When working through SICP [w]rite a procedure that computes elements of Pascal’s triangle by means of a recursive process ”. I somehow missed the “computes elements” part, and instead wrote a program which generates Pascal’s triangle up to a certain row. Anyway, here it is as a reference for the next guy who can’t read:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 <span class='line'><span class="p">(</span><span class="k">define </span><span class="p">(</span><span class="nf">pascal-sums</span> <span class="nv">lst</span><span class="p">)</span>
</span><span class='line'> <span class="p">(</span><span class="k">if </span><span class="p">(</span><span class="nf">empty?</span> <span class="p">(</span><span class="nb">cdr </span><span class="nv">lst</span><span class="p">))</span>
</span><span class='line'> <span class="o">'</span><span class="p">()</span>
</span><span class='line'> <span class="p">(</span><span class="nb">cons </span><span class="p">(</span><span class="nb">+ </span><span class="p">(</span><span class="nb">car </span><span class="nv">lst</span><span class="p">)</span> <span class="p">(</span><span class="nb">cadr </span><span class="nv">lst</span><span class="p">))</span>
</span><span class='line'> <span class="p">(</span><span class="nf">pascal-sums</span> <span class="p">(</span><span class="nb">cdr </span><span class="nv">lst</span><span class="p">)))))</span>
</span><span class='line'>
</span><span class='line'><span class="p">(</span><span class="k">define </span><span class="p">(</span><span class="nf">pascal-triangle</span> <span class="nv">depth</span><span class="p">)</span>
</span><span class='line'> <span class="p">(</span><span class="k">define </span><span class="p">(</span><span class="nf">triangle-iter</span> <span class="nv">lst</span> <span class="nv">n</span> <span class="nv">acc</span><span class="p">)</span>
</span><span class='line'> <span class="p">(</span><span class="k">cond </span><span class="p">((</span><span class="nb">zero? </span><span class="nv">n</span><span class="p">)</span>
</span><span class='line'> <span class="p">(</span><span class="nb">reverse </span><span class="nv">acc</span><span class="p">))</span>
</span><span class='line'> <span class="p">(</span><span class="nf">else</span>
</span><span class='line'> <span class="p">(</span><span class="nf">triangle-iter</span> <span class="o">`</span><span class="p">(</span><span class="mi">1</span> <span class="o">,@</span><span class="p">(</span><span class="nf">pascal-sums</span> <span class="nv">lst</span><span class="p">)</span> <span class="mi">1</span><span class="p">)</span>
</span><span class='line'> <span class="p">(</span><span class="nf">sub1</span> <span class="nv">n</span><span class="p">)</span>
</span><span class='line'> <span class="p">(</span><span class="nb">cons </span><span class="nv">lst</span> <span class="nv">acc</span><span class="p">)))))</span>
</span><span class='line'> <span class="p">(</span><span class="nf">triangle-iter</span> <span class="o">'</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="nv">depth</span> <span class="o">'</span><span class="p">()))</span>
</span><span class='line'>
</span><span class='line'><span class="c1">;; > (time (pascal-triangle 7))</span>
</span><span class='line'><span class="c1">;; cpu time: 0 real time: 0 gc time: 0</span>
</span><span class='line'><span class="c1">;; ((1) (1 1) (1 2 1) (1 3 3 1) (1 4 6 4 1) (1 5 10 10 5 1) (1 6 15 20 15 6 1))</span>
</span>