C++ remove charachters from array

ghaefb · 06-01-2005, 03:09 AM

Ok, it there a simple way to remove for example first few or the last few charachters from normal c++ array ?

Let's say I have: char a[20] = "test 123";
I want to remove the "test", so only "123" will be stored in array.

I did it like this:

Code:

char temp[20]:
int i = 0, j = 4;
while(line[i] != '\0')
{
  temp[i] = a[j];
  i++; j++;
}

But this doesn't seem like a good way... segfault risk.
Is there a better way, more elegant maybe ???

Thanks

infinity42 · 06-01-2005, 05:20 AM

My C++ is a bit rusty, but I'll give it a try:

Code:

/* 
	Chop out part of a string
	Infinity42 - 1-6-05 
*/

#include <string.h>
#include <stdio.h>
using namespace std;

int main() {
	char *in, *out;
	int start, stop, str_len;
	str_len=50;
	//use a safe way of getting the string here
	in=new char[str_len];
	strncpy(in,"A random string\0",15);
	//Get start and stop here
	start=2; stop=5;
	//probably should do more checking they are valid here
	if(start>strlen(in)||stop>strlen(in)||start<0||stop<0||(stop-start)<=0) {
		printf("Start or stop out of range\0");
		return -1;
	}
	out=new char[(stop-start)+1]; //leave an extra char for \0
	for(int i=start;i<=stop;i++) {
		strncat(out,&in[i],1);
	}
	strcat(out,"\0");
	printf("%s (%i)",out,strlen(out));
        delete[] out,in;
}

Hope that helps

ghaefb · 06-01-2005, 07:35 AM

Ok, this looks complicated.
Thanks for the help, bu I need someting more simple and I'm trying to use as less string functions as I can.

My code works, I'm just not sure if it's safe...

infinity42 · 06-01-2005, 08:18 AM

ah right, ok then. One way of making it safer it to test that you're not overunning the bounds, so:

Code:

char temp[20];
int i = 0, j = 4;
while(line[i] != '\0' && i<=20)
{
  temp[i] = a[j];
  i++; j++;
}

This will stop you running off the end of tmp[], but not a[]. Where are line[] and a[] coming from? If you could post a bigger fragment I might be able to help more.

Hope that helps

ghaefb · 06-01-2005, 08:37 AM

I see... Thanks this helps

My mistake, the code is: (example)

Code:

char line[150];
std::ifstream text("/proc/version");
text.getline(line,150,'\n');

char temp[20];
int i = 0, j = 4;
while(line[i] != '\0' && i<=20)
{
  temp[i] = line[j];
  i++; j++;
}
text.close();

So now the temp array contains what I need.

One more question.... it's a general question about arrays.
I see people using "char *line;" more often than 'char line[100];"
So should I use pointer array ? Why ?
Because I can delete it at the end maybe ??

Thanks alot

infinity42 · 06-01-2005, 09:31 AM

ah, that makes more sense

Should you be checking

Code:

line[j] != '\0'

not

Code:

line[i] != '\0'

I've just thought: you could do this (hacky and probably bad practice)

Code:

char line[150];
std::ifstream text("/proc/version");
text.getline(line,150,'\n');

char *temp;
temp=&line[4]
text.close();

To explain how this works I will also answer your questions about array pointers.

An array in C is simply a pointer to the start of it. So 'line' on it's own is a pointer to the value 'line[0]'. Using var[x] is equivalent to *(var+x). So &var[x] really just gives you var+x, but i think using the referencing operator looks neater. So what you have now is a pointer to the fourth element in the array. Just bear in mind that if you change 'line' you will change temp to, as they both point to the same section of memory. So in answer to your question: an array is just a reserved section of contiguous memory, and the array is just a pointer to the start of that memory. The [] operators just dereference a piece of the memory. This is why it is so easy to segfault when dealing with arrays, as var[10000] will just blindly try to do *(var+10000). I'm not good at explaining things, so i hope that made some sense.. just keep asking me questions until you understand it lol

Hope that helps...

ghaefb · 06-01-2005, 09:35 AM

I think I got it in some way...

So to sum up, I should use array pointers ?

jtshaw · 06-01-2005, 10:20 AM

It is typically considered better programming practice to use dynamic arrays (aka, create a pointer and new the memory) then static arrays. For simple things like this there is really no reason not to do it your way, but if there is any chance you ever might want to grow your array you really need to allocated it dyamically.

Quite honestly though, I've gotten into the habit of using std::vector<Type> instead of arrays in cases where I have no way of determining a logical starting size for the array.