Read integers from standard input

Question

A simple C problem, I want to read integers from standard input(user input), each line of standard input contains an integer, and an empty line denotes the end of input. I tried to use gets() and scanf(), but it's not working.

while (1){
            char a;
            scanf("%c",&a);
            printf("%c",a);
            if (a=='\r'||a=='\n'){
                break;
            }
        }

when i use scanf(), It terminates every time i press an enter;

while(1){
            char a[10];
            gets(a);
            if (a=='\r'||a=='\n'){
                break;
            }
        }

and when i use gets(), '\r' or '\n' can not be read into buffer, so it never break.

Could anyone help me with that, thanks in advance!

Answer 1

I want to read integers from standard input

Doing

while (1){
  char a;
  scanf("%c",&a);
  printf("%c",a);
  if (a=='\r'||a=='\n'){
    break;
  }
}

you read a line containing any characters, not only a number representation

Doing

while(1){
   char a[10];
   gets(a);
   if (a=='\r'||a=='\n'){
      break;
   }
}

a is a char* so in a=='\r'||a=='\n' you wrongly compare a pointer with characters, you want *a=='\r'||*a=='\n'

Additional remarks :

• never use gets, use fgets to read a line, so fgets(a, sizeof(a), stdin)
• but you still read a line containing any characters, not only a number

I want to read integers from standard input(user input), each line of standard input contains an integer, and an empty line denotes the end of input.

A proposal, a line containing only space and tabs before the end of line is considered empty, and the spaces and tabs before/after the number are bypassed without being considered invalid :

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

int main()
{
  while(1) {
    puts("enter number");

    char a[10];

    if (fgets(a, sizeof(a), stdin) == NULL) {
      puts("EOF");
      break;
    }

    errno = 0;

    char * endptr;
    long n = strtol(a, &endptr, 10);

    if (errno != 0) {
      puts("not a valid long");
    }
    else {
      /* check possible unexpected characters */
      char * p = endptr;

      for (;;) {
        if (*p == 0) {
          if (endptr == a) {
            puts("done");
            return 0;
          }
          printf("the number is %ld\n", n);
          break;
        }
        if (!isspace(*p++)) {
          puts("not (only) a valid long");
          break;
        }
      }
    }
  }
}

Compilation and execution :

pi@raspberrypi:/tmp $ gcc -pedantic -Wextra n.c
pi@raspberrypi:/tmp $ ./a.out
enter number
123
the number is 123
enter number
 123
the number is 123
enter number
12a
not (only) a valid long
enter number
a12
not (only) a valid long
enter number
a
not (only) a valid long
enter number

done

Answer 2

Is this an XY problem? You can use fgets and sscanf to do the job.

#include <stdio.h>

int main(void) {
    char buffer[100];
    int number;
    while(fgets(buffer, sizeof buffer, stdin) != NULL) {
        if(sscanf(buffer, "%d", &number) != 1) {    // or strtol perhaps
            break;
        }
        printf("Number: %d\n", number);
    }
    return 0; 
}

Answer 3

I think using getline might be a more general solution, here is some example code that you could modify for your purposes, "buf" would contain the input.

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

int main(int argc, char *argv[])
{
    char *buf = NULL;
    int size;
    unsigned int length;
    size = getline(&buf, &length, stdin);
    if (size != -1)
        /* do anything you want with the stuff that was entered here */
        /* for the example I just write it back to stdout! */
        puts(buf);
    else
        /* this would be your "end" condition */
        printf("Nothing read!\n");

    printf("Size: %d\n Length: %d\n", size, length);
    free(buf);
    return 0;
}