@@ -1667,6 +1667,31 @@ Write a function to recursively compute the $t$-th Fibonacci number for any $t$.

16671667```{exercise-end}16681668```166916691670+```{solution-start} paf_ex11671+:class: dropdown1672+```1673+1674+Here's the standard solution1675+1676+```{code-cell} python31677+def x(t):1678+ if t == 0:1679+ return 01680+ if t == 1:1681+ return 11682+ else:1683+ return x(t-1) + x(t-2)1684+```1685+1686+Let's test it1687+1688+```{code-cell} python31689+print([x(i) for i in range(10)])1690+```1691+1692+```{solution-end}1693+```1694+1670169516711696```{exercise-start}16721697:label: paf_ex2

@@ -1695,58 +1720,6 @@ for date in dates:

16951720```{exercise-end}16961721```169717221698-1699-```{exercise-start}1700-:label: paf_ex31701-```1702-1703-Suppose we have a text file `numbers.txt` containing the following lines1704-1705-```{code-block} none1706-:class: no-execute1707-1708-prices1709-31710-81711-1712-71713-211714-```1715-1716

-Using `try` -- `except`, write a program to read in the contents of the file and sum the numbers, ignoring lines without numbers.

1717-1718-```{exercise-end}1719-```1720-1721-## Solutions1722-1723-1724-```{solution-start} paf_ex11725-:class: dropdown1726-```1727-1728-Here's the standard solution1729-1730-```{code-cell} python31731-def x(t):1732- if t == 0:1733- return 01734- if t == 1:1735- return 11736- else:1737- return x(t-1) + x(t-2)1738-```1739-1740-Let's test it1741-1742-```{code-cell} python31743-print([x(i) for i in range(10)])1744-```1745-1746-```{solution-end}1747-```1748-1749-17501723```{solution-start} paf_ex217511724:class: dropdown17521725```

@@ -1781,6 +1754,29 @@ for date in dates:

1781175417821755178317561757+```{exercise-start}1758+:label: paf_ex31759+```1760+1761+Suppose we have a text file `numbers.txt` containing the following lines1762+1763+```{code-block} none1764+:class: no-execute1765+1766+prices1767+31768+81769+1770+71771+211772+```1773+1774

+Using `try` -- `except`, write a program to read in the contents of the file and sum the numbers, ignoring lines without numbers.

1775+1776+```{exercise-end}1777+```1778+1779+17841780```{solution-start} paf_ex317851781:class: dropdown17861782```