How do I concat a character into a string?

Question

I'm trying to take a char and add it to a string. OR if that's not possible, add it to an array of characters?

I tried changed 'string updated = ""' to 'char updated[text_length]' but I was getting the same error - incompatible integer to pointer conversion error

if(argc == 2 && IsDigitsOnly(argv[argc-1])) {
    // convert to integer
    int x = atoi(argv[argc-1]);

    // prompt user for plaintext
    string plaintext = get_string("plaintext: ");
    int text_length = strlen(plaintext);
    int ascii = 'a';
    string updated= "";


    for (int i = 0; i < text_length; i++)
    {
        if (isalpha(plaintext[i]))
        {
            if (isupper(plaintext[i]))
            {
                // GETTING ERROR HERE -- trying to pass 'letter' in strcat
                // This gives me an error: incompatible integer to pointer conversion error
                int n = (plaintext[i] + x) % 90;
                char letter = n;
                strcat(updated, letter);
            }
            else
            {
                ascii = (ascii + x) % 122;
            }
        }
    }

    printf("%s\n", updated);
}

David Ranieri · Accepted Answer · 2022-12-12T09:07:49.083

1

Based on the string and get_string() call, I assume that you are using cs50. In this case:

You can not write to a string literal (updated).
srtcat wants a pointer to char (not a char) as second parameter.

Switch to:

char updated[SOME_SIZE] = "";
...
char letter = n;
strcat(updated, (char[]){letter, 0});

Notice that strcat is prone to buffer overflows, consider replacing it with snprintf, something like:

char updated[SOME_SIZE] = "";
...
char letter = n;
size_t len = strlen(updated);

snprintf(updated + len, sizeof updated - len, "%c", letter);

edited Dec 12 '22 at 09:07

answered Dec 12 '22 at 08:32

David Ranieri

39,972
7
52
94

Thanks for your help. Would you mind explaining what's going on in this code "strcat(updated, (char[2]){letter, 0});" - trying to learn C at the moment – matt.mcg Dec 12 '22 at 08:34
Of course, is a compound literal: https://en.cppreference.com/w/c/language/compound_literal – David Ranieri Dec 12 '22 at 08:35

chux - Reinstate Monica · Answer 2 · 2022-12-12T09:11:04.053

How do I concat a character into a string?

// Since C99, append by forming a compound literal.
strcat(some_string, (char []){some_chracter, '\0'});

Yet in OP's case, concatenating a character to a string is a weak approach.

For this task, strcat(updated, ...) is slow as each time a letter needs to be appended, updated is longer and longer. See Schlemiel the Painter's Algorithm.

A more idiomatic approach would keep track of the length of updated and use that as an index in appending letter.

int text_length = strlen(plaintext);
int ascii = 'a';
// string updated= "";
char updated[text_length + 1]; // `text_length + 1` is the maximum size needed.
// Add
int updated_used  = 0;

for (int i = 0; i < text_length; i++)
{
    if (isalpha(plaintext[i]))
    {
        if (isupper(plaintext[i]))
        {
            int n = (plaintext[i] + x) % 90;
            char letter = n;
            // strcat(updated, letter);
            updated[updated_used++] = letter;
        }
        else
        {
            ascii = (ascii + x) % 122;
        }
    }
}

// Add
updated[updated_used] = '\0';

Both int n = (plaintext[i] + x) % 90; and ascii = (ascii + x) % 122; look wrong.

I suspect OP wants to shift within uppercase letters with

int n = (plaintext[i] - 'A' + x) % 26 + 'A';

and for lower case:

int n = (plaintext[i] - 'a' + x) % 26 + 'a';

How do I concat a character into a string?

2 Answers2