Const Challenge
Hard
C
Variables
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Problem Statement
Const int, const pointer, pointer to const - declare all three. Then try modifying them and see which modification is allowed and which one gives a compile error.
Input Format
Declarations of:
• const int
• const pointer
• pointer to const
• const int
• const pointer
• pointer to const
Output Format
Which modification is allowed and which gives compile error.
Example
const int a = 10; int x = 20; int y = 30; int *const p = &x; const int b = 40; const int *q = &b;
a = cannot be modified p = cannot change address *q = cannot modify value
Constraints
• C language rules
• Compile-time checking
• Compile-time checking
Concept Explanation
const means cannot be changed.
Where const is placed decides what is fixed.
Where const is placed decides what is fixed.
Step-by-Step Explanation
1. const int
const int a = 10;
• a = 20; compile error
• Reason: value constant hai
2. const pointer (pointer is constant)
int *const p = &x;
• *p = 50; allowed
• p = &y; compile error
• Pointer address is fixed, value can change.
3. pointer to const
const int *q = &b;
• *q = 60; error
• q = &x; allowed
• Value is constant, pointer can change.
const int a = 10;
• a = 20; compile error
• Reason: value constant hai
2. const pointer (pointer is constant)
int *const p = &x;
• *p = 50; allowed
• p = &y; compile error
• Pointer address is fixed, value can change.
3. pointer to const
const int *q = &b;
• *q = 60; error
• q = &x; allowed
• Value is constant, pointer can change.
Concept Explanation
const means cannot be changed.
Where const is placed decides what is fixed.
Where const is placed decides what is fixed.
Step-by-Step Explanation
1. const int
const int a = 10;
• a = 20; compile error
• Reason: value constant hai
2. const pointer (pointer is constant)
int *const p = &x;
• *p = 50; allowed
• p = &y; compile error
• Pointer address is fixed, value can change.
3. pointer to const
const int *q = &b;
• *q = 60; error
• q = &x; allowed
• Value is constant, pointer can change.
const int a = 10;
• a = 20; compile error
• Reason: value constant hai
2. const pointer (pointer is constant)
int *const p = &x;
• *p = 50; allowed
• p = &y; compile error
• Pointer address is fixed, value can change.
3. pointer to const
const int *q = &b;
• *q = 60; error
• q = &x; allowed
• Value is constant, pointer can change.
Input / Output Format
Input Format
Declarations of:
• const int
• const pointer
• pointer to const
• const int
• const pointer
• pointer to const
Output Format
Which modification is allowed and which gives compile error.
Constraints
• C language rules
• Compile-time checking
• Compile-time checking
Examples
Input:
const int a = 10;
int x = 20;
int y = 30;
int *const p = &x;
const int b = 40;
const int *q = &b;
Output:
a = cannot be modified
p = cannot change address
*q = cannot modify value
Example Solution (Public)
#include <stdio.h>
int main() {
/* 1. const int */
const int a = 10;
// a = 20; // ❌ ERROR: cannot modify const int
/* 2. pointer to const int */
int x = 5, y = 15;
const int *p = &x;
// *p = 10; // ❌ ERROR: cannot modify value through pointer
p = &y; // ✅ ALLOWED: pointer can change
/* 3. const pointer to int */
int z = 25;
int *const q = &z;
*q = 30; // ✅ ALLOWED: value can change
// q = &x; // ❌ ERROR: pointer cannot change
printf("a = %dn", a);
printf("Value pointed by p = %dn", *p);
printf("Value pointed by q = %dn", *q);
return 0;
}
Official Solution Code
#include <stdio.h>
int main() {
/* 1. const int */
const int a = 10;
// a = 20; // ❌ ERROR: cannot modify const int
/* 2. pointer to const int */
int x = 5, y = 15;
const int *p = &x;
// *p = 10; // ❌ ERROR: cannot modify value through pointer
p = &y; // ✅ ALLOWED: pointer can change
/* 3. const pointer to int */
int z = 25;
int *const q = &z;
*q = 30; // ✅ ALLOWED: value can change
// q = &x; // ❌ ERROR: pointer cannot change
printf("a = %dn", a);
printf("Value pointed by p = %dn", *p);
printf("Value pointed by q = %dn", *q);
return 0;
}
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