Converting Letters to Numbers in C

Question

Hello, I'm trying to write a code that would convert letters into numbers. For example A ==> 0 B ==> 1 C ==> 2 and so on. Im thinking of writing 26 if statements. I'm wondering if there's a better way to do this...

Thank you!

Answer 1

There is a much better way.

In ASCII (www.asciitable.com) you can know the numerical values of these characters.

'A' is 0x41.

So you can simply minus 0x41 from them, to get the numbers. I don't know c very well, but something like:

int num = 'A' - 0x41;

should work.

Answer 2

If you need to deal with upper-case and lower-case then you may want to do something like:

if (letter >= 'A' && letter <= 'Z')
  num = letter - 'A';
else if (letter >= 'a' && letter <= 'z')
  num = letter - 'a';

If you want to display these, then you will want to convert the number into an ascii value by adding a '0' to it:

  asciinumber = num + '0';

Answer 3

Another, far worse (but still better than 26 if statements) alternative is to use switch/case:

switch(letter)
{
case 'A':
case 'a': // don't use this line if you want only capital letters
    num = 0;
    break;
case 'B':
case 'b': // same as above about 'a'
    num = 1;
    break;
/* and so on and so on */
default:
    fprintf(stderr, "WTF?\n");
}

Consider this only if there is absolutely no relationship between the letter and it's code. Since there is a clear sequential relationship between the letter and the code in your case, using this is rather silly and going to be awful to maintain, but if you had to encode random characters to random values, this would be the way to avoid writing a zillion if()/else if()/else if()/else statements.

Answer 4

In most programming and scripting languages there is a means to get the "ordinal" value of any character. (Think of it as an offset from the beginning of the character set).

Thus you can usually do something like:

for ch in somestring:
    if lowercase(ch):
        n = ord(ch) - ord ('a')
    elif uppercase(ch):
        n = ord(ch) - ord('A')
    else:
        n = -1  # Sentinel error value
        # (or raise an exception as appropriate to your programming
        #  environment and to the assignment specification)

Of course this wouldn't work for an EBCDIC based system (and might not work for some other exotic character sets). I suppose a reasonable sanity check would be to test of this function returned monotonically increasing values in the range 0..26 for the strings "abc...xzy" and "ABC...XYZ").

A whole different approach would be to create an associative array (dictionary, table, hash) of your letters and their values (one or two simple loops). Then use that. (Most modern programming languages include support for associative arrays.

Naturally I'm not "doing your homework." You'll have to do that for yourself. I'm simply explaining that those are the obvious approaches that would be used by any professional programmer. (Okay, an assembly language hack might also just mask out one bit for each byte, too).

Answer 5

Since the char data type is treated similar to an int data type in C and C++, you could go with some thing like:

char c = 'A';   // just some character

int urValue = c - 65;

If you are worried about case senstivity:

#include <ctype.h> // if using C++ #include <cctype>
int urValue = toupper(c) - 65;

Answer 6

This is a way that I feel is better than the switch method, and yet is standards compliant (does not assume ASCII):

#include <string.h>
#include <ctype.h>

/* returns -1 if c is not an alphabetic character */
int c_to_n(char c)
{
    int n = -1;
    static const char * const alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    char *p = strchr(alphabet, toupper((unsigned char)c));

    if (p)
    {
        n = p - alphabet;
    }

    return n;
}

Answer 7

Aww if you had C++

For unicode definition of how to map characters to values

typedef std::map<wchar_t, int> WCharValueMap;
WCharValueMap myConversion = fillMap();

WCharValueMap fillMap() {
  WCharValueMap result;
  result[L'A']=0;
  result[L'Â']=0;
  result[L'B']=1;
  result[L'C']=2;
  return result;
}

usage

int value = myConversion[L'Â'];