从C中的文件读取特定的行和列

Question

Im trying to figure out how to read specific lines from file, ie I have file with 40 lines and I want to print only the lines 3,4,17,24 out from the file, by the line number. What I know right know is just how to print the whole file line by line with a condition of != EOF. What I'm trying to get at the end is a program that can read file's decided lines and print them out. Thanks a lot.

void Analyzer(char* filename, int *rows, int *cols) {
    char s[99];
    FILE *fin = fopen(filename, "r");
    if (fin == NULL)
    {
        printf("Cannot open file.\n");
        return;
    }
    while (fgets(s, 99, fin) != EOF) {
        printf("%s", s);
    }
    fclose(fin);
}

edit: the next step after that is getting specific colums from the decided lines, What I mean by cloumns is that every line has ',' between colums, my idea was to try with strtok(), but if you got any better idea i'd thank you.

Answer 1

Keep a count of lines read.

int linecounter = 0;
while (fgets(s, 99, fin) != NULL) {
    switch (++linecounter){
        case 3: case 4: case 17: case 24:
            printf("%s", s);
    }
}

I also fixed your bug; fgets() returns NULL not EOF on end of file.

Answer 2

A bit off-topic answer: It is a nice exercise to do this in C directly, but it is actually error prone. There are much better tools to filter lines from a file.

ed and sed would be my favorites here:

printf %s\\n 3 4 17 24 q| ed txtfile

sed `printf \ \-e%s\  3p 4p 17p 24p d` < txtfile

or with a list of numbers (output in ed with line numbers)

(cat lnums | while read l; do printf %sn\\n "$l"; done; printf %s\\n q)| ed txtfile

args=`sed -e 's/^[0-9]*$/-e &p/' -e '$a -e d' lnums | tr '\n' ' '`; sed $args txtfile