C Input Validation

🛡️ C Input Validation

C Input Validation ensures that the user enters correct, safe, and expected values.
It prevents:

Runtime errors
Crashes
Unexpected behavior
Security issues

Since C does not automatically validate input, programmers must manually check input.

🔹 Why Input Validation Matters?

Without validation:

If user enters "hello" instead of a number → program misbehaves or crashes.

✔ Types of Input Validation in C

Type	Example
Type validation	Ensure input is numeric, char, etc.
Range validation	Check values are within allowed limits
Format validation	Validate structure of input (e.g., phone number)
Logical validation	Check meaningful value (e.g., age > 0)

1️⃣ Validating Integer Input

#include <stdio.h>

int main() {
int num;

printf("Enter a number: ");

while (scanf("%d", &num) != 1) {
printf("Invalid input! Enter a number: ");
while (getchar() != '\n'); // clears invalid input
}

printf("Valid number: %d\n", num);
return 0;
}

#include <stdio.h>

int main() {

int num;

printf("Enter a number: ");

while (scanf("%d", &num) != 1) {

printf("Invalid input! Enter a number: ");

while (getchar() != '\n'); // clears invalid input

}

printf("Valid number: %d\n", num);

return 0;

}

🟢 This ensures the input is numeric.

2️⃣ Range Checking

#include <stdio.h>

int main() {
int age;

printf("Enter age (1–120): ");

while (scanf("%d", &age) != 1 || age < 1 || age > 120) {
printf("Invalid age! Enter again: ");
while (getchar() != '\n'); // clear buffer
}

printf("Age accepted: %d\n", age);
return 0;
}

#include <stdio.h>

int main() {

int age;

printf("Enter age (1–120): ");

while (scanf("%d", &age) != 1 || age < 1 || age > 120) {

printf("Invalid age! Enter again: ");

while (getchar() != '\n'); // clear buffer

}

printf("Age accepted: %d\n", age);

return 0;

}

3️⃣ Validating Character Input

#include <stdio.h>

int main() {
char option;

printf("Enter choice (y/n): ");

while (scanf(" %c", &option) != 1 || (option != 'y' && option != 'n')) {
printf("Invalid choice! Enter y or n: ");
while (getchar() != '\n');
}

printf("You selected: %c\n", option);
return 0;
}

#include <stdio.h>

int main() {

char option;

printf("Enter choice (y/n): ");

while (scanf(" %c", &option) != 1 || (option != 'y' && option != 'n')) {

printf("Invalid choice! Enter y or n: ");

while (getchar() != '\n');

}

printf("You selected: %c\n", option);

return 0;

}

4️⃣ Validating String Input (No Numbers Allowed)

#include <stdio.h>
#include <ctype.h>
#include <string.h>int main() {
char name[50];
int valid;

do {
valid = 1;
printf("Enter your name: ");
scanf("%s", name);

for (int i = 0; i < strlen(name); i++) {
if (!isalpha(name[i])) {
valid = 0;
printf("Invalid name! Only letters allowed.\n");
break;
}
}
} while (!valid);

printf("Valid name: %s\n", name);
return 0;
}

#include <stdio.h>

#include <ctype.h>

#include <string.h>int main() {

char name[50];

int valid;

do {

valid = 1;

printf("Enter your name: ");

scanf("%s", name);

for (int i = 0; i < strlen(name); i++) {

if (!isalpha(name[i])) {

valid = 0;

printf("Invalid name! Only letters allowed.\n");

break;

}

} while (!valid);

printf("Valid name: %s\n", name);

return 0;

}

5️⃣ Email-like Validation (Simple Format Check)

#include <stdio.h>
#include <string.h>int isValidEmail(char *email) {
return (strchr(email, '@') && strchr(email, '.'));
}

int main() {
char email[100];

printf("Enter email: ");
scanf("%s", email);

if (isValidEmail(email))
printf("Valid email.\n");
else
printf("Invalid email format.\n");

return 0;
}

#include <stdio.h>

#include <string.h>int isValidEmail(char *email) {

return (strchr(email, '@') && strchr(email, '.'));

}

int main() {

char email[100];

printf("Enter email: ");

scanf("%s", email);

if (isValidEmail(email))

printf("Valid email.\n");

else

printf("Invalid email format.\n");

return 0;

}

🧼 Clearing Input Buffer (`getchar()`)

Important when using scanf() to avoid leftover input:

🧠 Best Practices

✔ Always validate user input before using it
✔ Clear input buffer after invalid entries
✔ Use safer input function (fgets()) for strings
✔ Provide clear error messages
✔ Apply logical and range checks

🏁 Summary Table

Validation Type	Example
Type check	`scanf("%d")` validation
Range check	Age between 1–120
Logical check	Division by zero prevention
Format check	Email, phone number

C Input Validation