Try to use += on string to stack char(s) in C - c

Question

I know it sounds real stupid, but It is cuz I just lightly learned Java and Python last year...
now I am try to do C properly with CS50.
on week 2, there is Caesar encipher text program task.
I tried to use += on string to stack char(s) one by one.
And of course it didn't work.
but is there any other way to do it similarly?
here is the code
#include <stdio.h>
#include <cs50.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
string encipher(string x, int y);
int main(int argc, string argv[])
{
string key = argv[1];
bool num_flag = false;
for (int i = 0; i < strlen(key); i++)
{
int k = isdigit(key[i]);
if (k == 0)
{
num_flag = true;
}
}
if (num_flag == true)
{
printf("Usage: ./caesar key\n");
}
else
{
int keys = atoi(argv[1]);
string c = get_string("plaintext: ");
string ciphertext = encipher(c, keys);
printf("ciphertext: %s\n", ciphertext);
}
}
string encipher(string x, int y)
{
string cip = "";
for (int i = 0; i < strlen(x); i++)
{
int low = islower(x[i]);
int upper = isupper(x[i]);
if (low != 0)
{
char lo = x[i] - 97;
char c = (lo + y) % 26;
char l = c + 97;
cip += l;
}
else if (upper != 0)
{
char up = x[i] - 65;
char c = (up + y) % 26;
char u = c + 65;
cip += u;
}
else
{
cip += x[i];
}
}
return cip;
}
.
I changed code to pass the test, but not perfect either...
#include <stdio.h>
#include <cs50.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
string encipher(string x, int y);
int main(int argc, string argv[])
{
if(argc != 2)
{
printf("Usage: ./caesar key\n");
}
else
{
string key = argv[1];
bool num_flag = false;
for (int i = 0; i < strlen(key); i++)
{
int k = isdigit(key[i]);
if (k == 0)
{
num_flag = true;
}
}
if (num_flag == true)
{
printf("Usage: ./caesar key\n");
}
else
{
int keys = atoi(argv[1]);
string c = get_string("plaintext: ");
string cipher = encipher(c, keys);
printf("ciphertext: %s\n", cipher);
}
}
}
string encipher(string x, int y)
{
for (int i = 0; i < strlen(x); i++)
{
int low = islower(x[i]);
int upper = isupper(x[i]);
if (low != 0)
{
char lo = x[i] - 97;
char c = (lo + y) % 26;
char l = c + 97;
x[i] = l;
}
else if (upper != 0)
{
char up = x[i] - 65;
char c = (up + y) % 26;
char u = c + 65;
x[i] = u;
}
}
return x;
}
Thanks for helping newbie!

Answer 1

with strncat() you can add chars to an existing string.
( strncat(char DESTINATION STRING, const char SOURCE STRING, AMOUNT OF CHARS YOU WANT TO ADD FROM THE SOURCE) )

Answer 2

In C you can't add string like other high-level languages. You have to use a special function in string.h. Like strcat(first_string, second_string);
An alternative is: char *strncat(char *dest, const char *src, size_t n) to add the first n characters (you can interprete size_t like int).