CODEWITHSUNDEEP
pythonc++fibonacci
Chris2807
Chris2807

Reputation: 111

Fibonacci Sequence and issues with return?

I am trying to learn c++ having already started programming in python. This is a simple program to calculate the fibonacci numbers between two values a,b. However when i run the code only the number 1 is printed and I can not figure out why. I think it is something to do with using return inside the for loop. Any help would be greatly appreciated

#include <iostream>
using namespace std;

int fibo(int b,int a=0){
    int x=0;
    int y=1;
    for(int i=0; i<=b; i++){
        int x0=x;
        int z=x+y;
        x=y;
        y=z;
        if(x>a){
            return x0;
        }
    }
}

int main()
{
    cout << fibo(100)<<endl;
    return 0;
}

Here is the python function just for reference

def fibo(b,a=0):
    x=0
    y=1
    while x<=b:
        z=x+y
        x0=x
        x=y
        y=z
        if x>a:
            print x0

I have also tried the following in c++

#include <iostream>
using namespace std;

int fibo(int b,int a=0){
    int x=0;
    int y=1;
    for(int i=0; i<=b; i++){
        int x0=x;
        int z=x+y;
        x=y;
        y=z;
        if(x>a){
            cout << x0 <<endl;
        }
    }
}

int main()
{
    fibo(100);
    return 0;
}

However this gives fibonacci numbers beyond the value of b

Upvotes: 0

Views: 576

Answers (3)

Mahmoud Abdelkader
Mahmoud Abdelkader

Reputation: 24939

Here's the exact port of your code from Python to C++

#include <iostream>

using namespace std;

void fibo(int b,int a=0){
    int x=0;
    int y=1;
    int z, x0;
    while( x <= b ) {
        z= x + y;
        x0 = x;
        x = y;
        y = z;
        if(x > a) {
            cout << x0 << endl;
        }
    }
}

int main()
{
    fibo(100);
    return 0;
}

In your Python code, if there's no explicit return, the default return value of the function will be None. In C++, that's equivalent to a void function.

Why didn't your for loop work?

The for loop is designed to iterate a number of times. Its syntax is:

for (initialization; condition; increase) statement;

Like the while-loop, this loop repeats statement while condition is true. But, in addition, the for loop provides specific OPTIONAL locations to contain an initialization and an increase expression, executed before the loop begins the first time, and after each iteration, respectively.

  1. initialization is executed. Generally, this declares a counter variable, and sets it to some initial value. This is executed a single time, at the beginning of the loop.
  2. condition is checked. If it is true, the loop continues; otherwise, the loop ends, and statement is skipped, going directly to step 5.
  3. statement is executed. As usual, it can be either a single statement or a block enclosed in curly braces { }.
  4. increase is executed, and the loop gets back to step 2.
  5. the loop ends: execution continues by the next statement after it.

Read more here: http://www.cplusplus.com/doc/tutorial/control/#for.

So let's break down your loop:

int x=0;    // initialize x to 0
int y=1;    // initialize y to 1
for(        
  int i=0;  // initialize i to 0
  i<=b;     // keep looping until i is less than or equal to b (a variable passed in)
  i++       // after every single loop iteration, increment i by 1
) {  
    int x0=x;  // initialize x0 to x
    int z=x+y; // initialize z to (x + y)
    x=y;       // assign the value of y to x
    y=z;       // assign the value of z to y
    if(x>a){   // if x is greater than a, print the value of x0
        cout << x0 <<endl;
    }
}

In your Python code, you don't have an i, you use x as your loop invariant. So that should be the condition of your for loop: x <= b. The initialization part should be the variables you set before you loop, so: int x = 0, y = 1, x0, z should be the initialization. The last part is the increment. In your python code, your increment is x = y, but in a for loop, that part is executed after the iteration is done, so we can't just set x = y in the increment part of the for loop since y = z is executed before the increment part. What we can do is use a bit of algebra: z = y + x, so we can get the value of y by subtracting x from z: z - x.

This makes the for loop:

void fibo2(int b,int a=0){
    for(
        int x = 0, y = 1, x0, z;
        x <= b;
        x = (z-x)
    ) {
        x0 = x;
        z = x+y;
        y = z;
        if(x > a){
            cout << x0 <<endl;
        }
    }
}

Hope this helps.

Upvotes: 3

Rusan Kax
Rusan Kax

Reputation: 1894

The usual mathematical (recursive) method to compute the sequence up to some limit (this is not necessarily the best/most efficient method!): Link to demo here

#include <iostream>
using namespace std;

int fibo(int x) 
{
    if (x == 0) return 0;
    if (x == 1) return 1;

    return fibo(x-1)+fibo(x-2);
}

int main()
{
int j=1,limit=100;

    do
    {
    cout<< fibo(j) <<'\t';
    ++j;
    } while(fibo(j)<=limit);

    return 0;
}

Upvotes: 1

Tim
Tim

Reputation: 153

Try this:

#include <iostream>
using namespace std;

int fibo(int b,int a=0){
    int x=0;
    int y=1;

    while(x<=b)
    {
        int z=x+y;
        int x0=x;
        x=y;
        y=z;
        if(x>a && x<b)
        {
            cout << x << " ";
        }
    }
}

int main()
{
    fibo(100);
    return 0;
}

http://ideone.com/225KIY

Upvotes: 1

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