C Programming for the Absolute Beginner phần 7 pptx

main()

{

char myString[21] = {0};

int iSelection = 0;

int iRand;

srand(time(NULL));

iRand = (rand() % 4) + 1; // random #, 1-4

while ( iSelection != 4 ) {

printf("\n\n1\tEncrypt Clear Text\n");

printf("2\tDecrypt Cipher Text\n");

printf("3\tGenerate New Key\n");

printf("4\tQuit\n");

printf("\nSelect a Cryptography Option: ");

scanf("%d", &iSelection);

switch (iSelection) {

case 1:

printf("\nEnter one word as clear text to encrypt: ");

scanf("%s", myString);

encrypt(myString, iRand);

break;

case 2:

printf("\nEnter cipher text to decrypt: ");

scanf("%s", myString);

decrypt(myString, iRand);

break;

case 3:

iRand = (rand() % 4) + 1; // random #, 1-4

printf("\nNew Key Generated\n");

174 C Programming for the Absolute Beginner, Second Edition

break;

} //end switch

} //end loop

} //end main

void encrypt(char sMessage[], int random)

{

int x = 0;

//encrypt the message by shifting each characters ASCII value

while ( sMessage[x] ) {

sMessage[x] += random;

x++;

} //end loop

x = 0;

printf("\nEncrypted Message is: ");

//print the encrypted message

while ( sMessage[x] ) {

printf("%c", sMessage[x]);

x++;

} //end loop

} //end encrypt function

void decrypt(char sMessage[], int random)

{

int x = 0;

Chapter 7 • Pointers 175

x = 0;

//decrypt the message by shifting each characters ASCII value

while ( sMessage[x] ) {

sMessage[x] = sMessage[x] - random;

x++;

} //end loop

x = 0;

printf("\nDecrypted Message is: ");

//print the decrypted message

while ( sMessage[x] ) {

printf("%c", sMessage[x]);

x++;

} //end loop

} //end decrypt function

SUMMARY

• Pointers are variables that contain a memory address that points to another variable.

• Place the indirection operator (*) in front of the variable name to declare a pointer.

• The unary operator (&) is often referred to as the “address of” operator.

• Pointer variables should always be initialized with another variable’s memory address,

with 0, or with the keyword NULL.

• You can print the memory address of pointers using the %p conversion specifier.

• By default, arguments are passed by value in C, which involves making a copy of the

incoming argument for the function to use.

• Pointers can be used to pass arguments by reference.

• Passing an array name to a pointer assigns the first memory location of the array to the

pointer variable. Similarly, initializing a pointer to an array name stores the first address

of the array in the pointer.

• You can use the const qualifier in conjunction with pointers to achieve a read-only

argument while still achieving the pass by reference capability.

176 C Programming for the Absolute Beginner, Second Edition