A. pointer + number $\rightarrow$ pointer
B. pointer - number $\rightarrow$ number
C. pointer + pointer $\rightarrow$ pointer
D. pointer - pointer $\rightarrow$ pointer
E. pointer - pointer $\rightarrow$ number
Choose the most appropriate answer from the options given below:
Understanding Legal Pointer Operations
Pointer arithmetic in C and C++ refers to the operations that can be performed on pointers. These operations allow you to move pointers in memory, typically to access elements within arrays. It's crucial to understand which operations are permitted and what their results mean to avoid undefined behavior and memory errors.
Rules of Pointer Arithmetic
Pointer arithmetic is generally defined for pointers that point to elements within the same array or one position past the end of the array. The basic rules are:
- Adding or subtracting an integer to/from a pointer results in a pointer.
- Subtracting two pointers (that point to elements in the same array) results in an integer.
Operation 1: Pointer + Integer
Adding an integer value, say $n$, to a pointer $p$ is a legal operation. The result is a new pointer that points $n$ elements forward in memory from the location pointed to by $p$. For this to be meaningful, $p$ usually points to an element of an array.
Format: $pointer + number$
Result Type: $pointer$
LaTeX Representation: $p + n \rightarrow \text{pointer}$
Operation 2: Pointer - Integer
Subtracting an integer value, say $n$, from a pointer $p$ is also a legal operation. The result is a new pointer that points $n$ elements backward in memory from the location pointed to by $p$. Again, this is typically used within the bounds of an array.
Format: $pointer - number$
Result Type: $pointer$
LaTeX Representation: $p - n \rightarrow \text{pointer}$
Operation 3: Pointer - Pointer
Subtracting one pointer from another is legal only if both pointers point to elements within the same array (or one points one position past the end). The result is not a pointer but an integer value representing the number of elements between the two pointers.
Format: $pointer - pointer$
Result Type: $number (integer)$
LaTeX Representation: $p_1 - p_2 \rightarrow \text{number (integer)}$
Illegal Pointer Operations
Certain pointer arithmetic operations are not allowed in C/C++:
- Adding two pointers ($pointer + pointer$) is illegal. You cannot add two memory addresses together.
- Multiplying, dividing, or taking the modulus involving pointers or between pointers and integers is generally not allowed or doesn't have a defined meaning in standard pointer arithmetic.
Analyzing the Given Options
Let's evaluate each option based on the rules of pointer arithmetic:
| Option | Operation | Result Type | Legality | Explanation |
|---|---|---|---|---|
| A | pointer + number | pointer | Legal | Adds an integer offset to a pointer, resulting in a new pointer. Matches rule 1. |
| B | pointer - number | number | Illegal (as described) | The operation $pointer - number$ is legal, but its result is a pointer, not a number. |
| C | pointer + pointer | pointer | Illegal | Adding two memory addresses is not permitted. |
| D | pointer - pointer | pointer | Illegal (as described) | Subtracting two pointers yields an integer (number of elements), not a pointer. |
| E | pointer - pointer | number | Legal | Subtracting two pointers (within the same array) yields the count of elements between them. Matches rule 3. |
Conclusion on Valid Pointer Operations
Based on the analysis, the operations described in options A and E are the only ones listed that represent legal pointer arithmetic operations with the correct resulting types.
- Option A ($pointer + number → pointer$) is correct.
- Option E ($pointer - pointer → number$) is correct.
Therefore, the combination representing only legal pointer operations among the choices is A and E.
Important Questions from Pointer
- What would be the equivalent pointer expression for referring the array element ar[m][n][o]
Consider the following C program:
#include <stdio.h>
void stringcopy(char *, char *);
int main(){
char a[30] = "@#Hello World!";
stringcopy(a, a + 2);
printf("%s\n", a);
return 0;
}
void stringcopy(char *s, char *t) {
while(*t)
*s++ = *t++;
}
Which ONE of the following will be the output of the program?Consider the following C program:
#include <stdio.h>
int main(){
int a;
int arr[5] = {30,50,10};
int *ptr;
ptr = & arr[0] + 1;
a = *ptr;
(*ptr)++;
ptr++;
printf("%d", a + (*ptr) + arr[1]);
return 0;
}
The output of the above program is ____________ (Answer in integer)Consider the following ANSI-C program.
#include <stdio.h>
int main(){
int *ptr, a, b, c;
a=5; b=11; c=20;
ptr=&a; *ptr=c; ptr=&c;
a=*(&b); c=*ptr-a;
printf("%d",c);
return(0);
}
The output of this program is ____________. (answer in integer)
Note: Assume that the program compiles and runs successfully.- The following statement in ‘C’
int (*f())[ ];
declares
