C++ merging text using pointers - c++

Question

I am trying to learn some of this beautiful language but I've got stuck on this. Problem is: Why does the last count shows only Witaj PJC not Witaj Cpp PJC? As you see function app has to append transformed 2nd word to 1st one.
Thanks for any help.
If you could give me any good tutorial about pointers I would appreciate that. Thanks!
#include <iostream>
#include <string.h>
using namespace std;
void app(char *str2, char *str1){
for(int i =0; i < strlen(str2); i++){
*(str2++);
}
for(int i =0; i < strlen(str1); i++){
*(str1++);
}
for(int i =0; i < strlen(str1); i++){
*(str2)=*(str1);
*(str2)++;
*(str1)--;
}
}
int main()
{
char *str1 = "ppC ";
char str2[20] = "Witaj";
cout << str2 << endl; // Witaj
app(str2, str1);
cout << str2 << endl; // Witaj Cpp shows WitCpp
app(str2, "CJP ");
cout << str2 << endl; // Witaj Cpp PJC shows WitPJ
return 0;
}

Answer 1

Your problem is this sort of loops:
for(int i =0; i < strlen(str2); i++){
*(str2++);
}
You can't move your pointer with str2++ and expect that strlen(str2) still returning the lenght of the original one.
For loop variables, in each iteration:
i str2 strlen(str2) condition
Iteration 1 0 Witaj 5 0 < 5 Ok
Iteration 2 1 itaj 4 1 < 4 Ok
Iteration 3 2 taj 3 2 < 3 Ok
Iteration 4 3 aj 2 3 < 2 Exit at 3rd character!!
Thus.. you only "move" your pointer 3 bytes.
Change your app function for that one:
void app(char *str2, char *str1){
int nstr2 = strlen(str2);
int nstr1 = strlen(str1);
for(int i =0; i < nstr2; i++){
*(str2++);
}
for(int i =0; i < nstr1; i++){
*(str1++);
}
for(int i =0; i < nstr1; i++){
*(str2++)=*(--str1);
}
}
Anyway... this program is only for academic porpouses or you are thinking use it professionally?

Answer 2

And for some functioning code for just string appending, i scribbled this...
Note that you should make a const call instead, and if you want to reverse one of the strings (a bit unclear from your question) it should be done prior to appending.
Example of string append (rather unsafely and rudimentary) using a new allocation:
char* app(char *str2, char *str1){
char* appendedstring = (char*)malloc(sizeof(char)*20);
char *temp = str1;
char *temp2 = str2;
int stringlen1 = strlen(str1);
int stringlen2 = strlen(str2);
//Copy string 1
for (int i = 0; i < stringlen2; i++){
appendedstring[i] = *temp2;
temp2++;
}
//Append string 2
for (int i = 0; i < stringlen1 + 1; i++){
appendedstring[stringlen2 + i] = *temp;
temp++;
}
return appendedstring;
}
int main()
{
int t;
char *str1 = "ppC ";
char str2[20] = "Witaj";
cout << str1 << endl;
cout << str2 << endl; // Witaj
char* newstr = app(str2, str1);
cout << newstr << endl; // Witaj Cpp shows WitCpp
char* newstr2 = app(str2, "CJP ");
cout << newstr2 << endl; // Witaj Cpp PJC shows WitPJ
return 0;
}