C programming: File

Write a below source code, save it as "file.1c", and confirm whether it can run properly.

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

int main(void)
{
  FILE *fout;
  int	c;

  if((fout=fopen("file1.txt", "w")) == NULL){
    printf("Cannot open output file\n");
    exit(1);
  }

  printf("Input string\n");
  printf("Ctrl-d if you want to exit\n");
  while ((c=getchar()) != EOF){
    putc(c, fout);
  }

  fclose(fout);

  exit(0);
}

(Execution result)
$
$ ./a.out
Input string
Ctrl-d if you want to exit
aaaa
bbbbbbbbbbbbb
cccccc
dddddddd
eeee
$
$ cat file1.txt
aaaa
bbbbbbbbbbbbb
cccccc
dddddddd
eeee

Exercise 2

We want to read strings from "file1.txt" created in Exercise 1, and show them on screen by "printf()". Write a below source code, complement the expression hidden by "******" and save it as "file.2c". Confirm whether it can run properly.

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

int main(void)
{
  FILE *fin;
  char	c[256];

  if((fin=fopen("file1.txt", "r")) == NULL){
    printf("Cannot open input file\n");
    exit(1);
  }

  printf("Contents of file1.txt:\n");
  while (*****************************){
    printf("%s",c);
  }

  fclose(fin);

  exit(0);
}

(Execution result)
$
$ ./a.out
Contents of file1.txt:
aaaa
bbbbbbbbbbbbb
cccccc
dddddddd
eeee
$

Exercise 3

Write a below source code, save it as "file.3c", and confirm whether it can run properly.

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

int main(void)
{
  FILE *fout;
  int	dat;

  if((fout=fopen("file3.txt", "w")) == NULL){
    printf("Cannot open output file\n");
    exit(1);
  }

  printf("Input integer\n");
  printf("Ctrl-d if you want to exit\n");

  while (scanf("%d", &dat) != EOF){
    fprintf(fout,"%d ",dat);
  }

  fclose(fout);

  exit(0);
}

(Execution result)
$
$ ./a.out
Input integer
Ctrl-d if you want to exit
21
78
42
93
76
   <- Ctrl-d
$
$ cat file3.txt
21 78 42 93 76 $

Exercise 4

We want to read data from "file3.txt" created in Exercise 3, and show the values of the data multiplied by 3. Write a below source code, complement the expression hidden by "******" and save it as "file.4c". Confirm whether it can run properly.

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

int main(void)
{
  FILE *fin, *fout;
  int	dat;

  if((fin=fopen("file3.txt", "r")) == NULL){
    printf("Cannot open input file\n");
    exit(1);
  }

  if((fout=fopen("file4.txt", "w")) == NULL){
    printf("Cannot open output file\n");
    exit(1);
  }

  while (****************************){
    ********************************
  }

  fclose(fin);
  fclose(fout);

  exit(0);
}

(Execution result)
$
$ ./a.out
$
$ cat file4.txt
63 234 126 279 228 $