String Functions: Strcat() - c++

Question

I'm currently writing a program that uses string functions. I need some advice/hints on how I can display "Hello World" and its length with myStrcat() in main(). I'm new to programming and any support would be greatly appreciated.
My Code:
#include <iostream>
#include <cstring>
#include <string>
using namespace std;
int myStrlen(char str1[])
{
int i = 0;
for (i=0; str1[i] != '\0'; i++)
str1[i] = '\0';
return i;
}
int myStrcat(char str2[], char str3[])
{
}
int myStrcpy(char str4[], char str5[])
{
int i = 0;
for (i=0; str5[i] != '\0'; i++)
str4[i] = str5[i];
str4[i] = '\0';
return i;
}
int main()
{
const int SIZE = 11;
char s1[SIZE] = "Hello";
char s2[SIZE] = "World";
cout << "s1: " << " " << s1 << endl << endl; ///Should display "Hello"
cout << "The length of s1: " << myStrlen(s1) << endl << endl;
cout << "Doing strcat(s1, s2) " << endl;
myStrcat(s1, s2);
cout << "s1: " << " " << s1 << endl; /// Should display "Hello World"
cout << "The length of s1: " << myStrlen(s1) << endl << endl;
cout << "Doing strcpy(s1, s2) " << endl;
myStrcpy(s1, s2);
cout << "s1: " << " " << s1 << endl; /// Should display "World"
cout << "The length of s1: " << myStrlen(s1) << endl << endl;
My Output:
s1: Hello
The length of s1: 5
Doing strcat(s1, s2)
s1:
The length of s1: 0
Doing strcpy(s1, s2)
s1: World
The length of s1: 5
Line 6 and 7 are suppose to display Hello World and its length (which is 11).

Answer 1

You have a number of not just quite right beginning to each of your functions. Firstly, let's think about the returns for each. myStrlen should return size_t instead of int. C++ designates a size_type for counters, measuring, etc.. The remaining functions should return char* (or nullptr on failure).
Looking at your myStrlen function where you have
for (i=0; str1[i] != '\0'; i++)
str1[i] = '\0';
You are setting every character in str1 to the nul-character because you are applying the loop to the next expression. You should not be worrying about nul-terminating anything within myStrlen -- you are just counting characters. So you can rewrite it as follows:
size_t myStrlen (const char *str)
{
size_t l = 0;
for (; str[l]; l++) {}
return l;
}
Your myStrcpy looks workable, though you should always validate your input parameters are not nullptr before using them -- I leave that to you. Since you have a myStrlen function, you can simply use that along with memcpy to create your myStrcpy function as:
char *myStrcpy (char *dest, const char *src)
{
size_t len = myStrlen(src);
return (char *)memcpy (dest, src, len + 1);
}
(note: traditionally you have source (src) and destination (dest) parameters when copying or concatenating)
For your myStrcat function, you are just using the myStrlen function to find the offset in dest to append src, so you really just need a call to myStrlen and then a call to myStrcpy to copy src to that offset in dest, e.g.
char *myStrcat (char *dest, const char *src)
{
size_t len = myStrlen (dest);
return myStrcpy (dest + len, src);
}
In your main(), if you want a space between "Hello" and "World", then const int SIZE = 11; is one too-low to hold the concatenated string "Hello World" which would require 12-bytes (including the nul-terminating character). Do Not Skimp on buffer size. 128 is plenty small.
Remaining with your main() but updating SIZE = 12; and adding a space between "Hello" and "World" with an additional call to myStrcat, you could do the following:
int main (void)
{
const int SIZE = 12; /* too short by 1 if you add space between */
char s1[SIZE] = "Hello";
char s2[SIZE] = "World";
std::cout << "s1: " << " " << s1 << std::endl << std::endl;
std::cout << "The length of s1: " << myStrlen(s1) << std::endl << std::endl;
std::cout << "Doing strcat(s1, s2) " << std::endl;
myStrcat(s1, " ");
myStrcat(s1, s2);
std::cout << "s1: " << " " << s1 << std::endl;
std::cout << "The length of s1: " << myStrlen(s1) << std::endl << std::endl;
std::cout << "Doing strcpy(s1, s2) " << std::endl;
myStrcpy(s1, s2);
std::cout << "s1: " << " " << s1 << std::endl;
std::cout << "The length of s1: " << myStrlen(s1) << std::endl << std::endl;
}
(note: don't include using namespace std;, it is just bad form in this day and age)
Example Use/Output
$./bin/mystrcpy
s1: Hello
The length of s1: 5
Doing strcat(s1, s2)
s1: Hello World
The length of s1: 11
Doing strcpy(s1, s2)
s1: World
The length of s1: 5
Look things over and let me know if you have further questions.

Answer 2

First you should read Why is “using namespace std;” considered bad practice?
Don't use c style strings if you are starting programming. Use std::string. It's much simpler to use.
#include <iostream>
#include <string>
int myStrlen(const std::string &str) {
return str.length();
}
int myStrcat(std::string &str1, const std::string &str2) {
str1 += str2;
str1.length();
}
int myStrcpy(std::string &str1, const std::string &str2) {
str1 = str2;
return str1.length();
}
int main() {
std::string s1 = "Hello";
std::string s2 = "World";
std::cout << "s1: " << s1 << "\n\n"; ///Should display "Hello"
std::cout << "The length of s1: " << myStrlen(s1) << "\n\n";
std::cout << "Doing strcat(s1, s2) " << '\n';
myStrcat(s1, s2);
std::cout << "s1: " << s1 << '\n'; /// Should display "Hello World"
std::cout << "The length of s1: " << myStrlen(s1) << "\n\n";
std::cout << "Doing strcpy(s1, s2) " << '\n';
myStrcpy(s1, s2);
std::cout << "s1: " << s1 << '\n'; /// Should display "World"
std::cout << "The length of s1: " << myStrlen(s1) << "\n\n";
return 0;
}