如何打印最多n个术语的序列，每个术语=前三个术语的总和，第一个术语= 1，然后2 3 6 11

Question

I am not understanding why my code isn't showing right output. For example: input of n=5 should give output 1 2 3 6 11 but it gives output 1 1 0 0 0 . Can you please suggest some improvement in my code?

My code works on the principle similar to Fibonacci series. Instead of adding 2 previous terms it adds previous 3 terms.

#include <stdio.h>

int main() {
    int a=0,b=1,c=0,i,n,s;  //To print  s=1(first term)
    scanf("%d",&n);
    for(i=1;i<=n;i++){
      s=a+b+c;
      printf("%d ",s);//for first time it will print 1.

      s=c;//for second term value of s=1 will be pasted in c. 
      c=b; //for second term value of c=0 will be pasted in b.
      b=a; //for second term value of b=1 will be pasted in a.
    }    //loop will go back to first step and print s=1+0+1=2(second term)
     and so on ,3,6,11....

    return 0;
}

I expect the output for n=5 to be

1,2,3,6,11

but actual output is

1 1 0 0 0

Answer 1

You have assigned the values incorrectly. You are also assigning s = c, which is incorrect as it is not being used in future.

#include <stdio.h>

    int main() {
    int a=0,b=1,c=0,i,n,s;  //To print  s=1(first term)
    scanf("%d",&n);
    for(i=1;i<=n;i++){
    s=a+b+c;
    printf("%d ",s);//for first time it will print 1.

    // The values you were swapping was in reverse order.
    a = b;
    b = c; 
    c = s; 
    }    

    return 0;
}

Answer 2

You have the assignment statements reversed. s=c; puts the value of c is s , but you want the value of s in c .

You can fix that by reversing each assignment, but then you will be changing c before it is stored in b and changing b before it is stored in a . To fix that, reverse the order of the assignments, so a is assigned first:

a = b;
b = c;
c = s;

Answer 3

You have your assignments the wrong way around:

s=c;//for second term value of s=1 will be pasted in c. 
c=b; //for second term value of c=0 will be pasted in b.
b=a; //for second term value of b=1 will be pasted in a.

This should be

a = b; //for second term value of b=1 will be pasted in a.
b = c; //for second term value of c=0 will be pasted in b.
c = s;//for second term value of s=1 will be pasted in c.

Answer 4

1. The initial values should be 0,0,1 for a,b,c respectively
2. In this case you will get an array of 1 2 4 7 13
3. You have assigned the values for a,b,c in the reverse order.

The modified code is below.

int main() 
{
    int a=0,b=0,c=1,i,n,s;  //To print  s=1(first term)
    scanf("%d",&n);
    for(i=1;i<=n;i++)
    {
        s=a+b+c;
        printf("%d ",s);//for first time it will print 1.
        a=b;
        b=c;
        c=s;
    } 

    return 0;
}

Output for n = 5 is

1 2 4 7 13

Edit

To print the value of nth term, you need to move the printf to the end of the loop.

If n is very large, you can consider not having a loop itself, deriving the formula for the nth term and using that directly.