File parsing in c, extract specific line if it exists

Question

I have a dynamically updated text file with names of people, I want to parse the file to extract "Caleb" and the string that follows his name. However, his name may not always be in the list and I want to account for that.

I could do it in Java, but not even sure what to do in C. I could start by reading in the text file line by line, but then how would I check if "Caleb" is a substring of the string I just read in and handle the case when he isn't? I want to do this without using external libraries - what would be the best method?

Barnabas: Followed by a string
Bart: Followed by a string
Becky: Followed by a string
Bellatrix: Followed by a string
Belle: Followed by a string
Caleb: I want this string
Benjamin: Followed by a string
Beowul: Followed by a string
Brady: Followed by a string
Brick: Followed by a string

returns: "Caleb: I want this string" or "Name not found"

score 1 · Answer 1 · answered Oct 31 '20 at 14:57

but then how would I check if "Caleb" is a substring of the string

The heart of the question as I read it. strstr does the job.

char *matchloc;
if ((matchloc = strstr(line, "Caleb:")) {
    // You have a match. Code here.
}

However in this particular case you really want starts with Caleb, so we do better with strncmp:

if (!strncmp(line, "Caleb:", 6)) {
    // You have a match. Code here.
}

Gonçalo Bastos · Answer 2 · 2020-11-13T13:39:18.063

So if you want to check if the user caleb exists, you can simple made a strstr, with your array of strings, and if exists you can make a strtok, to get only the string!

I dont know how you are opening the file, but you can use getline to get line by line!

You can do something like this:

#include <string.h>
#include <stdio.h>
#include <stdlib.h>

int main(){

FILE *file;
char *fich="FILE.TXT";
char *line = NULL;
char *StringFile[100];
size_t len = 0;
ssize_t stringLength;
const char s[2] = ":"; //Divide string for this
char *token;
int check =0;
char *matchloc;


file=fopen(fich, "r"); 
    if(file==NULL){
        fprintf(stderr, "[ERROR]: cannot open file <%s> ", fich);
        perror("");
        exit(1);
    }

while((stringLength = getline(&line, &len, file)) != -1){
    if(line[strlen(line)-1] == '\n'){
        line[strlen(line)-1] = '\0';  //Removing \n if exists
    }   

    if((matchloc = strstr(line, "Caleb:"))){
        check = 1;
        strcpy(*StringFile, line);
        token = strtok(*StringFile, s);

        while( token != NULL ) {
            token = strtok(NULL, s);
            printf("%s\n", token);
            break;
        }
        break;
    }
}

if(check==0){
       printf("Name not found\n");
} 

    return 0;
}

The code, can have some errors, but the idead is that! when founds the name, copy the line to array and the splits it.

File parsing in c, extract specific line if it exists

2 Answers2