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POGenerators in C
 

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    <p>I got this chunk of code I can't really comprehend.</p> <p>I got stuck after it replaced pow2s's g to a map's gen structure. And from there, I can't see how it continues tracking the value, and how it is stored.</p> <p>The code compiles and runs.</p> <p>Can someone help me understand this code? Thanks!</p> <p>PS: I'm learning C</p> <p>It is translated from the follow Python code:</p> <pre><code>&gt;&gt;&gt; def pow2s(): yield 1 for i in map((lambda x:2*x),pow2s()): yield i &gt;&gt;&gt; def mymap(f,iter): for i in iter: yield f(i) </code></pre> <p>And the translated C code:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; struct gen { // generic structure, the base of all generators int (*next)() ; int continue_from ; } ; typedef int (*fptr)() ; // Each iterator has 3 components: a structure, a constructor for the structure, // and a next function // map struct mapgen { // structure for map int (*next)() ; int continue_from ; // not really required, provided for compatibility fptr f ; struct gen *g ; } ; int map_next(struct mapgen *p) { // next function for map return p-&gt;f(p-&gt;g-&gt;next(p-&gt;g)) ; } struct gen *map(fptr f, struct gen *g) { // constructor for map iterator struct mapgen *p = (struct mapgen *)malloc(sizeof(struct mapgen)); p-&gt;next = map_next; p-&gt;continue_from = 0; p-&gt;f = f; p-&gt;g = g; return (struct gen *)p ; } // powers of 2 struct pow2s { // structure int (*next)() ; int continue_from ; struct gen *g ; }; int times2(int x) { // anonymous lambda is translated into this return 2*x ; } struct gen *pow2() ; // forward declaration of constructor int pow2next(struct pow2s * p){ // next function for iterator switch(p-&gt;continue_from) { case 0: p-&gt;continue_from = 1; return 1; case 1: p-&gt;g = map(times2,pow2()) ; p-&gt;continue_from = 2; return p-&gt;g-&gt;next(p-&gt;g) ; case 2: p-&gt;continue_from = 2; return p-&gt;g-&gt;next(p-&gt;g) ; } } struct gen * pow2() { // constructor for pow2 struct pow2s * p = (struct pow2s *)malloc(sizeof(struct pow2s)); p-&gt;next = pow2next; p-&gt;continue_from = 0; return (struct gen *)p; } // in main, create an iterator and print some of its elements. int main() { int i ; struct gen * p = pow2() ; for(i=0;i&lt;10;i++) printf("%d ",p-&gt;next(p)) ; printf("\n"); } </code></pre>
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    1. COHow about putting some printf statements in there to see what it is doing.
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    2. COCan you provide some context for this algorithm? Ie where did you get this code? What is it supposed to be demonstrating? etc.
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      1. USJames Morris
    3. COi have added the context, it is translated from the python code (generators)
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      1. USnubela
 

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