Arrays

Hi all, This is my 1st post. I know i am asking for help inmy 1st post itself. Couldn't help it coz i just started C++ and well i came across a problem that i am unable to solve. I will appreciate if someone can help me. Not with an answer. Maybe at least the direction i should look towards. I need to do solve a problem where the function

void changeStr(string& str1, string str2) and for
test 1

Test Case 1:
string str1 = "ABCDEFGHI";
string str2 = "123456789"
then after the function call, str1 = “ABC123DEF456GHI789”

Test Case 2:
string str1 = "ABCDEFGHITU";
string str2 = "123456789";
then after the function call, str1 = “ABC123DEF456GHI789TU”

Test Case 3:
string str1 = "ABCDEFGHI";
string str2 = "123456789IP";
then after the function call, str1 = “ABC123DEF456GHI789IP”

Test Case 4
string str1 = "ABCDEFGHITU";
string str2 = "123456789IP";
then after the function call, str1 = “ABC123DEF456GHI789TUIP”

My program which does not work for test 1 to 3 but works for test 4 only.

Code:

#include <iostream>
#include<string>
using namespace std;

void changeStr(string& str1, string str2);	

string str1 = "ABCDEFGHITU";
string str2 = "123456789IP";

int main(){
	
        changeStr(str1,str2);
	return 0;
}
void changeStr(string& str1, string str2)

{
	str1.insert(3,str2,0,3);
	str1.insert(9,str2,3,3);
	str1.insert(15,str2,6,3);
	str1.insert(20,str2,9,2);
	cout<<endl;
	cout << str1;
}

 

i managed to solve it though. But i feel it is not a good solution. Comments pls.

Code:

/*A program that reads in 2 strings and replaces the content of 1st string 
by alternately appending 3 characters from each string to the new str 1.*/

#include <iostream>
#include <string>
using namespace std;

void changeStr(string& str1, string str2);		//Function Prototype

string str1 = "ABCDEFGHITU";		//Initialise Str 1
string str2 = "123456789IP";		//Initialise Str 2
int a = str1.length();				//initialise a to the length of str 1
int b = str2.length();				//initialise b to the length of str 2


int main(){
	
	changeStr(str1,str2);
	return 0;
}

//changeStr(str1,str2);
 void changeStr(string& str1, string str2)
{	
	cout <<"String 1: "<<str1<<endl;	//display original string 1
	cout <<"String 2: "<<str2<<endl;	//display original string 2

	if ((a<=10)&& (b<=10)){
	str1.insert(3,str2,0,3);	//insert 3 chars from pos[0]of str 2 to str 1 from pos[3]
	str1.insert(9,str2,3,3);
	str1.insert(15,str2,6,3);
	}
	else if((a<=10)&& (b>=10)){
	str1.insert(3,str2,0,3);
	str1.insert(9,str2,3,3);
	str1.insert(15,str2,6,3);
	str1.insert(18,str2,9,3);
	}
	
	if((a>10)&& (b<=10)){
	str1.insert(3,str2,0,3);
	str1.insert(9,str2,3,3);
	str1.insert(15,str2,6,3);
	str1.insert(20,str2,9,2);
	}
	else if((a>10)&& (b>=10)){
	str1.insert(3,str2,0,3);
	str1.insert(9,str2,3,3);
	str1.insert(15,str2,6,3);
	str1.insert(20,str2,9,2);
	}
	cout <<"After function call, str1: ";
	cout << str1;			//display the moified new string 1
	cout <<endl;
}