Manually Concatenating two C-Strings

[sirjames2004]

Thanks a lot for the assistance earlier with my mjcStrCpy function. Got a question in regards to the concatenation function I'm trying to write now though. I got it to work pretty much. My only problem is that I can't assign the new string I created in the function, which contains the other two, back to the first string. He doesn't want us to output it within the function, and I can't return it either because the prototype he's making us use is a void return-type function. So I don't know of how else to retain the new, concatenated string.

Only the second argument to the function (second string) is a const pointer, so I should be able to change what the first one can point to. Please see my notes below, and once again thank you so much in advance!

// Author: Matt
// File: mjccstring.cpp
// Operating System: Linux
// Compiler: g++
// Written: 2/2013
// Assigned: 2/7/2013
// Due: 2/15/2013
// Purpose: 	This program will take two strings as input from the user, and perform various C-String related tasks with them

#include <iostream>
#include <iomanip>

// mjccstring.cpp
//−−−−−−−−−−−−−−−−
// (3 pts) Use pointer arithmetic (no subscripts).
// Return the number of characters before the null
// Example: iiiStrLen("xyz") would return 3
int mjcStrLen(const char * const);

// (5 pts) Use subscripts.
// Concatenate (append) the second string onto the end of the first one
// (assume that the first has appropriate space)
// example:
//   first string = "abc"
//   second string = "def"
//   resulting string = "abcdef"
void mjcStrCat(char [], const char []);

//initializes variables and calls the other functions
int main()
{
	char cstring1[] = "This is the first string, ";
	char cstring2[] = "and this is the second string";
	std::cout << "C-String 1 is:  " << cstring1 << std::endl;
	std::cout << "C-String 2 is:  " << cstring2 << std::endl;
	std::cout <<  cstring1 << " contains " << mjcStrLen(cstring1) << " characters" << std::endl;
	std::cout <<  cstring2 << " contains " << mjcStrLen(cstring2) << " characters" << std::endl;
	mjcStrCat(cstring1, cstring2);
	std::cout << "In main, after mjcStrCat exits, C-String 1 is:  " << cstring1 << std::endl;
	return 0;
}

//uses a while loop to count the number of characters in a C-string
int mjcStrLen(const char * const test_string)
{
	int length = 0;
	int index = 0;
	while(*(test_string + index)) //will be true until it hits the null character
	{
		length++;
		index++;
	}
	return length;
}

//concatenates string1 and string2 into a new string, then string1 will point to it
void mjcStrCat(char test_string1[], const char test_string2[])
{
	//gathers length of both strings, and creates a new string with the necessary space
	int size_of_string1 = mjcStrLen(test_string1);
	int size_of_string2 = mjcStrLen(test_string2);
	int size_of_newstring = size_of_string1 + size_of_string2;
	char new_string[size_of_newstring];
	//copy both strings to new string
	int index_of_newstring = 0;
	for(index_of_newstring; index_of_newstring < size_of_string1; index_of_newstring++)
		new_string[index_of_newstring] = test_string1[index_of_newstring];
	for(int index_of_string2 = 0; index_of_string2 < size_of_string2; index_of_string2++)
	{
		new_string[index_of_newstring] = test_string2[index_of_string2];
		index_of_newstring++;
	}
	//pointing first string to new string
	std::cout << "New String is:  " << new_string << std::endl;
	std::cout << "Inside function, before assignment, C-String 1 is:  " << test_string1 << std::endl;
	test_string1 = new_string;
	std::cout << "Inside function, after assignment, C-String 1 is:  " << test_string1 << std::endl;
}

[MiiNiPaa]

Line 63: char new_string[size_of_newstring]; — non-standart extention to language, will not work everywhere, and you will get point deducted (probably) for this. Also it is a local variable which will be destroed when function ends.
Do dynamic memory allocation: char new_string = new char[size_of_newstring]
then this: test_string1 = new_string; should do the trick.

BTW you can simplify your length function:

int mjcStrLen(const char * const test_string)
{
	int length = 0;
	while(test_string[length++]) ;
	return length;
}

[sirjames2004]

Thanks. I finally got something that works. May or may not be the most efficient, but when something's two weeks past due, I'll take it you know? Once again, appreciate the help

void mjcStrCat(char test_string1[], const char test_string2[])
{
	//begin operation at the end of string1 and the beginning of string2
	int index_of_string1 = mjcStrLen(test_string1);
	int index_of_string2 = 0;
	//looping through string2 and assigning it's characters to the end of string1
	for(index_of_string1; index_of_string1 < 256; index_of_string1++)
	{
		test_string1[index_of_string1] = test_string2[index_of_string2];
		index_of_string2++;
	}
}