Control Structure

Computations in imperative-language programs are accomplished by evaluating expressions and assigning the resulting values to variables. However, there are few useful programs that consist entirely of assignment statements. At least two additional linguistic mechanisms are necessary to make the computations in programs flexible and powerful: some means of selecting among alternative control flow paths (of statement execution) and some means of causing the repeated execution of statements or sequences of statements. Statements that provide these kinds of capabilities are called control statements. A control structure is a control statement and the collection of statements whose execution it controls.

Selection Statements

A selection statement provides the means of choosing between two or more execution paths in a program. Such statements are fundamental and essential parts of all programming languages, as was proven by Böhm and Jacopini. Selection statements fall into two general categories: two-way and n-way, or multiple selection.

Two-Way Selection Statements

Although the two-way selection statements of contemporary imperative languages are quite similar, there are some variations in their designs. The general form of a two-way selector is as follows:

if control_expression

    then clause

    else clause

Multiple-Selection Statements

The multiple-selection statement allows the selection of one of any number of statements or statement groups. It is, therefore, a generalization of a selector. In fact, two-way selectors can be built with a multiple selector. The need to choose from among more than two control paths in a program is common. Although a multiple selector can be built from two-way selectors and go-to, the resulting structures are cumbersome, unreliable, and difficult to write and read. Therefore, the need for a special structure is clear.

Iterative Statements

An iterative statement is one that causes a statement or collection of statements to be executed zero, one, or more times. An iterative statement is often called a loop. Every programming language from Plankalkül on has included some method of repeating the execution of segments of code. Iteration is the very essence of the power of the computer. If some means of repetitive execution of a statement or collection of statements were not possible, programmers would be required to state every action in sequence; useful programs would be huge and inflexible and take unacceptably large amounts of time to write and mammoth amounts of memory to store. The first iterative statements in programming languages were directly related to arrays. This resulted from the fact that in the earliest years of computers, computing was largely numerical in nature, frequently using loops to process data in arrays.

Counter-Controlled Loops

A counting iterative control statement has a variable, called the loop variable, in which the count value is maintained. It also includes some means of specifying the initial and terminal values of the loop variable, and the difference between sequential loop variable values, often called the step size. The initial, terminal, and step size specifications of a loop are called the loop parameters. Although logically controlled loops are more general than counter controlled loops, they are not necessarily more commonly used. Because counter-controlled loops are more complex, their design is more demanding.

Logically Controlled Loops

In many cases, collections of statements must be repeatedly executed, but the repetition control is based on a Boolean expression rather than a counter. For these situations, a logically controlled loop is convenient. Actually, logically controlled loops are more general than counter-controlled loops. Every counting loop can be built with a logical loop, but the reverse is not true. Also, recall that only selection and logical loops are essential to express the control structure of any flowchart.

User-Located Loop Control Mechanism

In some situations, it is convenient for a programmer to choose a location for loop control other than the top or bottom of the loop body. As a result, some languages provide this capability. A syntactic mechanism for user-located loop control can be relatively simple, so its design is not difficult. Such loops have the structure of infinite loops but include user-located loop exits. Perhaps the most interesting question is whether a single loop or several nested loops can be exited.

Iteration Based on Data Structures

A general data-based iteration statement uses a user-defined data structure and a user-defined function (the iterator) to go through the structure’s elements. The iterator is called at the beginning of each iteration, and each time it is called, the iterator returns an element from a particular data structure in some specific order. For example, suppose a program has a user-defined binary tree of data nodes, and the data in each node must be processed in some particular order.

A user-defined iteration statement for the tree would successively set the loop variable to point to the nodes in the tree, one for each iteration. The initial execution of the user-defined iteration statement needs to issue a special call to the iterator to get the first tree element. The iterator must always remember which node it presented last so that it visits all nodes without visiting any node more than once. So an iterator must be history sensitive. A user-defined iteration statement terminates when the iterator fails to find more elements.

Unconditional Branching

An unconditional branch statement transfers execution control to a specified location in the program. The most heated debate in language design in the late 1960s was over the issue of whether unconditional branching should be part of any high-level language, and if so, whether its use should be restricted. The unconditional branch, or go-to, is the most powerful statement for controlling the flow of execution of a program’s statements.

However, using the go-to carelessly can lead to serious problems. The go-to has stunning power and great flexibility (all other control structures can be built with go-to and a selector), but it is this power that makes its use dangerous. Without restrictions on use, imposed by either language design or programming standards, go-to statements can make programs very difficult to read, and as a result, highly unreliable and costly to maintain.