C fscanf returning null

Question

I have an issue with my homework where part of it requires scanning and reading some specific input. The related C code is:

typedef struct Train {
    int train_number;
    int priority;   // 0 is low priority, 1 is high priority
    int direction;  // 0 is Westbound, 1 is Eastbound
    float loading_time;
    float crossing_time;
    int loaded;
} Train;

#define MAX_TRAIN_COUNT 777
Train station[MAX_TRAIN_COUNT];

int main(int argc, char* argv[]) {
    //read the input file
    FILE *input;
    char c;
    int train_count = 0;
    input = fopen(argv[1], "r");

    while((c = getc(input)) != EOF) {
        if(c == '\n')
            train_count++;
    }

    int i;
    for (i = 0; i < train_count; i++) {
        char dir;
        int load, cross;
        fscanf(input, "%c %d %d\n", &dir, &load, &cross);
        printf("%c %d %d\n", dir, load, cross);
    }
    fclose(input);

The input is 3 rows consisting of one char and two integers separated by spaces.

e 10 6
W 6 7
E 3 10

The output I get is:

 4195728 0

Where there is a space before the 4195728. I cannot seem to find a solution to fix this.

What seems to be the problem?

Check your io operations instead of assuming they succeed. Assumption is the mother of all.... Anyway, you didn't rewind your file. All those `fscanf` calls are failing, and you don't know because you never bother to check. Unrelated, from what I see the line-counting portion of this is not necessary if you do decide to start doing properly checked IO. — WhozCraig, Mar 02 '18 at 09:16
In `fscanf(input, "%c %d %d\n", &dir, &load, &cross);` add a space before `%c`. Please see [scanf() leaves the new line char in the buffer](https://stackoverflow.com/questions/5240789/scanf-leaves-the-new-line-char-in-the-buffer). — Weather Vane, Mar 02 '18 at 09:18
It's much more robust to read whole lines using `fgets()`, then use `sscanf()` to parse what you got. Also check the return value of the scanning function, before relying on the values having been scanned. — unwind, Mar 02 '18 at 09:19
Btw, `int loaded = 0;` really works in regular C as a member decl? I must have been sleeping when that was added to the standard. — WhozCraig, Mar 02 '18 at 09:21
Thanks WhozCraig, I didn't know about the rewind thing. Got it working now. About the int loaded = 0; thing, I must have been sleeping lol. — Willytheboy, Mar 02 '18 at 09:29
BTW: change `char c;` to `int c;` for proper functionality for all input and avoid an infinite loop when `char` is an _unsigned char_. — chux - Reinstate Monica, Mar 02 '18 at 15:25
@WeatherVane Pre-pending a space before `"%c"` is a good general advice, yet not needed here in this case given the final `"\n"`. `input` needs to be re-wound.and other problems exist too - yet not the usual missing leading space before `%c`. — chux - Reinstate Monica, Mar 02 '18 at 15:31

chux - Reinstate Monica · Answer 1 · 2018-03-02T15:58:59.263

Code fails to rewind the file before reading the data a 2nd time. Lack of checking input function success contributed to not exposing the problem. @WhozCraig.

int i;
rewind(input); // add
for (i = 0; i < train_count; i++) {

  // fscanf(input, "%c %d %d\n", &dir, &load, &cross);
  if (fscanf(input, "%c %d %d\n", &dir, &load, &cross) != 3) Handle_Error();

A better approach would handle reading the lines of the data file in a helper function.

Using fgets() is much better at handling troublesome input than fscanf(). @unwind

int ReadTrainData(Train *data, FILE *stream) {
  char buffer[200];
  if (fgets(buffer, sizeof buffer, stream) == NULL) {
    return EOF;
  }
  // Various ways to parse data.
  // This one look for a completed scan and checks that `n` was changed.
  int n = 0;
  //              v-- add space to consume optional leading white-space (if desired)
  sscanf(buffer, " %c %d %d %n", &data.direction, &data.loading_time,
      &data.crossing_time, &n);
  if (n == 0) return 0; // data incomplete
  if (buffer[n]) return 0; // extra junk in line

  // maybe valid data
  if (strchr("NSEWnsew", &data.direction) == NULL) return 0;
  if (data.loading_time < 0 || data.loading_time > TRAIN_TIME_MAX) return 0;
  // add other validation tests as needed

  return 1;
}

Sample usage

size_t train_count = 0;
if (input) {
  Train data = {0};
  while(ReadTrainData(&data, input) == 1) {
    train_count++;
    printf("%c %d %d\n", data.direction, data.loading_time, data.crossing_time);
  }
  fclose(input);
}

C fscanf returning null

1 Answers1