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Question

Currently, I have some code that will read each line from a text file and print it in the console. (EG Bread 2.10, Milk 3.30)

How would I modify this only read the second index of each line(Bolded for clarity) (EG 2.10, 3.30 and 5.50) I'd then like to multiply those numbers by 0.1 and overwrite the same file.

File Format

Bread 2.10

Milk 3.30

Fish 5.50

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

int main()
{
    int c;
    FILE *file;
    file = fopen("oText.txt", "r");
    if (file) {
        while ((c = getc(file)) != EOF){
            putchar(c);
        }
        fclose(file);
    }
}

Answer 1

In its very simplest form, you want to read each line into a character array (a buffer) and then locate the last space in the buffer, and output from the next character to the end of the line. That will output the 2nd field in your case.

Rather than using character-oriented input functions like getchar() or fgetc() , line-oriented input functions like fgets() or POSIX getline() read an entire line at a time into a sufficiently sized buffer. (rule: don't skimp on buffer size!!) The line-oriented input function also read and include the '\n' at the end of each line in the buffer they fill. Generally you will want to trim the trailing '\n' from any string you are storing -- but you can also use it to your advantage to provide the '\n' if all you are doing is outputting the values as soon as they are read.

To find the last ' ' (space) in your buffer, the strrchr (string char reverse) function (provided in string.h ) will return a pointer to the last character you specify (or NULL if that character is not found in the buffer) This makes your job very easy. Just read each line into a buffer, then call strrchr (buf, ' ') to obtain a pointer to the last space in your buffer, then simply print from the next character to the end of line.

Reading from stdin (which you can simply redirect your file on stdin ), your code reduces to:

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

#define MAXC 1024   /* if you need a constant, #define one (or more) */

int main (void) {

    char buf[MAXC];     /* buffer (array of char) to hold line */

    while (fgets (buf, MAXC, stdin)) {          /* read each line */
        char *lastspace = strrchr (buf, ' ');   /* locate last space */
        if (lastspace != NULL)                  /* if space located */
            fputs (++lastspace, stdout);        /* output from next char on */
    }
}

( note: how fputs is used to output the line relying on the '\n' that was read and included in buf by fgets )

Example Input File

Not knowing whether you had blank lines in your input file as shown, they were left in place (and note: they are gracefully skipped as strrchr will return NULL when the only thing in buf is the '\n' )

$ cat dat/breadmilk.txt
Bread 2.10

Milk 3.30

Fish 5.50

Example Use/Output

Running the code above with the file contents passed in on stdin results in the following:

$ ./bin/breadmilkcost < dat/breadmilk.txt
2.10
3.30
5.50

A Flexible Approach

Also note that because your are locating the last space in the file instead of just trying to read the second word, etc.., this method would work equally well and have the same output even if your file contained:

Bread (two loaves) - 2.10

Milk (one carton) - 3.30

Fish (Alaskan Sockeye) - 5.50

Look things over and look up the man pages for fgets() and strrchr() to make sure you understand how things are working. If you have continuing questions, just drop a comment below and I'm happy to help further.