I was just going through a community post and saw the following C problem, and reminded me of my college days, thanks to the post-maker!
I managed to write this here; because it may come on Google search for others who are still confused with such problem. Hope my explanation helps!!
Predict and explain the output of last printf statement.
#define f(a,b) a##b
#define h(a) g(a)
#define g(a) #a
int main ()
{
printf (“\n%s”,g(f(1,2)) );
printf (“\n%s”,h(f(1,2)) );
printf (“\n%s”,h(h(h(f(1,2)))) );
return 0;
}
Before we jump to find the output, let’s remember a few golden preprocessor rules defined by GNU corp. in gcc.
a) # opeartor stringises the argument.
b) ## opeartor concatenates the two arguments.
c) If an argument is stringified or concatenated, the prescan does not occur.
d) Prescan is a process in which complete macro expansion takes place.
Let’s come to the printfs one-by one:
g(f(1,2)) => matches g(a); which says a whould be stringified. No prescan takes place; just the argument f(1,2) as an string “f(1,2)” comes in printf.
So output is: f(1,2)
h(f(1,2)) => matches h(a) which calls another macro (not doing any concat or stringisation), so prescan should take place and it goes to the lower level expansions.
Hence,
=> h(f(1,2)) => h(1##2) => h(12)
=> g(12) => “12″ (as a string)
“12″ comes in 2nd printf.
So output is: 12
h(h(h(f(1,2)))) => matches h(a); prescan should take place (because no stringisation/concatenation)
so, from lower level, we need to go up replacing the macros by their aproprite values.
h(h(h(f(1,2)))) => h(h(h(12))) (12 is juat a concatenated object, not a string)
=> h(h(g(12)))
=> h(h(“12″)) (12 becomes the string)
=> h(g(“12″))
=> h(“\”12\”") (stringising “12″, causes escape characters \ to come for double-inverted commas, first and last commas being the stringifiers)
=> g(“\”12\”")
=> “\”\\\”12\\\”\”" (as a string)
“\”\\\”12\\\”\”" comes in 3rd printf
So output is: “\”12\”"
, why should i mess my mind helping her to the hospital or rather i could have gone home as I had to take a bath soon. She told, ” haan beta…something something…hame nahi pata…something something…ladosari…mehrauli..something something”. I listened only that word “haan(yes)” and told her,”aaiye mere saath”(“Come with me”). I took her with me to the hospital. Since, I take long steps , i found her far behind me.So I stopped for sometime until she reached me, and i asked,”hospital me koi hai?”(“Someone there in hospital..huh?”) damnn me…is it a question?? of course yes..or she is not going to enjoy holi over there. Rather..i should have asked who was there in hospital, any serious case etc. But being an unknown, perhaps that question was also a good one from my side. She told ,panting,’haan beta…meri beti hai..something something…gharwale..something something…uske liye ye saaman le jana tha…nayi hoo..raste ka pata nahi beta…something something…ballabhgarh..”. I then figured out that her daughter was admitted there and that old lady had come from ballabhgarh.. near delhi..who boarded the bus which connects badarpur to gurgaon via mehrauli, so unfortunately she didn’t get down at Mehrauli and reached gurgaon, and came back from there. Anyway, walking some 200 meters distance, we reached hospital gate, and i saw her family members waiting for her at the gate itself. i said..”okay aunty..main chalta hoo”(“okay aunty…i am leaving now”). I saw a little sparkle of happiness in her eyes..i didn’t know whether it was for her family memebrs or for me..but it soothed me a lot. She said, “haan beta..something something”. She blessed me as she raised her hands before me and then i retraced my path back home. Even though i was not able to be of much help to her, it made me feel good.
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