1. a + b_samebase
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
int verifyInteger(){
int keeptrying = 1;
int i=0;
int isFloat=0;
int isChar=0;
int intNum;
char str[100];
do{
fflush(stdin);
i=0;
isFloat=0;
isChar=0;
intNum = 0;
scanf("%s", str);
for(i=0; i<strlen(str);i++){
if (!((str[i]>='0')&&(str[i]<='9'))){
if (str[i]=='.'){
isFloat++;
}
else{
isChar++;
}
}
}
// printf("\nfloat: %d, char: %d", isFloat, isChar);
intNum=atoi(str);
if ((isChar || isFloat>0)||(intNum < 0)){
printf("Invaild input, try again: ");
}
else {
//
printf("This is valid int.\n");
//
printf("input as int %lld\n", intNum);
return intNum;
keeptrying = 0;
}
}while(keeptrying == 1);
}
int convertToDecimal(int num, int base) {
int decimalNum = 0, remainder, count = 0;
while (num != 0) {
remainder = num % 10;
num /= 10;
decimalNum += remainder * pow(base, count);
count++;
}
return decimalNum;
}
int convertToBase(int num, int base) {
int baseNum = 0, remainder, count = 0;
while (num != 0) {
remainder = num % base;
num /= base;
baseNum += remainder * pow(10, count);
count++;
}
return baseNum;
}
int main() {
int base, numA, numB;
int check = 0;
printf("Enter the base(base <= 10): ");
do{
base = verifyInteger();
if(base <= 10){
check=1;
}else{
printf("The base must be smaller or equal to 10, try again: ");
check=0;
}
}while(check==0);
printf("Enter number A: ");
numA = verifyInteger();
printf("Enter number B: ");
numB = verifyInteger();
int decimalA = convertToDecimal(numA, base);
int decimalB = convertToDecimal(numB, base);
int sum = decimalA + decimalB;
int baseSum = convertToBase(sum, base);
printf("The sum of %d and %d in base %d is: %d\n", numA, numB, base, baseSum);
return 0;
}
2. Encode:
#include <stdio.h>
#include <ctype.h>
char encrypt(char plaintext) {
if (plaintext >= 'A' && plaintext <= 'Z') {
int p = plaintext - 'A';
int encrypted = (3 * p + 7) % 26;
char ciphertext = encrypted + 'A';
return ciphertext;
}
return plaintext;
}
int main() {
char plaintext[100];
printf("Enter the plaintext string: ");
fgets(plaintext, sizeof(plaintext), stdin);
char ciphertext[sizeof(plaintext)];
for (int i = 0; plaintext[i] != '\0'; i++) {
ciphertext[i] = encrypt(toupper(plaintext[i]));
}
ciphertext[sizeof(plaintext) - 1] = '\0';
printf("Plaintext: %s\n", plaintext);
printf("Ciphertext: %s\n", ciphertext);
}
3. Decode
#include <stdio.h>
#include <ctype.h>
char decrypt(char ciphertext) {
if (ciphertext >= 'A' && ciphertext <= 'Z') {
int c = ciphertext - 'A';
int inverse = 9; // 3^-1 = 9 (mod 26)
int decrypted = (inverse * (c - 7 + 26)) % 26;
char plaintext = decrypted + 'A';
return plaintext;
}
return ciphertext;
}
int main() {
char ciphertext[100];
printf("Enter the ciphertext string: ");
fgets(ciphertext, sizeof(ciphertext), stdin);
char decryptedtext[sizeof(ciphertext)];
for (int i = 0; ciphertext[i] != '\0'; i++) {
decryptedtext[i] = decrypt(toupper(ciphertext[i]));
}
decryptedtext[sizeof(ciphertext) - 1] = '\0';
printf("Decrypted text: %s\n", decryptedtext);
return 0;
}