Array Details and Advanced Concepts

Programs often need to do the same thing to every item in a collection. Suppose you have an array of names and want to print each one. Before reading any explanation, try to predict what this loop prints, and in what order.

A for-each loop works as a cycle. Each trip around the cycle makes the loop variable hold a new value from the array, and the body runs with that value. The loop checks on each trip whether values remain. It does not check whether a counter has reached a particular number.

Trace the example one pass at a time. On pass one, name holds Ada and the body prints Ada. On pass two, name holds Grace and the body prints Grace. On pass three, name holds Linus and the body prints Linus. When no values remain, the loop stops. The printed output is therefore exactly the three stored values, in stored order.

The general form of a for-each loop is a type, then a variable name, then a colon, then the array or collection to be walked through. The body in curly braces repeats once for each element. In the example above, the type is String, the variable is name, and the array is namelist.

Read aloud, the header in the example says: for each String, called name, in the array namelist, do the following. Read this way, the header describes a traversal. The loop visits the array in turn, and at each visit a single value is made available under the name you chose.

The most important fact about a for-each loop is that no array index appears in it. The loop control variable represents one of the values in the array. It does not represent that value's position. This is the sharpest contrast with the traditional for loop, which usually keeps an integer counter and uses that counter to fetch each element.

Use the traditional for loop when your code needs position information, such as printing the index beside each name or comparing neighboring elements. Use the for-each loop when you only need each value in turn.

An array stores its values in ordered positions. In the example, namelist has three stored values. A traditional loop reaches each value by asking for the value at a position. A for-each loop does not ask for positions at all, but the values are still visited in the same stored order.

The following two programs print the same array. The for-each version needs only the header and the body. The traditional version must declare a counter, state where it starts, state when it stops, and fetch each element by its position.

When a for-each loop's output differs from what you expected, the problem is usually in one of three places: the header, the body, or the data in the array. Check them in that order, because the loop faithfully reports whatever the array holds.