Appending a char to a char* - c

Question

I'm writing a program and I need to append a char to a char*. I have a char** that represents lines of text. I'm trying to take a single char at a time from char**, and add it to a char*.
So basically, I tried strcat(char*, char[i][j]), but strcat rejects char[i][j] since it's not a pointer. I think I need to use sprintf(char*, "%c", char[i][j]), but I'm having trouble understanding how to append with sprintf. I don't want it to overwrite what's already in my char*.
Any tips?

Answer 1

You almost got it!
strncat (char *, & char[i][j], 1);
Calling strncat like that will copy exactly one char from your char** at position [i][j]. Just remember that it will also append the null character after that.

Answer 2

given that you know that buf is defined as
char buf [MAXSIZE];
Let us assume that is has been initialized to an initial string and you want to add mychar to it (a single character)
i = strlen(buf);
if (i < MAXSIZE-1)
{
buf[i] = mychar; // Make the new character the last element of buf
buf[i+1] = '\0' // end the new string with the null character
}
This will append the new character and push the ending null character to ensure that it is a valid string. This is what strcat does when the appended entry is also a string or what strncat does when the count is 1. Note that the new strlen(buf) will now be i+1

Answer 3

Assuming buf is a 0 terminated c-string, you can use snprintf():
#include <stdio.h>
#include <string.h>
int main(void)
{
size_t b_len;
char buf[256];
char ch = '!';
strcpy(buf, "Hello world");
b_len = strlen(buf);
snprintf(buf + b_len, sizeof buf - b_len, "%c", ch);
printf("%s", buf);
char *p = malloc(256); /* check if malloc failed */
strcpy(p, "Hello world");
size_t len = strlen(p);
snprintf(p + len, 256 - len, "%c", ch);
printf("%s", p);
return 0;
}
If buf happens to be pointer (such as an malloced pointer) then you can't use sizeof (it can only be used on an array to get the size).

Answer 4

I have a very hard time understanding your question, instead of posting an example of what you want to achieve you explained something that seems to be based on your way of solving a problem with very little training and/or experience in the c language. So it's hard to write a good answer but definitely I will try to show you a way of doing something similar to what apparently you want to do.
The thing is, I highly doubt that strcat() is appropriate for this or for anything except, concatenating two strings. More than two, require something better than strcat(). And for a single char it's definitely less appropriate.
The following code appends all the characters in the string literal array into a single string, check it out
int
main(int argc, char **argv)
{
char string[100];
char lines[3][14] = {
"First line...",
"Another line.",
"One more line"
};
size_t i;
i = 0;
for (size_t j = 0 ; ((j < sizeof(lines) / sizeof(lines[0])) && (i < sizeof(string) - 1)) ; ++j)
{
for (size_t k = 0 ; ((k < 13) && (i < sizeof(string) - 1)) ; ++k)
string[i++] = lines[j][k];
}
string[i] = '\0';
puts(string);
return 0;
}