wraparound c++ for ASCII - c++

Question

I'm Matt, first time posting. I'm in school and learning c++ at the moment and I'm stuck on this problem. I can't seem to find the solution so I'm looking for help.
#include <iostream>
using namespace std;
int main()
{
char ch;
cin >> ch;
if(ch <= 122){
cout << ++ch;
cout << ++ch;
}else if (ch > 122){
cout << static_cast<char>(97)++ch;
cout << static_cast<char>(97)++ch;
}
}
The program is very basic. All it needs to do is be fed a lowercase letter and the program just needs to spit out the next two characters. My problem is that after 'z' I don't know how to wrap back around to 'a'. I've tried to static_cast but it says I can'. I've tried reassigning the variable and it says I can't. I've tried several other basic things but none seem to work.
Thank you in advance for the help!

Answer 1

First, don't use magic numbers like 122 and 97. Use the actual character value.
Second, just declare a string abcdefghijklmnopqrstuvwxyz, and index into that string. This eliminates the need for 122, 97, or any other number. Not only that, you can probably see how to do the problem much easier when dealing with indices such as 0, 1, 25, etc. instead of 122, 97, etc.
Once you do that, a little bit of insight shows that the next two characters will be at position (if positions start at 0), (index + 1) % 26 and (index + 2) % 26. The % is the modulus operator, and it returns the remainder after a division is done.
For example, if the current character is y, the yis located at position 24 of the string. So
(24 + 1) % 26 = 25 % 26 = 25
and
(24 + 2) % 26 = 26 % 26 = 0
So the next two characters are situated at position 25 and position 0, which are z and a.
Take another example: z:
(25 + 1) % 26 = 26 % 26 = 0
and
(25 + 2) % 26 = 27 % 26 = 1
So the next characters after z are a and b.
Basically, when you get an assignment where the data "wraps around" to 0, then the word "remainder" or "modulo arithmetic" should immediately come to mind.
So the final program would look like this:
#include <iostream>
int main()
{
char ch;
const char * alphabet = "abcdefghijklmnopqrstuvwxyz";
std::cin >> ch;
int position1 = ch - 'a'; // get position of input character
int position2 = (position1 + 1) % 26; // get position of next character
int position3 = (position1 + 2) % 26; // get position of next next character
// output results
std::cout << ch << alphabet[position2] << alphabet[position3];
}
Live Example

Answer 2

(assuming that the input is: 'a' - 'z')
Keep It Simple
Solution 1:
#include <iostream>
int main()
{
char ch = 0;
std::cin >> ch; // "fed a lowercase letter"
// "spit out the next two characters"
if (ch < 'y')
std::cout << ++ch << ++ch;
else if (ch == 'y')
std::cout << "za";
else // (ch=='z')
std::cout << "ab";
}
Solution 2:
#include <iostream>
int main()
{
const char * lut = "abcdefghijklmnopqrstuvwxyzab";
char ch = 0;
std::cin >> ch; // "fed a lowercase letter"
ch -= 'a'; // lowercase letter to index
// "spit out the next two characters"
std::cout << lut[++ch] << lut[++ch];
}

Answer 3

Try to use the following code for your problem.
#include <iostream>
using namespace std;
int main()
{
char ch = '\0';
cin >> ch; // "fed a lowercase letter"
char c_next;
c_next = (ch-'a'+1)%26+'a';
cout <<c_next;
c_next = (ch-'a'+2)%26+'a';
cout << c_next;
return 0;
}

Answer 4

Here is one way at going about tackling your problem that is clean and elegant. It is very readable that uses a look up table, converts caps to lowercase using a bit of modulo arithmetic; it also leverages some of the newer features of modern C++ such as range loops.
#include <iostream>
#include <ccytpe> // needed for ::tolower
int main() {
// ascii a-z [97,122]
char alphabet[26] = {}; // 0 initizlize this will be a look up table
int i = 97;
for( auto & c : alphabet ) {
c = static_cast<char>( i );
i++;
}
// check to see if our table is correct
for( auto & c : alphabet ) {
std::cout << c << " ";
std::cout << '\n';
}
std::cout << '\n';
// Alphabet Seems to be fine.
char c = {};
std::cout << "Please enter a lower case character: ";
std::cin >> c;
if( c >= 'A' && c <= 'Z' ) {
::tolower( c ); // make sure that it's not in caps
} else if( c >= 'a' && c <= 'z' ) {
// nothing to do
} else {
std::cout << "Error: input value\n";
return -1;
}
// Now that we have the correct inputs we can show your next two characters.
// Since we know that the ascii table has a range of [97,122] for
// lower case letters and that our array index starts at 0; what we can do
// is a little bit of arithmetic to take the input character and set that
// to the index value of the array above. Then use the array indexing to
// output the next 2 characters. To do this we simply just need to subtract 97 or 'a'
c = c - 'a';
// Now we can print the two lines using the adjusted c value with
// a little bit of modulo arithmetic using the stride, size, or
// length of the alphabet.
int stride = 26;
std::cout << alphabet[++c % stride] << '\n';
std::cout << alphabet[++c % stride] << '\n';
// And we are done!
return 0;
}
This is what the code would look like without the comments and the code to print the whole alphabet:
#include <iostream>
#include <cctype>
int main() {
char alphabet[26] = {};
int i = 97;
for( auto & c : alphabet ) {
c = static_cast<char>( i );
i++;
}
char c = {};
std::cout << "Please enter a lower case character: ";
std::cin >> c;
if( c >= 'A' && c <= 'Z' ) {
::tolower( c );
} else if( c >= 'a' && c <= 'z' ) {
// nothing to do
} else {
std::cout << "Error: input value\n";
return -1;
}
c = c - 'a';
int stride = 26;
std::cout << alphabet[++c % stride] << '\n';
std::cout << alphabet[++c % stride] << '\n';
return 0;
}