$$ is the compounding frequency (in units of 1 / year)

• $$t$$ is the investment time (in units of years)

• $$A$$ is the final amount

Note

I am expressing this differently than you text, which seems to use somewhat non-standard notation.

Looking at this expression, we need to learn how to raise something to a power. In C++, this is done by the std::pow() function, which is part of cmath.

Let’s try this out:

#include <cmath>
#include <iostream>

int main() {

std::cout << std::pow(2.0, 3.0) << std::endl;

}


We need to tell C++ what type of data we are going to be storing. We’ll talk about data types next, but for now we’ll use double, which is double precision floating point format (more on that later).

Important

C++ is statically typed – this means we need to declare what type of data is to be stored in each variable we use before we can use that variable. For instance:

double P;


creates a variable P that can store a double precision number.

We will also want to take input. For this we need to use std::cin. Let’s look how this works:

#include <iostream>

int main() {
double P = 0.0;
std::cout << "Enter the principal (P): ";
std::cin >> P;

std::cout << "P = " << P << std::endl;
}


Notice that we initialize P to 0.0 just to be safe (we’ll see a more compact way of initializing later).

Now we will look at the complete code. We shouldn’t be expected to be able to write something like this yet, but this will show us some of the C++ constructs that we will learn in the next weeks.

Listing 1 interest.cpp
#include <iostream>
#include <cmath>

int main() {

double P = 0.0;
double r = 0.0;
double n = 0.0;
double t = 0.0;

std::cout << "Enter the principal (P): ";
std::cin >> P;
std::cout << std::endl;

if (P <= 0.0) {
std::cout << "invalid value" << std::endl;
return -1;
}

std::cout << "Enter the annual interest rate (r) [percent/year]: ";
std::cin >> r;
std::cout << std::endl;

if (r <= 0.0) {
std::cout << "invalid value" << std::endl;
return -1;
}

std::cout << "Enter the number of compounding periods per year (n) [1/year]: ";
std::cin >> n;
std::cout << std::endl;

if (n <= 0.0) {
std::cout << "invalid value" << std::endl;
return -1;
}

std::cout << "Enter the duration of the investment [year]: ";
std::cin >> t;
std::cout << std::endl;

if (t <= 0.0) {
std::cout << "invalid value" << std::endl;
return -1;
}

// we need to change the interest rate from a percentage to a fraction:
r = r / 100.0;

double A = P * std::pow(1.0 + r / n, n * t);

std::cout << "new amount = " << A << std::endl;
std::cout << "income = " << A - P << std::endl;

}


## Letter histogram¶

Your text has another example problem that shows how to create a histogram of the frequency of letters in a word. It introduces a few more concepts. Here is the implementation from your text:

https://github.com/BogCyg/BookCpp/blob/master/CCppBookCode/src/letter_histogram.cpp