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C++ Discussion Thread
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Jul 13th, 2009

[C++] Problem with changing contents of char[]

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Hi,
I'm quite new to C++ and I'm trying to write a function which would change uppercase letter in char[] to lowercase. Inside this function everything is fine but when I leave it, contents of char[] is not changed. Any ideas why? Also when I try to delete local pointers I'm getting error. Thanks for any help with this.

c++ Syntax (Toggle Plain Text)
void to_lower(const char* s)
{
	if (!s) return; //if s doesn't point to anything
	int size = 5;
	char* new_cstring = new char[size];
	for (int i = 0; i < size; new_cstring[i] = 0, ++i); //initialize array with 0
	int counter = 0;
	//checks if there are still characters in the array
	while (*s)
	{
		//if it's an UPPERCASE letter
		if((int)*s >= 65 && (int)*s <= 90)
		{
			new_cstring[counter] = ((int)*s + 32); //changes this letter to lowercase
		}
		else
		{
		new_cstring[counter] = *s;
		}
		++counter;
		++s;
		if (counter == size - 1)
		{
			//creating new array to store contents of new_cstring
			char* arr = new char[counter];
			for (int i = 0; i < counter; arr[i] = 0, ++i); //initializes arr with 0
			int i = counter;
			for (; i >= 0; arr[i] = new_cstring[i], --i); //copies from old array to new one
			new_cstring = new char[counter + 10]; //creates new bigger array
			for(int i = 0; i < (counter + 10); new_cstring[i] = 0, ++i); //initialize new_string with 0
			for (int i = 0; i < counter; new_cstring[i] = arr[i], ++i);//copies from arr to new_cstring
			size = counter + 10;
			//delete[] arr;
		}
	}
	s = new_cstring;
 
}
void to_lower(const char* s)
{
	if (!s) return; //if s doesn't point to anything
	int size = 5;
	char* new_cstring = new char[size];
	for (int i = 0; i < size; new_cstring[i] = 0, ++i); //initialize array with 0
	int counter = 0;
	//checks if there are still characters in the array
	while (*s)
	{
		//if it's an UPPERCASE letter
		if((int)*s >= 65 && (int)*s <= 90)
		{
			new_cstring[counter] = ((int)*s + 32); //changes this letter to lowercase
		}
		else
		{
		new_cstring[counter] = *s;
		}
		++counter;
		++s;
		if (counter == size - 1)
		{
			//creating new array to store contents of new_cstring
			char* arr = new char[counter];
			for (int i = 0; i < counter; arr[i] = 0, ++i); //initializes arr with 0
			int i = counter;
			for (; i >= 0; arr[i] = new_cstring[i], --i); //copies from old array to new one
			new_cstring = new char[counter + 10]; //creates new bigger array
			for(int i = 0; i < (counter + 10); new_cstring[i] = 0, ++i); //initialize new_string with 0
			for (int i = 0; i < counter; new_cstring[i] = arr[i], ++i);//copies from arr to new_cstring
			size = counter + 10;
			//delete[] arr;
		}
	}
	s = new_cstring;
	
}

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atch

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Jul 13th, 2009

Re: [C++] Problem with changing contents of char[]

>>s = new_cstring;

That is only changing the local copy of the string, not the original. Do not create a new character array, but work directly with the pointer that is in the parameter to that function.

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Jul 13th, 2009

Re: [C++] Problem with changing contents of char[]

you need to pass the reference to the pointer to make it work. You are just modifying the copy of the pointer here. So the changes will not be visible once you leave the fn.

fn signature could be

c++ Syntax (Toggle Plain Text)
void to_lower(const char*& s)
void to_lower(const char*& s)

while calling you could do

c++ Syntax (Toggle Plain Text)
char s[] = "Test";
const char* p = s;
to_lower(p);
cout << p << endl;
char s[] = "Test";
const char* p = s;
to_lower(p);
cout << p << endl;

Agni

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Jul 13th, 2009

Re: [C++] Problem with changing contents of char[]

Or those of you that like using std:strings etc.

C++ Syntax (Toggle Plain Text)
#include <algorithm>
#include <cctype>
#include <iostream>
#include <string>
 
int lowercase ( int c )
{
    return std::tolower ( (unsigned char)c );
}
 
int main()
{
    std::string foo ( "THIS IS A TEST" );
 
    std::cout<< foo <<'\n';
    std::transform ( foo.begin(), foo.end(), foo.begin(), lowercase );
    std::cout<< foo <<'\n';
 
    std::cin.get();
}
#include <algorithm>
#include <cctype>
#include <iostream>
#include <string>

int lowercase ( int c )
{
    return std::tolower ( (unsigned char)c );
}

int main()
{
    std::string foo ( "THIS IS A TEST" );

    std::cout<< foo <<'\n';
    std::transform ( foo.begin(), foo.end(), foo.begin(), lowercase );
    std::cout<< foo <<'\n';
    
    std::cin.get();
}

iamthwee

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Jul 13th, 2009

Re: [C++] Problem with changing contents of char[]

Ok, thanks I'll try it.

atch

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