concatenating two strings c++ - c++

Question

im trying to stick two strings together without using + operator,also using loops to to that.the problem is after when it read two strings it couldnt print the second string and only the first string appears.
here is my code
this code is like copying two strings in one.
char str1[MAX];
char str2[MAX];
cout<<"Enter The first String:\n";
cin.getline(str1,MAX,'\n');
cout<<"Enter the second String:\n";
cin.getline(str2,MAX,'\n');
char str3[2*MAX]; int k=0;
for(int i=0;i<MAX;i++)
{ str3[k]=str1[i]; k++; }
for(int j=0;j<MAX;j++)
{ str3[k]=str2[j]; k++; }
str3[k]='\0';
cout<<endl<<"Here is the concatenated string:\n";
cout<<str3<<endl;

Answer 1

It is better to write such code using pointers.
So I would substitute this wrong code
char str3[2*MAX]; int k=0;
for(int i=0;i<MAX;i++)
{ str3[k]=str1[i]; k++; }
for(int j=0;j<MAX;j++)
{ str3[k]=str2[j]; k++; }
str3[k]='\0';
for the following
char str3[2 * MAX];
char *p = str3;
char *q = str1;
while ( *p = *q++ ) ++p;
q = str2;
while ( *p++ = *q++ );
Also the same can be written using for loops. For example
char str3[2 * MAX];
char *p = str3;
for ( char *q = str1; *p = *q++; ++p );
for ( char *q = str2; *p++ = *q++; );

Answer 2

Your code goes past the end of str1 and str2, including their null terminators. Once the null terminator of srt1 is copied, C string inside str3 is considered complete, so str2 part is ignored.
You need to modify the first loop to stop once it sees '\0' in str1, and copy str2 from that point on. Do the same for the second loop. Your code adds null termination already, so the result will be correct:
for(int i=0;i<MAX && str1[i] != '\0';i++)
{ str3[k]=str1[i]; k++; }
for(int j=0;j<MAX && str2[j] != '\0';j++)
{ str3[k]=str2[j]; k++; }
Note: I am assuming that this is a learning exercise for which you are not allowed to use std::string.

Answer 3

Correct code should be:-
char str3[2*MAX];
int k=0;
for(int i = 0; str[i] != '\0'; i++)
{
str3[k]=str1[i];
k++;
}
for(int j=0 ; str2[j] != '\0'; j++ )
{
str3[k] = str2[j];
k++;
}
str3[k]='\0';
You were not taking into account null terminator and hence reading past that.

Answer 4

Change this :
for(int i=0; str1[i] ;i++)
{ str3[k]=str1[i]; k++; }
for(int j=0; str2[j] ;j++)
{ str3[k]=str2[j]; k++; }
You have to stop concatenating when str1 or str2 ends that is when str1[i] or str2[j]
will be 0 ('\0'). But you were looping through MAX. That's why your program produced wrong output.
Hope you understand now :)

Answer 5

#dasblinkenlight explains why this doesn't work in her/his answer.
Here's yet another solution using the standard C(++) library function strcat defined in . See http://www.cplusplus.com/reference/cstring/strcat/
#include <iostream>
#include <cstring>
using namespace std;
int main(int argc, char** argv) {
char str1[MAX];
char str2[MAX];
cout << "Enter The first String:" << endl;
cin.getline(str1, MAX);
cout << "Enter the second String:" << endl;
cin.getline(str2, MAX);
char str3[2 * MAX];
strcat(str3, str1);
strcat(str3, str2);
cout << endl << "Here is the concatenated string:" << endl
<< str3 << endl;
}