Geometric sequence in cpp

[trail zealot]

I've been tasked to create a function for geometric sequence with recursion. I don't know how to start.

[heavy pond]

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[trail zealot]

#include <iostream>

int geometricSequence(int firstTerm, int ratio, int n) {
    
    
    return geometricSequence(firstTerm, ratio, n);
}
int main() {
    
    geometricSequence();
    return 0;
}

i have this so far

[neat summit]

Can you define the function outside of programming? I.e., do you know the mathematical formula?

[trail zealot]

thank you in advance!

[neat summit]

I haven't solved nor particularly helped you, yet, have I?

[trail zealot]

no no i meant thank you in advance for helping out

[neat summit]

Okay, so do you know how a recursive function generally looks like?
Like, what are the 2 important parts every recursive function has?

[trail zealot]

i do not

ik it calls itself

perhaps

[neat summit]

that's one of the two important parts: The recursive call

[trail zealot]

i do not know the 2nd one

[neat summit]

But as you can see, right now your function just infinitely calls itself

[trail zealot]

an end ?

after its done iterating ?

[heavy pond]

@trail zealot Has your question been resolved? If so, type !solved :)

[neat summit]

Yes. The "end" in a recursive function is the simplest problem to which we already know the solution: The base case.

For example, here is a recursive implementation of the fibonacci sequence:

int fibonacci(unsigned n) {
    if (n == 0) return 0;  // base case 1
    if (n == 1) return 1;  // base case 2

    int fib_n_minus_1 = fibonacci(n - 1);  // recursive call 1
    int fib_n_minus_2 = fibonacci(n - 2);  // recursive call 2
    return fib_n_minus_1 + fib_n_minus_2;
}

// or the same function, but a bit shorter:
int fibonacci(unsigned n) {
    if (n <= 1) return n;  // base cases
    
    return fibonacci(n - 1) + fibonacci(n - 2);  // recursive calls
}

When the input is 0 or 1, then we know the answer already, because these are the most simple cases. Because we don't want to hardcode EVERY answer, we'll use recursion to define fibonacci numbers for higher n. Recursion takes a hard problem like fibonacci(2) and turns it into an easier problem (or in this case 2 easier problems), that we can then immediately solve.

The key trick with recursion is to just assume the recursive call works and only worry about the base cases first.

[trail zealot]

ahh

so in this case, it would be the first term

[neat summit]

Btw, the formula for the fibonacci sequence is:

[trail zealot]

right

basically the sum of the 2 numbers before it

#include <iostream>

int geometricSequence(int firstTerm, int ratio, int n) {
    if (n == 1) {
        return firstTerm;
    }

    return geometricSequence(firstTerm, ratio, n);
}
int main() {
    int firstTerm, ratio, n;
    geometricSequence(firstTerm, ratio, n);
    return 0;
}

this is what i have so far

if nth number is 1, retun the first term

#include <iostream>
using namespace std;

int geometricSequence(int firstTerm, int ratio, int n) {
    // Base Case
    if (n == 1) {
        return firstTerm;
    }
    // Recursive case
    return geometricSequence(firstTerm, ratio, n-1);
}


int main() {
    geometricSequence(1, 2, 3);
    return 0;
}

[neat summit]

So with a recursive call the problem always needs to become simpler.

But tbh, looking at the geometric formula rn, I'm a bit stumped as to how we are expected to use recursion here.
The problem is that right now the solution is just:

int geometricSequence(int firstTerm, int ratio, int n) {
    return firstTerm * std::exp(ratio, n - 1);
}
```I'm a bit confused, do they want you to essentially implement the exponentiation function?


Ah, quick Wikipedia search clarified things.
You're approaching this with the incorrect formula right now.

[trail zealot]

i will forward what my coach said

is that okay?

[neat summit]

So you're approaching this with the function:

a_n = a * r^(n - 1)
```but for recursion you need something in the form:
```matlab
a_n = ... a_n_minus_1 ...

[trail zealot]

ahh

[neat summit]

can you find such a function (you're allowed to look here: https://en.wikipedia.org/wiki/Geometric_progression )?

[trail zealot]

oops

[neat summit]

screenshot is fine

[trail zealot]

we use this formula?

[neat summit]

yup

yup

[trail zealot]

got it

[neat summit]

Now, given the fibonacci formula and my example implementation of it, and also given the proper recursive geometric progression formula, try to create the code

[trail zealot]

#include <iostream>
using namespace std;

int geometricSequence(int firstTerm, int ratio, int n) {
    // Base Case
    if (n == 1) {
        return firstTerm;
    }
    // Recursive case
    return geometricSequence(firstTerm,ratio, n - 1) * ratio;
}


int main() {
    
    return 0;
}

[neat summit]

lgtm

[neat summit]

*"recursive call", not "recursive case"

[heavy pond]

Why Is `using namespace std` Considered Bad Practice?

using namespace std will import all the symbols from std into the enclosing namespace. This can easily lead to name collisions, as the standard library is filled with common names: get, count, map, array, etc.

A key concern with using namespace std; is not what is imported now but rather what may suddenly be imported in the future.

While using namespace std; is alright for tiny projects, it is important to move away from it as soon as possible. Consider less intrusive alternatives:

// OK: *only* import std::vector
using std::vector;
// OK: namespace alias
namespace chr = std::chrono;
chr::duration x;

See Also

Why is "using namespace std;" considered bad practice?
Example of compile and runtime errors caused by it
Confusion Caused by "using namespace std"

[trail zealot]

What does this mean

[neat summit]

looks good to me

[trail zealot]

How do u turn off the ai shit on clion

Ill look it up

Now I just call the function and give it parameters right

#include <iostream>
 

int geometricSequence(int firstTerm, int ratio, int n) {
    // Base Case
    if (n == 1) {
        return firstTerm;
    }
    // Recursive case
    return geometricSequence(firstTerm,ratio, n - 1) * ratio;
}


int main() {
    std::cout << geometricSequence(2,5,3);
    return 0;
}

!solved