Scanf two numbers at a time from stdout - c

Question

I have a program that outputs a huge array of integers to stdout, each integer in a line. Ex:
103
104
105
107
I need to write another program that reads in that array and fill up the spaces where the number isn't an increment of 1 of the previous number. The only different between numbers is going to be 2 (105,107), which makes it easier.
This is my code to do that logic:
printf("d",num1);
if ((num2-num1) != 1)
numbetween = num1 + 1;
printf("%d", numbetween);
printf("%d", num2);
else(
printf("%d",num2);
)
So the output of this program will now be:
103
104
105
106
107
My issue is reading the numbers. I know I can do while (scanf("%hd", &num) != EOF) to read all the lines one at a time. But to do the logic that I want, I'm going to need to read two lines at a time and do computation with them, and I don't know how.

Answer 1

You could always just read the first and last numbers from the file, and then print everything in between.
int main( void )
{
// get the first value in the file
int start;
if ( scanf( "%d", &start ) != 1 )
exit( 1 );
// get the last value in the file
int end = start;
while ( scanf( "%d", &end ) == 1 )
;
// print the list of numbers
for ( int i = start; i <= end; i++ )
printf( "%d\n", i );
}

Answer 2

Read first num then add missing if needed when you read next int
#include <stdio.h>
#include <stdlib.h>
int main()
{
int previous = 0;
int num;
scanf("%hd", &previous);
while (scanf("%hd", &num) != EOF) {
for (int i = previous; i < num; i++) {
printf("%d\n" , i);
}
previous = num;
}
printf("%d\n" , previous);
return 0;
}
this input
100
102
103
105
107
110
returns this output
100
101
102
103
104
105
106
107
108
109
110

Answer 3

While you can read the first and last, to fill the range, what you are really doing is finding the min and max and printing all values between them inclusively. Below the names are left first and last, but they represent min and max and will cover your range regardless whether the values are entered in order. Taking that into consideration, another approach insuring you cover the limits of the range of int would be:
#include <stdio.h>
int main (void) {
int num = 0;
int first = (1U << 31) - 1; /* INT_MAX */
int last = (-first - 1); /* INT_MIN */
/* read all values saving only first (min) and last (max) */
while (scanf (" %d", &num) != EOF) {
first = num < first ? num : first;
last = num > last ? num : last;
}
/* print all values first -> last */
for (num = first; num <= last; num++)
printf ("%d\n", num);
return 0;
}
Input
$ cat dat/firstlast.txt
21
25
29
33
37
41
45
49
53
57
61
65
69
73
77
81
85
89
93
97
101
Output
$ ./bin/firstlast < dat/firstlast.txt
21
22
23
24
25
26
27
28
29
<snip>
94
95
96
97
98
99
100
101
Note: you can change the types to conform to your expected range of data.