Thread: help using strings and mapping

ok now that you say that...then my encrypt function is wrong. I need to take each character to see if it is a letter, then change it to lower case, then encrypt by mapping using 'a' as index 0 and so on. I don't think I am implemeenting it right and am stuck.

Code:

void encrypt(char secret[],char message[],char coded[]){    
  int count; 
  char letter;     
  for(count = 0;count < MAXLEN;count++){     
    letter = message[count];   
    if(isalpha(letter)){
      letter = tolower(letter);  
      letter = secret[letter - 'a']; 
      coded[count] = letter; 
    }    
  }  
}

ok...so now my program should do what i want? but i am also getting a compilation error/warning for this line

Code:

void encrypt(char secret[],char message[],char coded[]){    
  int count; 
  char letter;     
  for(count = 0;count < MAXLEN;count++){     
    letter = message[count];   
    if(isalpha(letter)){
      letter = tolower(letter);  
      letter = secret[letter - 'a']; 
      coded[count] = letter; 
    }    
  }  
}

it is saying that the array subscript is of type 'char' ?

any thoughts

ok...so now my program should do what i want? but i am also getting a compilation error/warning for this line

Code:

void encrypt(char secret[],char message[],char coded[]){    
  int count; 
  char letter;     
  for(count = 0;count < MAXLEN;count++){     
    letter = message[count];   
    if(isalpha(letter)){
      letter = tolower(letter);  
      letter = secret[letter - 'a']; 
      coded[count] = letter; 
    }    
  }  
}

it is saying that the array subscript is of type 'char' ?

could I do something like this too

Code:

void encrypt(char secret[],char message[],char coded[]){    
  int count; 
  char letter;     
  for(count = 0;count < MAXLEN;count++){     
    letter = message[count];   
    if(isalpha(letter)){
       letter =tolower(letter);
    }    
    if (letter == 'a') 
        coded[count] = secret[letter - 'a'];
    else if (letter == 'b') 
        coded[count] = secret[letter - 'a'];
    else if (letter == 'c') 
        coded[count] = secret[letter - 'a'];
    else if (letter == 'd') 
        coded[count] = secret[letter - 'a'];
    else if (letter == 'e') 
        coded[count] = secret[letter - 'a'];
    else if (letter == 'f') 
        coded[count] = secret[letter - 'a'];
    else if (letter == 'g') 
        coded[count] = secret[letter - 'a'];
    else if (letter == 'h') 
        coded[count] = secret[letter - 'a'];
    else if (letter == 'i') 
        coded[count] = secret[letter - 'a'];
    else if (letter == 'j') 
        coded[count] = secret[letter - 'a'];
    else if (letter == 'k') 
        coded[count] = secret[letter - 'a'];
    else if (letter == 'l') 
        coded[count] = secret[letter - 'a'];
    else if (letter == 'm') 
        coded[count] = secret[letter - 'a'];
     else if (letter == 'n') 
        coded[count] = secret[letter - 'a'];
     else if (letter == 'o') 
        coded[count] = secret[letter - 'a'];
     else if (letter == 'p') 
        coded[count] = secret[letter - 'a'];
     else if (letter == 'q') 
        coded[count] = secret[letter - 'a'];
     else if (letter == 'r') 
        coded[count] = secret[letter - 'a'];
     else if (letter == 's') 
        coded[count] = secret[letter - 'a'];
     else if (letter == 't') 
        coded[count] = secret[letter - 'a'];
     else if (letter == 'u') 
        coded[count] = secret[letter - 'a'];
     else if (letter == 'v') 
        coded[count] = secret[letter - 'a'];
     else if (letter == 'w') 
        coded[count] = secret[letter - 'a'];
     else if (letter == 'x') 
        coded[count] = secret[letter - 'a'];
     else if (letter == 'y') 
        coded[count] = secret[letter - 'a'];
     else if (letter == 'z') 
        coded[count] = secret[letter - 'a']; 
  }  
}

Erm, that's kinda the same as:

Code:

coded[count] = secret[letter - 'a'];

No IFs needed with that. OF course, I haven't read the thread from the beginning, but...

so do i not need to use isalpha even though I don't want it to change the punctuation marks. I only want to change the letters to lower case and then encrypt them. the punctuation statys the same in the encryption. i do have the ctype.h included. so does what i have for the encryption function work with changing each alphabetic character??

the alphabet code

You need a final condition (a final "else") for all non-letters. Otherwise, array coded[] will have missing values.

Todd

Originally Posted by dwks

Elysia, your code is not portable. (Neither was the OP's.) 'a' through 'z' are not guaranteed to be contiguous. ('0' through '9' are, however.)

Sure, but I was pointing out that that huge IF statement could be reduced to a single line.

Originally Posted by dwks

Elysia, your code is not portable. (Neither was the OP's.) 'a' through 'z' are not guaranteed to be contiguous. ('0' through '9' are, however.)

I think it's easy for us to get carried away with these beginning C students with too much minutia. You are exactly and precisely correct in pointing out that Elysia and the Original Poster are leveraging the ASCII character set and Latin-1 Code Page when checking for a-z in a contiguous sequence. However, in this intro to C class, I suspect they won't be porting their code to an IBM mainframe, converting it from ASCII to EBCDIC at the same time, nor will they being using Greek, Hebrew, German or Cyrillic characters (or otherwise deviating from Latin-1), when prompting the users for input, thus it will work just fine as designed.

Todd