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--- tanszek:oktatas:techcomm:luhn_algortithm_to_protect_credit_card_numbers [2024/11/05 16:47] – knehez
+++ tanszek:oktatas:techcomm:luhn_algortithm_to_protect_credit_card_numbers [2025/10/27 20:20] (current) – [Steps of the Luhn Algorithm] knehez
@@ Line 40: / Line 40: @@
 card_number = "4539148803436467"
 print("Valid" if luhn_check(card_number) else "Invalid")
+</sxh>
+Here is a C implementation of the algorithm:
+<sxh c>
+#include <stdio.h>
+#include <string.h>
+#include <ctype.h>
+// Function to validate a Visa card number using the Luhn Algorithm
+int isValidVisaCard(const char* cardNumber) {
+    int length = strlen(cardNumber);
+    int sum = 0;
+    int isSecond = 0;
+    // Traverse the card number in reverse
+    for (int i = length - 1; i >= 0; i--) {
+        int digit = cardNumber[i] - '0';
+        if (isSecond) {
+            // Double every second digit
+            digit *= 2;
+            if (digit > 9) {
+                // Subtract 9 if the doubled value is greater than 9
+                digit -= 9;
+            }
+        }
+        // Add digit to the sum
+        sum += digit;
+        // Toggle the isSecond flag
+        isSecond = !isSecond;
+    }
+    // If the sum is divisible by 10, the card number is valid
+    return (sum % 10 == 0);
+}
+int main() {
+    char cardNumber[20];
+    printf("Enter the Visa card number: ");
+    scanf("%19s", cardNumber);
+    if (isValidVisaCard(cardNumber)) {
+        printf("The card number is valid.\n");
+    } else {
+        printf("The card number is invalid.\n");
+    }
+    return 0;
+}
 </sxh>
@@ Line 56: / Line 107: @@
 . **Calculate the Checksum with the Luhn Algorithm**:
-   - Apply the Luhn algorithm to calculate the checksum digit (the last digit), which will make the number valid.
+   * Apply the Luhn algorithm to calculate the checksum digit (the last digit), which will make the number valid.
 === Example Code to Generate a Valid Card Number ===