Reading file containing multiple data formats - c

Question

I'm supposed to read a file in C with a structure that looks like this
A:
1
2
3
4
B:
1 1
2 2
3 3
4 4
C:
1 1 1
2 2 2
3 3 3
4 4 4
The file is always separated into three parts and each part starts with an identifier (A:, B:,..).
Identifier is followed by unspecified number of rows containing data. But in each part the format of the data is different. Also it's not just integers but that's not important in this question.
I don't have a problem reading the file. My question is what would be an optimal way to read such a file? It can contain thousands of rows or even more parts than just three. The result should be for example string arrays each containing rows from a different part of the file.
I didn't post any code because I don't need/want you to post any code either. Idea is good enough for me.

Answer 1

You could read the file line by line and check every time if a new section starts. If this is the case, you allocate new memory for a new section and read all the following lines to the data structure for that new section.
For dynamic memory allocation, you will need some counters so you know how many lines per section and how many sections in total you have read.
To illustrate the idea (no complete code):
typedef struct {
int count;
char **lines;
} tSection;
int section_counter = 0;
tSection *sections = NULL;
tSection *current_section = NULL;
char line[MAXLINE];
while (fgets(line, MAXLINE, file)) {
if (isalpha(line[0])) { // if line is identifier, start new section
sections = realloc(sections, sizeof(tSection)*(section_counter+1));
current_section = &sections[section_counter];
current_section->lines = NULL;
current_section->count = 0;
section_counter++;
}
else { // if line contains data, add new line to structure of current section
current_section->lines = realloc(current_section->lines, sizeof(char*)*(current_section->count+1));
current_section->lines[current_section->count] = malloc(sizeof(char)*MAXLINE);
strcpy(current_section->lines[current_section->count], line);
current_section->count++;
}
}

Answer 2

If each section in the file has a fixed format and the section header has a fixed format, you can use fscanf and a state machine based approach. For example in the code below, the function readsec reads a section based on the parameters passed to it. The arguments to readsec depends on which state it is in.
void readsec(FILE* f, const char* fmt, int c, char sec) {
printf("\nReading section %c\n",sec);
int data[3];
int ret=0;
while ((ret=fscanf(f, fmt, &data[0], &data[1], &data[2]))!=EOF){
if (ret!=c) {
return;
}
// processData(sec, c, &data); <-- process read data based on section
}
}
int main() {
FILE * f = fopen("file","r");
int ret = 0;
char sect = 0;
while ((ret=fscanf(f, "%c%*c\n", &sect))!=EOF){
switch (sect) {
case 'A':
readsec(f, "%d", 1, 'A');break;
case 'B':
readsec(f, "%d %d", 2, 'B');break;
case 'C':
readsec(f, "%d %d %d", 3, 'C');break;
default:break;
}
}
return 0;
}