c - scanf in a while loop reads on first iteration only -

note: please notice not duplicate of why scanf() causing infinite loop in code? , i've seen question issue there checks ==0 instead of !=eof. also, problem different, "infinite loop" there still waits user input, not exit.

i have following while loop:

while ((read = scanf(" (%d,%d)\n", &src, &dst)) != eof) {                 if(read != 2 ||                          src >= n || src < 0 ||                          dst >= n || dst < 0) {                         printf("invalid input, should (n,n)");                 } else                         matrix[src][dst] = 1;         } 

the intention of read input in format (int,int), stop reading when eof read, , try again if invalid input received.

the probelm is, scanf works first iteration, after there infinite loop. program not wait user input, keeps assuming last input same.

read, src, , dst of type int.

i have looked @ similar questions, seem fail checking if scanf returns 0 instead of checking eof, , answers tells them switch eof.

you need use

int c; while((c=getchar()) != '\n' && c != eof); 

at end of while loop in order clear/flush standard input stream(stdin). why? answer can seen below:

the scanf format string(" (%d,%d)\n") have requires user type

1. an opening bracket(()
2. a number(for first %d)
3. a comma(,)
4. a number(for last %d)

the space(first character of format string of scanf) , newline character(\n last character of format string of scanf) considered whitespace characters. lets see c11 standard has whitespace characters in format string of fscanf(yes. said fscanf because equivalent scanf when first argument stdin):

7.21.6.2 fscanf function

[...]

5. a directive composed of white-space character(s) executed reading input first non-white-space character (which remains unread), or until no more characters can read. directive never fails

so, whitespace characters skips/discards whitespace characters, if any, until first non-whitespace character seen in quote above. means space @ start of format string of scanf cleans leading whitespace until first non-whitespace character , \n character same.

when enter right data per format string in scanf, execution of scanf not end. because \n hadn't found non-whitespace character in stdin , stop scanning when finds one. so, have remove it.

the next problem lies when user types else not per format string of scanf. when happens, scanf fails , returns. rest of data caused scanf fail prevails in stdin. character seen scanf when called next time. can make scanf fail. causes infinite loop.

to fix it, have clean/clear/flush stdin in each iteration of while loop using method shown above.