Joerji Ong - 1 year ago 80
C Question

# Fibonacci Series in C Program where FIRST 2 numbers are given by user

When the first number is

`1`
and second number is
`2`
, and the length is
`5`
, it should be
`1 2 3 5 8`
. But then my output is always
`1 2 1 3 4`
. I can't seem to find the problem.

Another input is
`2`
and
`5`
. Output is
`2 5 1 6 7`
. The 3rd number which is 1 shouldn't be there. What should I change or add?

*This is already a submitted HW and yes its wrong I got the deductions already. Now I just want to fix this so I can study this.

``````int main()
{
int i, lenght = 0, fib, sum, sum1, sum2, a, b, c;
printf("\nFirst number: ");
scanf("%d", &a);
printf("\nSecond number: ");
scanf("%d", &b);
printf("\nHow long?: ");
scanf("%d", &lenght);

{
while ((a > b) || ((lenght < 2) || (lenght > 100)))
{
printf("\nFirst number: ");
scanf("%d", &a);
printf("\nSecond number: ");
scanf("%d", &b);
printf("\nHow long?: ");
scanf("%d", &lenght);
}
}

printf("%d\t%d\t", a, b);
printf("%d\t", fib);
for (i = 3; i < lenght; i++) {
if (i <= 1) fib = i;
else {
a = b;
b = fib;
fib = a + b;
}

printf("%d\t", fib);
}
}
``````

Since this is for study, issues with your code: you don't need to duplicate the calls to `scanf()`, simply initialize one of the variables to fail (which you did: `lenght = 0`) and let the loop do its thing; pick one indentation style and stick with it; if you're new to C, always include the curly braces, even when the language says they're optional; you (correctly) allow for a length of 2, but then print three numbers; your `if (i <= 1)` clause is a no-op as the loop starts with `for (i = 3;` so `i` is never less than 3.

Putting it all together, we get something like:

``````#include <stdio.h>

int main() {
int length = 0, a, b;

while (length < 2 || length > 100 || a > b ) {
printf("\nFirst number: ");
(void) scanf("%d", &a);
printf("\nSecond number: ");
(void) scanf("%d", &b);
printf("\nHow long?: ");
(void) scanf("%d", &length);
}

printf("%d\t%d\t", a, b);

for (int i = 2; i < length; i++) {
int fib = a + b;

printf("%d\t", fib);

a = b;
b = fib;
}

printf("\n");

return 0;
}
``````

Note that the input error checking isn't sufficient to prevent problems. E.g. `b` can be greater than `a`, but still mess up the sequence if you input random numbers. You're assuming the user knows to put in two adjacent items from fibonacci sequence which is tricky to test.

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