Scanf and two strings - c

Question

My task is read two strings of digits and save them in different arrays.
I decided to use scanf function, but program can read only first string.
This is my bad-code.
int main()
{
int firstArray[50], secondArray[50], i, j;
/* fill an array with 0 */
for(i=0; i<50; ++i)
{
firstArray[i]=secondArray[i]=0;
}
i=j=0;
while((scanf("%d", &firstArray[i]))== 1) { ++i; }
while((scanf("%d", &secondArray[j]))== 1) { ++j; }
/* Print this. */
for(i = 0; i < 20; ++i)
{
printf("%d ", firstArray[i]);
}
putchar('\n');
for(j = 0; j < 20; ++j)
{
printf("%d ", secondArray[j]);
}
return 0;
}
I just don't understand how scanf function works. Can someone please explain?

Answer 1

scanf ignores blank characters (including new line). Thus your scan will read entire input into firstArray if you have no "non blank" separator.
If file/data has ; at end of first line it will stop the read into firstArray there, and never read anything into secondArray - as you never consume the ;.
/* This will never be 1 as ; is blocking */
while((scanf("%d", &secondArray[i])) == 1) {
So: if you separate with i.e. ; you will have to read / check for this before you read into secondArray.
You could also add something like:
char c;
/* this can be done more tidy, but only as concept */
while((scanf("%d", &firstArray[i])) == 1 && i < max) {
++i;
if ((c = getchar()) == '\n' || c == ';')
break;
}
Also instead of initializing array to 0 by loop you can say:
int firstArray[50] = {0}; /* This set every item to 0 */
Also take notice to ensure you do not go over your 50 limit.

Answer 2

You say strings of digits and you read %d. The format scans the input for the longest sequence representing an integer (signed) value. Two "digit strings" are consumed by the first while loop.
EDIT Instead of "strings of digits" you should say "strings of integers". In this case it is a little bit more subtle since the first while can consume all the integers, unless they are separated by something that is not a possible integer (e.g. a ;).
So, to make the following to work, you must separate the two "lines" with something that can't be parsed as integer and which is not considered "white character". Not the better solution, but one the possible.
#include <stdio.h>
#include <ctype.h>
int main()
{
int firstArray[50] = {0};
int secondArray[50] = {0};
int i, j, l1, l2;
int tmp;
i = j = 0;
// read integers, but not more than size of array
while( scanf("%d", &firstArray[i]) == 1 && i < sizeof(firstArray) ) {
++i;
}
// consume non digits
for(tmp = getchar(); tmp != EOF && !isdigit(tmp); tmp = getchar());
// on EOF you should exit and stop processing;
// we read one more char, push it back if it was a digit
if (isdigit(tmp)) ungetc(tmp, stdin);
while( scanf("%d", &secondArray[j]) == 1 && j < sizeof(secondArray) ) {
++j;
}
l1 = i; // preserve how many ints were read
l2 = j;
/* Print this. */
for(i = 0; i < l1; ++i)
{
printf("%d ", firstArray[i]);
}
putchar('\n');
for(j=0; j < l2; ++j)
{
printf("%d ", secondArray[j]);
}
return 0;
}
EDIT A solution that maybe fits your need better is to read the lines (one per time) into a buffer and sscanf the buffer.

Answer 3

You cannot use scanf to do that.
Read the documentation.
Observations:
with scanf if you enter a digit your loop runs forever
there is no check on size 50 limit of your arrays
if you press return then it ignores that line because does not match your pattern
if you enter a letter the pattern does not match and loop breaks
So use some other function, maybe gets, atoi or strtol. And remember to check the size 50 limit of your arrays.

Answer 4

Actually, there is one special point in C's arrays.
Though you declare an array's size. say int arr[5]; You can store values beyond the size of 5. It doesn't show any error but leads to undefined behavior (Might overwrite other variables).
Please Refer this question: Array size less than the no. of elements stored in it
In you case, that was your problem. The compiler had never passed beyond the first while statements. Thus, you didn't get any output. In fact, it didn't even compile the whole code yet!
while((scanf("%d", &firstArray[i]))== 1) { ++i; }
So, you could write this while statement like this:
while( scanf("%d", &firstArray[i]) ==1 && i<50 )
i++;
or else:
while(i<50 )
{
scanf("%d", &firstArray[i]);
i++;
}
or else:
for (i=0; i<50; i++)
scanf("%d", &firstArray[i]);