CODEWITHSUNDEEP
cio
emanyalpsid
emanyalpsid

Reputation: 329

Reading lines from file using input redirection in C

What I'm trying to do is to read a file line by line using scanf. I'm using scanf because the input file needs to be redirected when compiled ex. ./a.out < inputFile

It was successful in the beginning, using:

while(scanf("%[^\n]%*c", &line) == 1) {          
             printf("%s\n",line);
}

Which printed the file line by line, however when I want to read 2 ints(for example) before reading a bunch of lines ex:

0 4
IWANTTOREADTHISLINE
IWANTTOREADTHISLINE
IWANTTOREADTHISLINE
IWANTTOREADTHISLINE

By using scanf beforehand:

scanf("%d %d", &a, &b);
while(scanf("%[^\n]%*c", &line) == 1) {          
                 printf("%s\n",line);
    }

It doesn't work. What's the reason for this? or is there a better way to do it? Thanks!

Upvotes: 1

Views: 4631

Answers (2)

Scott Griffiths
Scott Griffiths

Reputation: 684

Is there a reason you are using input redirection? Since you asked if there was a better way, I would suggest simply passing the name of the file that you want to read as an argument. For instance, to read a file foo.txt, you would use

./a.out foo.txt

Here is an example of how this works:

#include <stdio.h>

int main(int argc, char* argv[]) {
  if(argc < 2) {
    printf("Please pass a filename as an argument\n");
    return -1;
  }

  FILE* fp;       // file pointer
  char buf[200];  // buffer to hold a line of text

  // open file, name stored in argv[1]
  if((fp = fopen(argv[1], "r")) == NULL) {
    printf("Error opening file %s\n", argv[1]);
    return -1;
  }
  while(fgets(buf, sizeof(buf), fp) != NULL) {
    // do stuff with the string, stored in buf
    printf("%s", buf);
  }

  // close the file and exit
  fclose(fp);
  return 0;
}

Note that argc is an argument counter, and argv[] is an array of arguments. You can pass multiple arguments to your program in this way. Cheers!

Upvotes: 0

GreenAsJade
GreenAsJade

Reputation: 14685

You need to scan in the trailing \n on the line with the ints.

momerath:~ mgregory$ cat foo.txt
0 4
IWANTTOREADTHISLINE
IWANTTOREADTHISLINE
IWANTTOREADTHISLINE
IWANTTOREADTHISLINE
momerath:~ mgregory$ cat foo.c
#include <stdlib.h>
#include <stdio.h>

int main(){

  int a,b;
  char line[100];

  scanf("%d %d\n", &a, &b); 
  while(scanf("%[^\n]%*c", &line) == 1) {          
    printf("%s\n",line);
  }
}
momerath:~ mgregory$ gcc foo.c
foo.c:10:28: warning: format specifies type 'char *' but the argument has type
      'char (*)[100]' [-Wformat]
  while(scanf("%[^\n]%*c", &line) == 1) {          
               ~~~~        ^~~~~
1 warning generated.
momerath:~ mgregory$ ./a.out < foo.txt
IWANTTOREADTHISLINE
IWANTTOREADTHISLINE
IWANTTOREADTHISLINE
IWANTTOREADTHISLINE
momerath:~ mgregory$

Upvotes: 4

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