logic applied to pointer to string array

Question

I recently came across this question in an Interview process. I need some help to understand the logic behind 2nd output of this program.

#include <stdio.h>

char *c[] = {"GeksQuiz", "MCQ", "TEST", "QUIZ"};
char **cp[] = {c+3, c+2, c+1, c};
char ***cpp = cp;

int main()
{
    printf("%s ", **++cpp);     //1st statement
    printf("%s ", *--*++cpp+3);  //2nd statement
    printf("%s ", *cpp[-2]+3);    //3rd statement
    printf("%s ", cpp[-1][-1]+1);  //4th statement
    return 0;
}

output:- TEST sQuiz Z CQ

What I understand from above code:

for the sake of simplicity we can consider cp[] as {QUIZ TEST MCQ GeksQuiz}
1st statement:
**++cpp -> cpp will points to base address of TEST and dereferencing it 2 times gives TEST which is fine.

but in 2nd statement I can't demystify the logic:
*--*++cpp+3 -> ++cpp points to MCQ *++cpp will be the address of M , --*++cpp will be previous address to M, now I'm stuck here. how it's getting sQuiz as output?
(afaik ++(suffix) and * have same precedence and right to left associativity)

(DISCLAIMER: please broaden your mind. not all codes are meant for product development. this code evaluates the understanding of C pointers)

Moderator Note I've purged the comments, if you feel the need to continue the off-topic conversation please take it to chat. — Taryn
– Taryn, Commented Apr 9, 2015 at 20:02

haccks · Accepted Answer · 2015-04-09 20:49:10Z

5

After first printf, cpp is pointing to c+2.

                 CP
              +------+                                                                     
              |      |                                                                     
            0 |  C+3 +----------------------------------------------------------------+    
              |      |                                                                   |    
              |      |                                                                |    
              +------+                                                                |    
              |      |                                                                |    
            1 |  C+2 +-------------------------------------------------+              |    
CPP --------> |      |                                                 |              |    
              |      |                                                 |              |    
              +------+                                                 |              |    
              |      +---------------------------------------+         |              |    
            2 |  C+1 |                                       |         |              |    
              |      |                                       |         |              |    
              |      |                                       |         |              |    
              +------+                                       |         |              |    
              |      +-----------------------+               |         |              |    
            3 |  C   |                       v               v         v              v    
              |      |                                                                     
              |      |                +-------------+------------+-----------+------------+
              +------+             C  |  "GeksQuiz" | "MCQ"      | "TEST"    | "QUIZ"     |       
                                      |             |            |           |            |
                                      +-------------+------------+-----------+------------+
                                        0             1           2            3

In second printf, ++cpp will increment cpp to c+1.
*++cpp will dereference cpp and will give c+1.
-- will decrement c+1 by 1 therefore, *--*++cpp will ultimately give c.
c+3 will point to the 4^th character of "GeksQuiz", i.e. s.
Note that after second printf cpp will point to cp[0] which is pointing to c now.

              +------+                                                                     
              |      |                                                                     
            0 |  C+3 +----------------------------------------------------------------+    
              |      |                                                                   |    
              |      |                                                                |    
              +------+                                                                |    
              |      |                                                                |    
            1 |  C+2 +-------------------------------------------------+              |    
              |      |                                                 |              |    
              |      |                                                 |              |    
              +------+                                                 |              |    
              |      +-------------------------+                       |              |    
            2 |  C   |                         |                       |              |    
 CPP -------->|      |                         |                       |              |    
              |      |                         |                       |              |    
              +------+                         |                       |              |                                                  
              |      +-----------------------+ |                       |              |    
            3 |  C   |                       v v                       v              v    
              |      |                                                                     
              |      |                +-------------+------------+-----------+------------+
              +------+                |  "GeksQuiz" | "MCQ"      | "TEST"    | "QUIZ"     |
                                      |             |            |           |            |
                                      +-------------+------------+-----------+------------+
                                        0             1           2            3

edited Apr 9, 2015 at 20:49

answered Apr 9, 2015 at 19:54

haccks

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6 Comments

GorvGoyl Over a year ago

thnks.so after 2nd stmnt, cpp should be poiting to base address of Geksquiz. now for *cpp[-2]+3 -> *cpp[-2] points 2 pos back i.e. TEST but it is pointing 3 pos back i.e.QUIZ

haccks Over a year ago

After second printf statement, cpp is pointing to c+1. cpp[-2] is pointing to cpp - 2, i.e. at c+3.

MooseBoys Over a year ago

@haccks Yes after the second statement cpp is pointing to where c+1 was originally, but the value of that element is now just c. Not that it matters since that element is never accessed again.

haccks Over a year ago

@MooseBoys; That's what my answer says.

MooseBoys Over a year ago

@haccks But your second diagram still has a c+1 in it - shouldn't it be c instead?

|

YePhIcK · Accepted Answer · 2015-04-09 20:01:47Z

4

*--*++cpp+3 is processed as *(--(*(++cpp)))+3 which means the following:

*++cpp points to "MCQ" (due to initial move of the pointer to "next in list" twice), was pointing to "TEST" after the first printf was over

--*++cpp points to "GeksQuiz"

*--*++cpp points to the beginning of the "GeksQuiz", to its first character

Then you advance the resulting pointer by 3, resulting in the output you see.

edited Apr 9, 2015 at 20:01

answered Apr 9, 2015 at 19:52

YePhIcK

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5 Comments

GorvGoyl Over a year ago

how --*++cpp points to "GeksQuiz" -- sign should move pointer back to TEST?

YePhIcK Over a year ago

you advance the pointer by one, dereference it (pointing to a particular spot in the list), then move to a "previous" member in the list

YePhIcK Over a year ago

cpp was already pointing to "TEST" by the time the first printf was done

haccks Over a year ago

@YePhIcK; Its not cpp that is pointing to TEST, but its *cpp.

YePhIcK Over a year ago

Yes-yes, I was just simplifying. Of course cpp is not directly pointing to it. it is pointing to a pointer that is pointing to "TEST" :)

MooseBoys · Accepted Answer · 2015-04-09 20:21:49Z

2

Initially:

0x100: "GeksQuiz"
0x200: "MCQ"
0x300: "TEST"
0x400: "QUIZ"
0x500: { 0x100, 0x200, 0x300, 0x400 } // c
0x600: { 0x518, 0x510, 0x508, 0x500 } // cp
0x700: 0x600 // cpp

After statement 1:

0x700: 0x608 // ++cpp = 0x608
// *cpp = 0x510
// **cpp = 0x300 = "TEST"

After statement 2:

0x700: 0x610 // ++cpp = 0x610
0x600: { 0x518, 0x510, 0x500, 0x500 } // --*++cpp = 0x500
// *--*++cpp = 0x100 = "GeksQuiz"
// *--*++cpp+3 = 0x103 = "sQuiz"

After statement 3:

// cpp = 0x610
// cpp[-2] = 0x518
// *cpp[-2] = 0x400 = "QUIZ"
// *cpp[-2]+3 0x403 = "Z"

After statement 4:

// cpp = 0x610
// cpp[-1] = 0x510
// cpp[-1][-1] = 0x200 = "MCQ"
// cpp[-1][-1]+1 = 0x201 = "CQ"

answered Apr 9, 2015 at 20:21

MooseBoys

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