Strip leading and trailing non alphabetical chars - c

Question

char *process(char *string) {
char *newWord = malloc(strlen(string) * sizeof(char) + 1);
if (newWord == NULL) {
fprintf(stderr, "Memory error.\n");
exit(1);
}
char *sptr = string;
char *nptr = newWord;
char *lastLetter = newWord;
// Skip leading non-alphabetical characters
while (!isalpha(*sptr) && *sptr != '\0') {
sptr++;
}
// Deal with empty string
if (*sptr == '\0') {
*newWord = '\0';
}
else {
// Process all letters and keep track of last letter to remove trailing special characters
while (*sptr != '\0') {
if (isalpha(*sptr)) {
*nptr = tolower(*sptr);
lastLetter = nptr;
nptr++;
}
sptr++;
}
// Remove trailing special characters by null-terminating after the last letter seen.
*(lastLetter + 1) = '\0';
}
return newWord;
}
I have this function that returns a word after trimming leading and trailing non alphabetical chars. My problem is I am running into trouble figuring out what I need to change in order for it to also not remove non alphas in the middle of words like:
word-word should return word-word and not remove the '-'.
Another example is words like didn't and don't It is removing the apostrophe. Any help?

Answer 1

You can do this more efficiently by making use of:
(a) The strdup() standard library function (which effectively does a malloc and strcpy in one fell swoop).
(b) 'Backward iteration' of the string, replacing non-alpha characters with a nul character until an alpha is found (stopping as soon as that happens).
char* process(char* string)
{
// First, we can 'forward iterate' until we find an ALPHA character ...
char* fp = string;
while (*fp && !isalpha(*fp)) ++fp;
// If we have found the NUL terminator, we have an empty string left...
if (!*fp) return NULL; // Nothing left!
// We don't need to check again for a valid (alpha) character: there WILL be at least one!
char* result = strdup(fp); // Allocate memory and copy current string!
// Now, we can 'backward iterate' until we get to an ALPHA...
char* bp = result + strlen(result) - 1;
while (!isalpha(*bp)) *bp-- = '\0'; // Replace with null character and THEN decrement
// Finally, convert to lowercase:
for (fp = result; *fp; ++fp) *fp = tolower(*fp);
return result;
}
Replacing the trailing non-alpha characters with nul characters is potentially 'wasting' memory (the returned buffer will likely be longer than the actual string it contains), but it is simple. One could add a further strdup call on the 'result' string to handle this, if that is problematical.
Here is a short main that you can use to test the above function:
int main()
{
char test[256];
printf("Enter a string: ");
scanf("%s", test);
char* answer = process(test);
if (answer) {
printf("Processed string: %s", answer);
free(answer);
}
else {
printf("Nothing left after processing!");
}
return 0;
}
Please feel free to ask for any further clarification and/or explanation.