What an Idiom!

There are certain common programming techniques in C that may not be obvious at first but that are common enough that you learn to recognize them. While you probably won't be using these idioms yourself this semester, you may well see them in other code you look at including possibly some of the case studies we'll see.

We've already seen one of these, the mid-test loop. One idiomatic thing about that loop is really a sort of anti-idiom. Namely, we can't use the switch statement to determine the condition upon which to exit the loop. The reason for this is that the break keyword is overloaded so that it's meaning in the switch prevents it from leaving the loop.

The first one is really just an application of the && operator.


for(i = 1; i <= MAX && isend(i); ++i) {
   ...
}

Here we have a function called isend that returns a true value if we've hit the end of the i 's we need to look at. One common way this is used is if we're doing something to each element of a list. So we'll process the first element, then the second, then the third and so on. Now, MAX is the limit on how big the list may be and it may well be the case that isend() is not defined to return a meaningful value if i > MAX . But if we've processed the whole list of MAX elements, then after the last time through the loop, we'll come back to test expression 2 with i equal to MAX + 1 . And we don't want isend() to be called with that value for i . Here the short-circuiting of && saves the day. In effect, the i <= MAX guards the call to isend() preventing it from being called with an invalid input.

That same sort of loop can be used in another way as well.


for(i = 1; i <= MAX && isit(i); ++i) ;
if(i <= MAX) {
   ...
}
else {
   ...
}

Here the function isit() will return true if the i th element in the list is the one we're looking for. Think about a list of students' information and we're looking for the GPA of a certain student and are looking that person up by name. Notice that the loop has no body; it is followed by a semicolon. The if statement is not in the body of the loop. That is, the loop doesn't repeat anything, it just has the side-effect of setting i to identify which element of the list we were searching for. And if that element was not in the list, then i will be greater than MAX . So in the true branch of the if , we'll put the code to process the element we found. The false branch will hold the handling of the error of not finding what we were looking for.

As you might expect, there are many other common techniques which experienced programmers use without giving them a second thought. While we won't be looking at any more of them here, we did want you to get a sense of what some of these were like.

You can move on to the next part .