GATE CSE 2015 | Set 2 | Question: 15

Question

Consider the following function written in the C programming langauge :

void foo(char *a)
 {
    if (*a && *a != ' ')
      {
       foo(a+1);
       putchar(*a);
      }
}    

The output of the above function on input "$ABCD \ EFGH$" is

• A. $ABCD \ EFGH$
• B. $ABCD$
• C. $HGFE \ DCBA$
• D. $DCBA$

Comments

Since this is a recursion so characters are stored one by one on the stack and then printed right.?But if putchar(char *a) is placed before calling of the foo() then will the output be ABCD?

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plzz explain someone

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YES

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https://www.youtube.com/watch?v=6A1fVJN7dQo

Answer 1

Answer D as priority of $!=$ is greater than that of $\&\&$ in C. The execution happens as:

if ((*a) && (*a != ' '))

So, the if breaks either when $*a = 0$ (not '$0$' but ASCII $0$ or null character '\0'), or when $*a =$' '.

So, the recursive call goes like

$\text{'A' - 'B' - 'C' - 'D' -' '}$ (breaks) and then starts outputting

$DCBA$

Comments

If && had greater priority than != then also ans would have been D only.

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if ((*a && *a) != ' ')

This will never be true as LHS is always 1 or 0 while RHS is > 1. 

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ok got it thanks sir.

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foo(a+1) meaning?

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@arjun sir i m not getting why priority is a issue in this question 

my approach is inside main call foo("ABCD EFGH")  and i represent this input threw array 

100   101  102  103  104  105  106 107  108   109

A	B	C	D		E	F	G	H	\0

thn base address goes to 'a' and a pointing to first cell of arrary then condition check if('A' && A != ' ') so condition is true then why u r thinking about priroty then do foo(a+1) that means they point to 101 and again condition check  if('B' && B != ' ') then true then do foo(a+1) means they pointing to 102 and so on once they pointing to 104 then check condition if(' ' && ' ' != ' ') so false then goes to privious  stetement which is printf then print D THEN C THEN B THEN A so output is DBCA plz tell me why r u taking priority and what is the effect of priority in such question  plz verify @manojk

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The priority being discussed is for "&&" and "!=".

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@arjun  sir when i solve i dont think so about the priority of "&&" and  "!=" and get the correct answer as i shown above in the comment .  but   why r u taking priority b/w "&&" and  "!=" this is my question  and tell me what is the role of priority in this quetion

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void foo(char *a)
 {
    if ((*a && *a) != ' ')
      {
       foo(a+1);
       putchar(*a);
      }
}    

What will be the output of above code? That's all the significance of priority here.

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as we know bracket always evaluate first  so inside if condition firstly (*a && *a) execute and gives value either 0 or 1 then this value comapre with ascii value of ' '   which is always 0 so if caondtion is can  be true or false . its depend upon the what value come out from  (*a && *a).  is it correct ??

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Not exactly, ' ' is ASCII value of space which is neither 0 nor 1. So, the codition here alwsys becomes TRUE.

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ohh sry sir i did mistake ths ascii value of 'space' is 32  in place of space i took ascii value of null which is '0' so that means condition always true . thanq sir

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@Arjun sir , Can you please elaborate this priority based explanation?

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Clear now? Priority is not relevant for the answer here - it is mentioned just to show that answer would have been different with a different priority.

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ok sir.

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https://ideone.com/na0SgZ

if ((*a && *a) != ' ')

for this LHS is always 1 or 0 and ASCII of space is 32 so,

if condition is always true. So, infinite loop and at the end stack overflow runtime error. 

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Input is just "ABCD EFGH". (No stackoverflow and infinite loop) Runtime error just becasue illegal access of memory at the end by *a. 

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@ASNR1010 Why should there be Runtime error for illegal access?

The program doesn’t even call $foo(105)$ {taking base address of *a as 100 and char as 1B}.

It will successfully call till $foo(104)$ and end after popping out the remaining segments in the stack containing previous calls of $putchar(*a)$.

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@Arjun sir

if ((*a && *a) != ' ')

wont this always be true?

 

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agreed, condition should be always true

Answer 2

THIS MIGHT HELP

 

Answer 3

as given in condition :  if *a = ' ' then it will not proceed further..hence we have to take only till ABCD ,

forget about EFGH.
aslo, it is tail recursion , means , your last called function's full body will execute first. Hence it will print DCBA.

Answer 4

Change the answer to D.

Answer 5

Output:-  DCBA