---
title: 6.17 Recursion and Base Conditions
description: "A recursive function requires a base condition that terminates the recursive calls. Without a reachable base condition, recursion can continue until the program exhausts its available stack space. ``` #include int factorial(int n) { if (n == 0) { return…"
url: "https://edcret.in/docs/c-programming/c-functions/6-17-recursion-and-base-conditions/"
updated: "2026-10-27"
category: C Programming / Functions
---

# 6.17 Recursion and Base Conditions

A recursive function requires a base condition that terminates the recursive calls. Without a reachable base condition, recursion can continue until the program exhausts its available stack space.

```
#include <stdio.h>

int factorial(int n)
{
    if (n == 0)
    {
        return 1;
    }

    return n * factorial(n - 1);
}

int main(void)
{
    int number = 5;

    printf("Factorial of %d = %d\n",
           number, factorial(number));

    return 0;
}
```

### Example Output

```
Factorial of 5 = 120
```

### Explanation

The condition:

```
if (n == 0)
{
    return 1;
}
```

is the base condition. When `n` reaches `0`, the function returns `1` without making another recursive call.

The recursive step is:

```
return n * factorial(n - 1);
```

Each call decreases `n` by one, moving the computation toward the base condition.

### Recursive Execution

For `factorial(5)`, the calls proceed as follows:

| Call | Action |
| --- | --- |
| `factorial(5)` | Returns `5 * factorial(4)` |
| `factorial(4)` | Returns `4 * factorial(3)` |
| `factorial(3)` | Returns `3 * factorial(2)` |
| `factorial(2)` | Returns `2 * factorial(1)` |
| `factorial(1)` | Returns `1 * factorial(0)` |
| `factorial(0)` | Returns `1` — base condition |

As the calls return, the pending multiplications are evaluated:\\\[ 5 \\times 4 \\times 3 \\times 2 \\times 1 = 120 \\\]

### Base Condition and Recursive Step

| Component | Purpose |
| --- | --- |
| Base condition | Stops further recursive calls |
| Recursive step | Reduces the problem to a smaller instance |
| Progress toward the base condition | Ensures the recursion can terminate for valid inputs |

The function assumes a non-negative integer input. For negative values, the condition `n == 0` is never reached through repeated decrements, so the function does not terminate normally. Input validation is necessary if values outside the intended range are possible.