Splitting a string into substrings using C - c

Question

I'm trying to make a program that splits the string based on a specific character.
Data Structure used:
typedef struct pieces {
char **members;
size_t len;
} pieces;
Function declarations:
pieces split (const char *s, const char c);
size_t charCount (const char *s, const char c);
char *slice (const char *s, int a, int b);
size_t indexOf (const char *s, const char c, size_t start);
charCount -> No. of times the char appeared in string.
indexOf -> Returns the index of a first occurrence of the given character inside the string, starting from the index start; i.e. indexOf("Stack Overflow", 'O', 0) == indexOf("Stack Overflow", 'O', 3)
I've implemented slice like this:
char *slice (const char *s, int a, int b)
{
if (a > b || a == b)
return NULL;
if (b > strlen(s)) // Only slice upto end if tried to slice out of index
b = strlen(s);
size_t len = b - a + 1;
char *slice = malloc(sizeof(char) * len);
for (size_t i = a; i < b; i++)
slice[i - a] = s[i];
slice[len - 1] = '\0';
return slice;
}
I'm confused on split function:
pieces split (const char *s, const char c)
{
// Is this the right way to make room for incoming slices ?
pieces arr;
arr.len = charCount(s, c) + 1;
arr.members = malloc(sizeof(char *) * arr.len);
// Should I do something like this to insert slices ?
for (size_t i = 0; i < strlen(s);)
{
int seperator_idx = indexOf(s, c, i);
char *piece = slice(s, i, seperator_idx);
arr.members[i] = piece; // Should I use strdup ??
i = seperator_idx + 1;
}
// What about the last slice ?
return arr;
}

Answer 1

There are some issues with the proposed prototypes:
pieces split(const char *s, const char c);
it is unclear if consecutive occurrences of c represent empty substrings or a single separator (as in strtok). Let's assume empty substrings should be accepted. const qualifying c is overkill and not meaningless in a prototype
size_t charCount(const char *s, const char c); same remark about const char c. Let's assume the null terminator is not part of the string so charCount("abc", '\0') is zero.
char *slice(const char *s, int a, int b); why are a and b typed int instead of size_t?
size_t indexOf(const char *s, const char c, size_t start); what should this function return in case c is not found the string starting from index start? Let's assume the offset of the end of string should be returned, as it is more convenient to implement slice.
With these conventions, indexOf and charCount can be written as:
#include <stddef.h>
size_t indexOf(const char *s, const char c, size_t start) {
while (s[start] && s[start] != c)
start++;
return start;
}
size_t charCount(const char *s, const char c) {
size_t count = 0;
while (*s) {
count += (*s++ == c);
}
return count;
}
Your slice function has multiple problems:
It should return an empty string if a == b,
it is confusing to name len something that is not the length of the substring. Either define len as size_t len = b - a; or use size_t size = b - a + 1;
it has undefined behavior if a is larger than strlen(s) and b > a.
you should gracefully return NULL in case of malloc() failure
Here is a modified version:
#include <stdlib.h>
/* return an empty string if a >= b */
char *slice(const char *s, size_t a, size_t b) {
size_t len = strlen(s);
if (a > len)
a = len;
if (b < a)
b = a;
char *slice = malloc(b - a + 1);
if (slice != NULL) {
for (size_t i = a; i < b; i++)
slice[i - a] = s[i];
slice[b - a] = '\0';
}
return slice;
}
The split function also has problems:
naming the pieces structure arr is confusing: it is not an array.
your allocation for arr.members is correct, but you should test if was allocated successful.
there is no need to strdup() the return value of slice, which was allocated with malloc().
you should use 2 separate index variables for the index i into the array arr.members and the index of the start of the substring.
the loop should be written with a test so split("", c) return a single empty string.
if indexOf returns the end of the string if c cannot be found, no special case is needed for the last slice.
Here is a modified version:
pieces split(const char *s, const char c) {
pieces arr;
arr.len = charCount(s, c) + 1;
arr.members = malloc(sizeof(*arr.members) * arr.len);
if (arr.members != NULL) {
for (size_t i = 0, start = 0; i < arr.len; i++) {
size_t end = indexOf(s, c, start);
arr.members[i] = slice(s, start, end);
start = end + 1;
if (arr.members[i] == NULL) {
/* free previous substrings and the members array */
while (i-- > 0) {
free(arr.members[i]);
}
free(arr.members);
arr.members = NULL;
break;
}
}
}
return arr;
}
Note these final remarks:
split as coded above works too if indexOf() returns (size_t)(-1) when the character is not found in the string.
recomputing the length of the string in slice() is wasteful. slice() should assume that the argument values are correct: 0 <= a < b <= strlen(s).
there is no direct way for split to return an error. Setting the members to NULL seems a workable solution.
instead of slice(), and assuming indexOf returns a valid offset into the string, you could use the POSIX standard function strndup():
arr.members[i] = strndup(s + start, end - start);

Answer 2

To split a string on 1 character, I would do something like:
#include <string.h>
#include <stdlib.h>
int count_words(char const *str, char const delim)
{
int count = 0;
int i = 0;
for (; str[i]; i++) {
// the next character is the beggining of a new string
if (str[i] == delim && str[i + 1] != delim)
count++;
}
// for safety
if (str[i - 1] != delim)
count++;
return count;
}
int word_length(char const *str, char const delim)
{
int length = 0;
// while we're on a valid character, increase the word length
while (str[length] && str[length] != delim)
length++;
return length;
}
pieces split(char const *str, char const delim)
{
// move the pointer until we're not on the delimiter
while (*str == delim)
str++;
// prepare the string array
pieces p;
p.len = count_words(str, delim);
p.members = malloc(p.len * sizeof(char *));
// for each string
for (int i = 0; i < p.len; i++) {
// copy the string
int length = word_length(str, delim);
p.members[i] = strndup(str, length);
// move the pointer until we're not on the delimiter
str += length;
while (*str == delim)
str++;
}
return p;
}