'Shift a letter down the alphabet?

I.E., you enter the number 5, and the character A and the output would yield F. I have no idea how to even start to go about this, any give me a push in the right direction?

Solution 1:^[1]

Look at the ASCII table and note the values of the characters.

Solution 2:^[2]

Try this:

#include <stdio.h>

char shift_char(char val, char shift)
{
  val = toupper(val);
  assert(isupper(val));
  char arr[26] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  return arr[ ( (toupper(val) - 'A' + shift) % 26) ];
}

You can get a little fancier if you want to preserve the case of the character. It also assumes, but does not verify shift is non-negative. That case may cause problems with the modulus operation you will need to guard against... or better yet prevent. Still, since this is tagged as homework, that's the sort of thing you should work through.

Solution 3:^[3]

If you can assume ASCII, it is easier.

Characters are no more than simple numbers: only the interpretation of said numbers changes. In ASCII all letters are sequential; so the number for 'A' + 5 is the number for 'F'; 'F' - 1 is 'E' ..., ...

int ch = 'J';
ch -= 2; putchar(ch);
ch -= 3; putchar(ch);
ch += 7; putchar(ch); putchar(ch);
ch += 3; putchar(ch);
puts("");

Just pay attention to wrapping!

If you can't assume ASCII, you need to convert characters yourself. Something like:

char charr[26] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
int ndx = 9; /* charr[9] is 'J' */
ndx -= 2; putchar(charr[ndx]);
ndx -= 3; putchar(charr[ndx]);
ndx += 7; putchar(charr[ndx]); putchar(charr[ndx]);
ndx += 3; putchar(charr[ndx]);
puts("");

Do not forget the wrapping

Solution 4:^[4]

Other people have pointed out that you can use ASCII. An easy way to handle wrapping is with modulus arithmetic:

char result, ch;
int offset;

... // Populate ch with the letter to be changed and offset with the number.

result = ch - 'a';
result = (result + offset) % 26; // 26 letters in the alphabet
result += 'a';

Solution 5:^[5]

char shift_char(char c, char shift)
{
    if(isalpha(c)) {
        if (c>='A' && c<='Z') {
          return 'A' + ( (c - 'A' + shift) % 26);
        } else if(c>='a' && c<='z') {
          return 'a' + ( (c - 'a' + shift) % 26);
        }
    }  
 return c;
}

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution	Source
Solution 1	Oded
Solution 2
Solution 3
Solution 4	nmichaels
Solution 5

'Shift a letter down the alphabet?

Solution 1:[1]

Solution 2:[2]

Solution 3:[3]

Solution 4:[4]

Solution 5:[5]