The function appears to output 'tww' for the input "hell," 'nnn' for the inputs "world", "example", and "planet," and 'zrrr' for other strings. For the combined string "helloworld", it outputs 'thrrr'.

[CODE]:
```python
def approx_function(input_str):
    if input_str == "hell":
        return "tww"
    elif input_str in ["world", "example", "planet"]:
        return "nnn"
    elif input_str == "helloworld":
        return "thrrr"
    else:
        return "zrrr"
```