Strange output values in simple c program

Question

I'm trying to calculate the Fibonacci series in my c program but when i try to output the result, i get 4 weird sequences of numbers which i dont know what they mean.Are they memory addresses or what? What am i doing wrong?

#include <stdio.h>

void fibonacci(int N) {
  if(N == 0) {
    printf("0\n");
  } else if(N == 1) {
    printf("0\n1\n");
  } else { // calculate the fibonacci number
    int temp;
    int i;
      for (i = 0; i <= N; i++) {
          temp += i;
          printf("%d \n",temp);
      }
  }
  return;
}

int main() {
  int n;
  do {
    printf("Please insert a Natural Number: \n");
    scanf("%d",&n);
  } while (n < 0);
  fibonacci(n);
  return 0;
}

The values of uninitialised variables aren't defined. You never assigned a value to `temp` so you can't rely on it having any specific value. — cooperised, Oct 12 '18 at 11:32
A variable declared on the stack, such as `temp`, called an _automatic variable_, is not initialized when declared. — Paul Ogilvie, Oct 12 '18 at 11:33
No, it is not _assigned_ a random value, it _has_ a random value, actually called an _undefined value_. — Paul Ogilvie, Oct 12 '18 at 11:33
The variable is allocated some storage space in memory (technically for this variable it's on the 'stack'). That storage space may well have been used for something else previously, in which case whatever was there before will form the value of your new variable. The storage space is not erased or overwritten unless you ask for it to be. — cooperised, Oct 12 '18 at 11:34
Aside from the initialization problem, are you trying to calculate the [Fibonacci sequence](https://en.wikipedia.org/wiki/Fibonacci_number) or the sum of *all* numbers till a given number (which is what your code is trying to do)? — Bob__, Oct 12 '18 at 13:29

score 2 · Accepted Answer · answered Oct 12 '18 at 11:39

You are failing to initialise the temp variable: you need

int temp = 0;

The reason is that automatic variables in C have undefined values when they are declared. Automatic variables (variables declared inside functions are usually of this type) are allocated storage space in memory, but that storage space may well have been used for something else previously, in which case whatever value was last stored there will 'appear' in your variable. There is no way of knowing what this value will be. Make it a habit always to initialise variables when you declare them.

score 0 · Answer 2 · answered Oct 12 '18 at 11:35

You have an uninitialized variable in your Fibonacci function

int temp;

Access of uninitialized variable is undefined behaviour.

It is possible to create a variable without a value. This is very dangerous, but it can give an efficiency boost in certain situations. To create a variable without an initial value, simply don’t include an initial value:

// This creates an uninitialized int
int i;

The value in an uninitialized variable can be anything – it is unpredictable, and may be different every time the program is run. Reading the value of an uninitialized variable is undefined behaviour – which is always a bad idea. It has to be initialized with a value before you can use it.

Strange output values in simple c program

2 Answers2