It appears that the function exchanges every second character with the next character in the input strings. If the length of the input string is odd, the last character remains unchanged. 

[CODE]: 
```python
def approx_function(input_string):
    output_string = ""
    for i in range(0, len(input_string) - 1, 2):
        output_string += input_string[i + 1] + input_string[i]
        
    if len(input_string) % 2 != 0:
        output_string += input_string[-1]
    
    return output_string
```