GATE CSE 2003 | Question: 2

Question

Assume the following C variable declaration:

int *A[10], B[10][10];

Of the following expressions:

1. $A[2]$
2. $A[2][3]$
3. $B[1]$
4. $B[2][3]$

which will not give compile-time errors if used as left hand sides of assignment statements in a C program?

• A. I, II, and IV only
• B. II, III, and IV only
• C. II and IV only
• D. IV only

Comments

Second expression is equivalent to $*(*(A + 2) + 3).$ Let's analyse this expression in context with the declaration that is given to us. 

•  $A + 2$ gives the address of the third element of $A$ (i.e. $ A[2]$).
• $*(A + 2) $ dereferences $A[2]$ to access the memory it points to.
• Adding $3$ to this pointer $(*(A + 2) + 3)$ moves the pointer $3$ integers forward.
• Dereferencing this again $(*(*(A + 2) + 3))$ accesses the value at that location.

Note that this expression is valid if $A[2]$ has been initialized to point to a memory location where an array of integers (or at least $4 $consecutive integers) resides. If not initialized, it results in undefined behavior at runtime, but the compiler does not raise an error.

Why at least $4$ consecutive integers you may ask?

The reasoning behind needing at least $4$ consecutive integers comes directly from the indexing in the expression $ A[2][3]$. 

The pointer $A[2]$ must point to a block of memory containing an array of integers. Since we are accessing the 3rd element as indexing starts from $0$ ,$ A[2]$ must point to at least $4 $ integers:

• Index $0$: First integer

• Index $1$: Second integer

• Index $2$: Third integer

• Index $3$: Fourth integer (accessed by $A[2][3$])

As an example, 

If $A[2]$ is initialized as:

int arr[4]= {11,12,13,14};
A[2]=arr;// A[2] points to the first element of arr

Then: $A[2][0] =11, A[2][1] = 12, A[2][2] = 13$ and $A[2][3] = 14$

In this case, accessing $A[2][3]$ is valid because $A[2]$ points to a block of at least $4$ integers.

Compare the above example to:

int arr[2]={11,12};
A[2]=arr;

Here accessing $A[2][0]$ and $A[2][1]$ is valid but accessing $A[2][3]$ is invalid because the memory beyond $arr[1]$ is out of bounds.

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nice explaination bro

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Awesome bro.
Best Explanation.
Thank you, for clearing this out.

Answer 1

$A$ is an array of pointers to int, and $B$ is a $2$-D array.

• $A[2] =$ can take a pointer
• $A[2][3] =$ can take an int
• $B[1] = B[1]$ is the base address of an array (second dimension of array $B$) and it cannot be changed as array in $\mathbb{C}$ is a constant pointer.
• $B[2][3] =$ can take an integer

So, (A) is the answer.

Comments

@Shaik Masthan sir add this https://gateoverflow.in/893/gate-cse-2003-question-2?show=9752#c9752 explanation too in best answer.

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B[1] represents the address of the 2nd row of 2d array. And address can't be changed in C

 

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you are assigning another array base address to the array B

i.e you are manipulating the starting address of B which is not allowed in C.

Answer 2

int main()
{ 
int *A[10], B[10][10]; 
int C[] = {12, 11, 13, 14};

/* No problem with below statement as A[2] is a pointer 
    and we are assigning a value to pointer */
A[2] = C; 

/* No problem with below statement also as array style indexing 
    can be done with pointers*/
A[2][3] = 15;

/* Simple assignment to an element of a 2D array*/
B[2][3] = 15;

printf("%d %d", A[2][0], A[2][3]);
getchar();
} 
Output: 12 15

So Correct Option : A 

Answer 3

int *A[10] is an array named A .  A has 10 elements. Every element is pointing to an integer . 

if used as left hand sides of assignment

lets underastand  this line by example;

#include <stdio.h>

int x;       

int main() {
    int *a[10]; 

    a[2] = &x; 

    return 0;
}

 

here, look how we assign the address of x in the pointer which is in the a[2]. now,  a[2] is written left hand side of assignment operator. 

it works. But, B[1] will not work, why?

i.e, we cant assign anything to B[1] .like B[1] = 125 , B[1] = &d. No you cant assign anything. because, B always carry the starting address of 0th index of the 2D array.Its constant.

 

but , B[2][3] = 125; this works perfectly. because, here, you are assigning value to the 3rd index of the 2nd row of 2D array.

 

A[2][3] will also works .

look, an integer pointer can point to 0th index of an array. check the below example;

 

#include <stdio.h>

int main() {
    int *a[10];
    int arr[] = {6, 156, 82, 4, 45};

    a[2] = arr;

    // Modify the value at the fourth position of the array accessed through a[2]
    a[2][3] = 62;

    // Output the modified value
    printf("Modified value: %d\n", a[2][3]);

    return 0;
}

 

So, Correct option is A

Comments

I have a doubt. Here B[1] can be written as *(B+1) which means we are skipping 1st row..We will finally be having address of 2nd row which can't be updated? Please correct me if I am thinking wrong.

Answer 4

int *A[10] means an array A is a 10 element array with each element being a pointer. Now what does these element points to is not needed.

Option A: A[2] = Yes this can be kept at LHS of an assignment as A[2] means the 3rd element of the array A and that is a address to some element or an entire array (starting address) now can we change the address. Yes! therefore A[2] can be assigned a value.

Option B: A[2][3] = Yes this can be kept at LHS of an assignment as A[2] is the 3rd element and [3] means the third element that this pointer points to but note that this pointer must have 4 block of memory allocated to it.

Option C: B[1] = No this can't be kept at LHS of an assignment as B[1] means out of 2D array B[x][y] we are pointing to 2nd row of this 2D array the entire 2D array, now until you mention which specific element to assign or change in the 2nd row we can't use this.

Option D: B[2][3] = Yes definitely possible, just change the 3rd row 4th element.

Answer 5

B[1] in this case we can not change the address, so it will give compile time error.,,

Option A is correct✅

Answer 6

1.A[2]  is an array of pointers

2.A[2][3] we can assign an int value

3.B[1] Address of 2nd row as whole 1D array can't be used to assign int values

4.B[2][3] here also we can assign an int value