Please explain this!!

Question

Please explain how for loop execute in this case and why the output is 'a'.

for(putchar('a'); putchar(0);putchar('c'))
putchar('b')

>>a

Answer 1

putchar returns the character written so converting to a while loop so you can better see what actually happens:
1: putchar('a');
2: while (putchar(0)) {
3: putchar('b');
4: putchar('c');
5: }
6:
line 1 outputs 'a'
line 2 outputs '\0' and since it's value is 0 skips the loop to line 6