How do I split a string by character position in c - c

Question

I'm using C to read in an external text file. The input is not great and would look like;
0PAUL 22 ACACIA AVENUE 02/07/1986RN666
As you can see I have no obvious delimeter, and sometimes the values have no space between them. However I do know how long in character length each value should be when split. Which is as follows,
id = 1
name = 20
house number = 5
street name = 40
date of birth = 10
reference = 5
I've set up a structure I want to hold this information in, and have tried using fscanf to read in the file.
However I find something along the lines of just isn't doing what I need,
fscanf(file_in, "%1d, %20s", person.id[i], person.name[i]);
(The actual line I use attempts to grab all input but you should see where I'm going...)
The long term intention is to reformat the input file into another output file which would be made a little easier on the eye.
I appreciate I'm probably going about this all the wrong way, but I would hugely appreciate it if somebody could set me on the right path. If you're able to take it easy on me in regard to an obvious lack of understanding, I'd appreciate that also.
Thanks for reading

Answer 1

Use fgets to read each line at a time, then extract each field from the input line. Warning: no range checks is performed on buffers, so attention must be kept to resize buffers opportunely.
For example something like this (I don't compile it, so maybe some errors exist):
void copy_substr(const char * pBuffer, int content_size, int start_idx, int end_idx, char * pOutBuffer)
{
end_idx = end_idx > content_size ? content_size : end_idx;
int j = 0;
for (int i = start_idx; i < end_idx; i++)
pOutBuffer[j++] = pBuffer[i];
pOutBuffer[j] = 0;
return;
}
void test_solution()
{
char buffer_char[200];
fgets(buffer_char,sizeof(buffer_char),stdin); // use your own FILE handle instead of stdin
int len = strlen(buffer_char);
char temp_buffer[100];
// Reading first field: str[0..1), so only the char 0 (len=1)
int field_size = 1;
int filed_start_ofs = 0;
copy_substr(buffer_char, len, filed_start_ofs, filed_start_ofs + field_size, temp_buffer);
}

Answer 2

scanf is a good way to do it, you just need to use a buffer and call sscanf multiple times and give the good offsets.
For example :
char buffer[100];
fscanf(file_in, "%s",buffer);
sscanf(buffer, "%1d", person.id[i]);
sscanf(buffer+1, "%20s", person.name[i]);
sscanf(buffer+1+20, "%5d", person.street_number[i]);
and so on.
I feel like it is the easiest way to do it.
Please also consider using an array of your struct instead of a struct of arrays, it just feels wrong to have person.id[i] and not person[i].id

Answer 3

If you have fixed column widths, you can use pointer arithmetic to access substrings of your string str. if you have a starting index begin,
printf("%s", str + begin) ;
will print the substring beginning at begin and up to the end. If you want to print a string of a certain length, you can use printf's precision specifier .*, which takes a maximum length as additional argument:
printf("%.*s", length, str + begin) ;
If you want to copy the string to a temporary buffer, you could use strncpy, which will generate a null terminated string if the buffer is larger than the substring length. You could also use snprintf according to the above pattern:
char buf[length + 1];
snprintf(buf, sizeof(buf), "%.*s", length, str + begin) ;
This will extract leading and trailing spaces, which is probably not what you want. You could write a function to strip the unwanted whitespace; there should be plenty of examples here on SO.
You could also strip the whitespace when copying the substring. The example code below does this with the isspace function/macro from <ctype.h>:
#include <stdlib.h>
#include <stdio.h>
#include <ctype.h>
int extract(char *buf, const char *str, int len)
{
const char *end = str + len;
int tail = -1;
int i = 0;
// skip leading white space;
while (str < end && *str && isspace(*str)) str++;
// copy string
while (str < end && *str) {
if (!isspace(*str)) tail = i + 1;
buf[i++] = *str++;
}
if (tail < 0) tail= i;
buf[tail] = '\0';
return tail;
}
int main()
{
char str[][80] = {
"0PAUL 22 ACACIA AVENUE 02/07/1986RN666",
"1BOB 1 POLK ST 01/04/1988RN802",
"2ALICE 99 WEST HIGHLAND CAUSEWAY 28/06/1982RN774"
};
int i;
for (i = 0; i < 3; i++) {
char *p = str[i];
char id[2];
char name[20];
char number[6];
char street[35];
char bday[11];
char ref[11];
extract(id, p + 0, 1);
extract(name, p + 1, 19);
extract(number, p + 20, 5);
extract(street, p + 25, 34);
extract(bday, p + 59, 10);
extract(ref, p + 69, 10);
printf("<person id='%s'>\n", id);
printf(" <name>%s</name>\n", name);
printf(" <house>%s</house>\n", number);
printf(" <street>%s</street>\n", street);
printf(" <birthday>%s</birthday>\n", bday);
printf(" <reference>%s</reference>\n", ref);
printf("</person>\n\n");
}
return 0;
}
There's a danger here, however: When you access a string at a certain position str + pos you should make sure that you don't go beyond the actual string length. For example, you string may be terminated after the name. When you access the birthday, you access valid memory, but it might contain garbage.
You can avoid this problem by padding the full string with spaces.