reading lines from text file into array

Question

I have a text file that has the following lines:

(0,0) -180.000  77.500  -999.000  -999.000  -999.000  2740.831  45.000  -0.001  -0.001  0.000 458.138 45.000  -999.000
(1,0) -179.500  77.500  -999.000  -999.000  -999.000  2740.831  45.000  -0.001  -0.001  0.000 458.138 45.000  -999.000
(2,0) -179.000  77.500  -999.000  -999.000  -999.000  2740.831  45.000  -0.001  -0.001  0.000 458.138 45.000  -999.000
(3,0) -178.500  77.500  -999.000  -999.000  -999.000  2740.831  45.000  -0.001  -0.001  0.000 458.138 45.000  -999.000
...
...
(359,0) -0.500  77.500  -999.000  -999.000  -999.000  2740.831  45.000  -0.001  -0.001  0.000 458.138 45.000  -999.000

I am trying to put each line of this text file (buf) into an individual element of an array (buffarray) using the following program:

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

#define PI 4*atan2(1,1)

int main(int argc, char *argv[]) {
    FILE *fp;
    char buf[200];
    char *token;
    char buffarray[223920];
    char filename[150];
    int i, j, k;

    sscanf(argv[1], "%s", filename);

    if ((fp = fopen(filename, "rt")) == NULL) {
        printf("Failed in fopen: %s\n", filename);
        return -1;
    }

    while (!feof(fp)) {
        fgets(buf, 200, fp);
        token = buf;
        printf("buf is %s\n", buf);     
        buffarray++ = token;
    }
}

How every when compiling this program I get an error message:

translate_ww3file.c: In function ‘int main(int, char**)’:
translate_ww3file.c:30:12: error: lvalue required as increment operand
   buffarray++ = token;
        ^

How do I resolve this issue? I ideally want to create another text file where the lines are rearranged so that lines 180 to 359 from the original text are printed first in the new text file and then lines 1 to 179 are printing out afterwards in the new text file.

One of many problems: [`while(!feof(fp))` always wrong](https://stackoverflow.com/questions/5431941) — user3386109, May 06 '19 at 23:37
Look closely at `buffarray++=token;`. Is this really what you want? — Code-Apprentice, May 06 '19 at 23:39
I suggest that you step back from the code and try to describe each step in the process **in words**. Break down each step into as much details as you can. — Code-Apprentice, May 06 '19 at 23:41
Note that `buffarray++=token;` has nothing to do with reading lines of a file. I think you mean that is what the loop should do, but I was asking what this one line is supposed to do. For now I suggest that you remove that line entirely and see what happens. — Code-Apprentice, May 06 '19 at 23:42
What I am trying to do is read each line of the array and put them into an array. I am able to successfully each each line but how to I put them into the array buffarray? — jms1980, May 06 '19 at 23:43
You already put each line into the array named `buf`. What do you want to do from there? Do you want the actual numerical values for each column? If so, you need to learn about `float` and `double`. — Code-Apprentice, May 06 '19 at 23:52
If all you want is to read all of the characters into one large array, then you should use `fread()` instead of `fgets()`. — Code-Apprentice, May 06 '19 at 23:55

score 3 · Accepted Answer · answered May 06 '19 at 23:57

Multiple problems:

the PI macro is not properly parenthesized. It should be #define PI (4*atan2(1,1))
while (!feof(fp)) is always wrong. Use while (fgets(buf, 200, fp)) instead.
You cannot increment an array, you want to concatenate the string at the end of the array with strcat(buffarray, token); but you must initialize buffarray[0] to '\0' before the loop.

Here is a corrected version:

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

#define PI  (4*atan2(1,1))

int main(int argc, char *argv[]) {
    FILE *fp;
    char buf[200];
    char *token;
    char buffarray[223920];
    char filename[150];

    if (argc < 2 || sscanf(argv[1], "%149s", filename) != 1) {
        printf("missing command line argument\n");
        return 1;
    }

    if ((fp = fopen(filename, "rt")) == NULL) {
        printf("Failed in fopen %s: %s\n", filename, strerror(errno));
        return 1;
    }

    *buffarray = '\0';
    while (fgets(buf, sizeof buf, fp)) {
        token = buf;
        printf("buf is %s\n", buf);     
        strcat(buffarray, token);
    }
    fclose(fp);

    printf("file contents:\n);
    fputs(buffarray, stdout);
    return 0;
}

Note: alternative to `#define PI (4*atan2(1,1))` --> `acos(-1)`. — chux - Reinstate Monica, May 07 '19 at 02:24
@chux: yes indeed. I wonder why the Standard does not mandate a definition in `` to the best approximation with the target floating point representation. `PI` is not used in the posted code anyway :) — chqrlie, May 07 '19 at 08:07
When I make the array in the above program. How can I have it set up so I can print out buffarray[i] for example. If I want to print out one element of the array how do I do it? — jms1980, May 08 '19 at 18:48
@jms1980: if your goal is to parse a text file, converting the text to numbers, the method is inappropriate. You probably should use `sscanf()` to convert the line into a set of values that you would store in an array of structures. Can you clarify the problem? — chqrlie, May 08 '19 at 18:57

reading lines from text file into array

1 Answers1