Where Is It?

One of the most common things we need to do with arrays is to search them. While there are several different approaches to the general problem of search, we're going to focus only on the simplest form of search.

If forced to put it into words we might describe the process like this:

Start at the first element
While we're not at the end of the list
   If this is the one we're looking for stop searching
   Otherwise move on to the next

By way of example, let's consider the case of searching for a student who has scored above 90 on the first assignment. We could implement this approach to the search with the following code:
i = 0;
while(i < num_stud)
   if(grades[i][0] > 90)

We might also combine the two tests:
Start at the first element
While we're not at the end and we haven't found
   what we're looking for:
      Move on to the next element and repeat

which would be implemented as follows:
i = 0;
while(i < num_stud && grades[i][0] <= 90)

Notice though, that this is exactly the same structure that we said was equivalent to the for statement. Building on this observation, we could code this as:
for(i = 0; i < num_stud && grades[i][0] <= 90; ++i) ;

Notice that there is no body to this loop; it has a semicolon at the end of the for() part. (Many recommend some stylistic convention to make it clear that such an empty body was intentional. For example some put the comment /* empty */ before the semicolon. Others put the semicolon on a line by itself. Some even do both.) The gist of this statement is that we aren't repeating anything for each student per se. Instead, we are interested in this statement for it's side effect: making i be the index of the first student that scored over 90.

After we're finished with the loop, we need to ask if we really found it or not. We could follow the loop with something like:

if(i >= num_stud) {
   /* error condition */
else {
   /* process the student we found */

(We sometimes see code where this test is placed inside the loop itself. For example:

for(i = 0; i < num_stud; ++i) {
   if(grades[i][0] > 90) {
      /* process the student we found */
      break ;

We do not recommend this style since it falsely implies that we might do the true branch of the if many times. If on the other hand, we wanted to do something to each of the students that scored over 90 (instead of just the first), then we would use this style.)

Another common searching task is typified by this code that looks for the student who did the best on the first assignment.

best = 0;
for(i = 1; i < num_stud; ++i)
   if(grades[i][0] > grades[best][0])
      best = i;
printf("Student %d did the best.\n", best);

The right way to think about this code is to notice that at each point in time best is the number of the student that has done the best out of all those we've look at so far.
Write a for statement in the one-line style that searches for the first student to receive a failing grade (< 60) on the last assignment (assuming 6 assignments). Use the variable i to count through the students.