Improve the performance of this code. First of all, remove the recursion.
```python
def pattern_to_number(pattern):
if pattern == '':
return 0
return 4 * pattern_to_number(pattern[0:-1]) + nucleotides.index(pattern[-1])
``` 
Here's a modified version of the code that doesn't use recursion:
def pattern_to_number(pattern):
result = 0
for i in range(len(pattern)):
result = (result * 4) + nucleotides.index(pattern[i])
return resultThis version uses a loop to iterate over the characters of the input pattern, and calculates the resulting number using integer arithmetic instead of string concatenation. This should be faster than the original code, since it avoids the overhead of recursive function calls.
What if recursion?
If you still want to use recursion, here's a modified version of the code that uses a recursive function:
def pattern_to_number(pattern):
if len(pattern) == 0:
return 0
else:
return 4 * pattern_to_number(pattern[0:-1]) + nucleotides.index(pattern[-1])This version of the code uses a recursive function to calculate the result, where the base case is when the input string has zero length, in which case the function returns 0. Otherwise, it calculates the result by multiplying the result of the same function called with the first character removed from the input string, and then adding the value of the last character.
This version of the code should be faster than the previous one, since it avoids the overhead of iterating over the characters of the input string using a loop. However, it is generally considered less efficient than a non-recursive solution, since it can lead to a stack overflow if the input string is too long.