How do I remove Chars from the end of a Char*? - c++

Question

I am writing a program in C++ that takes in an argument for a filename, the argument is a char*
ex: myFile.lan
I need to remove the last 3 digits of this char* ("lan") and change them to "asm" (ex: myFile.asm)
It seems really easy to add chars to a char pointer through strcpy, but does anyone know how I can remove chars from a char pointer?

Answer 1

If you're using C++, you should convert your argument to an std::string. This will protect you from going out of bounds and is more clear.
#include <string>
#include <iostream>
int main ()
{
// ... get the arg
const char* arg = "myFile.lan";
std::string filenameAsm(arg);
filenameAsm = filenameAsm.substr(0, filenameAsm.find_last_of("."));
filenameAsm += ".asm";
std::cout << filenameAsm; // prints myFile.asm
return 0;
}
What this code does is take only the part of the filename preceding the "." file extension delimiter (if it doesn't exist, it will take the whole filename) and append the desired ".asm" extension.

Answer 2

Working with a char * is basic 'C'. You can write anything into the memory space, but be careful about not going past the end of allocated space.
char * strings are all terminated with the null byte \0. So, to truncate, you could put a \0 at the appropriate location.
On the other hand, to overwrite characters, just use array syntax; e.g. if the string is length 10 and you want to change the last character, c_string[9] = 'X'; would change that character to an X.

Answer 3

You have to know how a string is ended in C. The length of your string is determined by the first occurrence of the \0 character. Thus, by moving this character backwards, your string becomes shorter. So you probably want to search your string for the first position of the dot, and then replace this dot with \0 (this depends on how exactly the string input looks like though. i'm assuming it's always filenames with a dot somewhere, but you know better).

Answer 4

Using the tools in string.h can simplify the task. strrchr will find the last '.' in the filename allowing you to manipulate the extension. Don't forget to test the lengths of new/old extension to prevent overwriting the \0 - null-terminating character (if the lengths differ, you can always concatenate or realloc the string size, but that's beyond the scope of this example). Look over the solution and let me know if you have any questions:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main (int argc, char **argv) {
if (argc < 3) {
fprintf (stderr, "\n error: insufficient input. Usage: %s <filename> <new_ext>\n\n", argv[0]);
return 1;
}
char *filename = strdup (argv[1]); /* copy argv[1] to prevent clobbering it */
char *p = strrchr (filename, '.'); /* pointer to last '.' in filename */
char *ext = strdup (p+1); /* make a copy of existing extension */
size_t esz = strlen (ext); /* length of existing extension in filename */
size_t nesz = strlen (argv[2]); /* length of new extension */
if (esz < nesz) {
fprintf (stderr, "\n error: invalid extension size. (%s > %s)\n\n", argv[2], ext);
return 1;
}
printf ("\n The original filename: %s\n", filename);
strncpy (p+1, argv[2], nesz); /* copy new extension to filename */
if (nesz < esz) /* if new extension is shorter than old */
*(p+1+nesz) = 0; /* null terminate after new extesion size */
printf (" The amended filename : %s\n\n", filename);
if (filename) free (filename); /* free memory allocated by strdup */
if (ext) free (ext);
return 0;
}
output:
$ ./bin/swapext myfile.lan asm
The original filename: myfile.lan
The amended filename : myfile.asm
$ ./bin/swapext myfile.lan c
The original filename: myfile.lan
The amended filename : myfile.c