Progress string parsing in C - c

Question

I have the following character string:
"..1....10..20....30...40....50...80..."
and I need to extract all numbers from it into array.
What is the best way to do it in C?

Answer 1

Perhaps the easiest way is to use the strtok() function (or strtok_r() if reentrancy is a concern):
char str[] = "..1...10...20";
char *p = strtok(str, ".");
while (p != NULL) {
printf("%d\n", atoi(p));
p = strtok(NULL, ".");
}
Once you have the results of calling atoi(), it should be a simple matter to save those integers into an array.

Answer 2

You can use a sscanf code with suppressed assignment (%*[.]) to skip over the dots (or any other character you want), and a scanned character count code %n to advance the string pointer.
const char *s = "..1....10..20....30...40....50...80...";
int num, nc;
while (sscanf(s, "%*[.]%d%n", &num, &nc) == 1) {
printf("%d\n", num);
s += nc;
}

Answer 3

Here is the correct way to do it, it is a little longer than the simplest way but it doesn't suffer from undefined behavior if the value read is out of range, works properly if the first character is not a dot, etc. You didn't specify whether the numbers could be negative so I used a signed type but only allow positive values, you can easily change this by allowing the negative sign at the top of the inner while loop. This version allows any non-digit characters to delimit integers, if you only want dots to be allowed you can modify the inner loop to skip only dots and then check for a digit.
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <errno.h>
#define ARRAY_SIZE 10
size_t store_numbers (const char *s, long *array, size_t elems)
{
/* Scan string s, returning the number of integers found, delimited by
* non-digit characters. If array is not null, store the first elems
* numbers into the provided array */
long value;
char *endptr;
size_t index = 0;
while (*s)
{
/* Skip any non-digits, add '-' to support negative numbers */
while (!isdigit(*s) && *s != '\0')
s++;
/* Try to read a number with strtol, set errno to 0 first as
* we need it to detect a range error. */
errno = 0;
value = strtol(s, &endptr, 10);
if (s == endptr) break; /* Conversion failed, end of input */
if (errno != 0) { /* Error handling for out of range values here */ }
/* Store value if array is not null and index is within array bounds */
if (array && index < elems) array[index] = value;
index++;
/* Update s to point to the first character not processed by strtol */
s = endptr;
}
/* Return the number of numbers found which may be more than were stored */
return index;
}
void print_numbers (const long *a, size_t elems)
{
size_t idx;
for (idx = 0; idx < elems; idx++) printf("%ld\n", a[idx]);
return;
}
int main (void)
{
size_t found, stored;
long numbers[ARRAY_SIZE];
found = store_numbers("..1....10..20....30...40....50...80...", numbers, ARRAY_SIZE);
if (found > ARRAY_SIZE)
stored = ARRAY_SIZE;
else
stored = found;
printf("Found %zu numbers, stored %zu numbers:\n", found, stored);
print_numbers(numbers, stored);
return 0;
}

Answer 4

I prefer the use of strtok in a for loop. Makes it feel more natural, though the syntax looks a little weird.
char str[] = "..1....10..20....30...40....50...80..."
for ( char* p = strtok( strtok, "." ); p != NULL; p = strtok( NULL, "." ) )
{
printf( "%d\n", atoi( p ) );
}