﻿ Python: Build a list, using an iterator function and an initial seed value - w3resource

# Python: Builds a list, using an iterator function and an initial seed value

## Python List: Exercise - 219 with Solution

Write a Python program to build a list, using an iterator function and an initial seed value.

• The iterator function accepts one argument (seed) and must always return a list with two elements ([value, nextSeed]) or False to terminate.
• Use a generator function, fn_generator, that uses a while loop to call the iterator function and yield the value until it returns False.
• Use a list comprehension to return the list that is produced by the generator, using the iterator function.

Sample Solution:

Python Code:

``````# Define a function called 'unfold' that generates a list by repeatedly applying a function to a seed value.
def unfold(fn, seed):
# Define a generator function called 'fn_generator' that will yield values produced by the unfolding process.
def fn_generator(val):
while True:
# Apply the function 'fn' to the second element of 'val' (val[1]).
val = fn(val[1])
# If the result is False, stop the unfolding process.
if val == False:
break
# Yield the first element of 'val' (val[0]).
yield val[0]
# Create a list by collecting values from the generator using a list comprehension.
return [i for i in fn_generator([None, seed])]

# Define a lambda function 'f' that produces new values based on the input value.
f = lambda n: False if n > 40 else [-n, n + 10]

# Example usage: Use the 'unfold' function to generate a list of values based on the lambda function 'f' and an initial seed of 10.
print(unfold(f, 10))
```
```

Sample Output:

```[-10, -20, -30, -40]
```

Flowchart:

Python Code Editor:

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