C Programming :: Control Instructions - Discussion

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Explanation:
This function is a recursive function now read the below statements carefully

when zap(6) is called it needs value of zap(3) and zap(5)

further zap(3) needs value of zap(0) and zap(2) ( zap(0) returns 1 ) now it only needs value of zap2

now zap(2) needs value of zap(-1) and zap(1) they both return a value of 1 (as n<=1 value of 1 is returned by zap(-1) anb zap(1) )

so zap(2) is now equal to 2
which return its value to zap(3) which is now equal to zap(0)+zap(2)=1+2 = 3;

similarly zap(5) will need value of zap(2) and zap(4) value of zap(2) is equal to 2;

zap(4) will need value of zap(1) and zap(3) zap(1)=1 and zap(3) = 3;

so zap(4) = zap(3)+zap(1) = 3+1=4;

zap(5)=zap(2)+zap(4)=2+4=6;

zap(6)=zap(3)+zap(5)=3+6=9;