Modify program to encrypt both uppercase and lowercase input

I have to write code for a random cipher problem. I have done it, but the program transforms only uppercase or lowercase (depending on what I choose) letters as input. What should I change so that the program transforms both upper and lower-case letters?

code:

   srand(time(0));     //seed for rand()
static char alphabet[]="ABCDEFGHIJKLMNOPQRSTUVWXYZ"; 
string alph="ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const int LENGTH=sizeof(alphabet)-1;     

int r;
char temp;
for (unsigned int i=0; i<LENGTH; i++)   //loop which shuffles the array
{
    r=rand() % LENGTH;  
    temp=alphabet[i];         
    alphabet[i] = alphabet[r];  
    alphabet[r]=temp;   
}

string text;
getline (cin, text);
for (unsigned int i=0; i<text.length(); i++)     //loop to encrypt
{
    if (isalpha(text[i]))
    {
        text[i]=alphabet[text[i] - 'A'];    //index alphabet with the value of text[i], adjusted to be in the range 0-25

    }
}