第4章アルゴリズムの言語による実現：低水準プログラミング言語

In imperative programming, the programmer writes a series of steps that must be followed in order. Source code spells out a precise series of operations that the computer must execute in order. Since the computer is told to take specific actions and execute these statements, the language is referred to as “imperative.” An imperative is an order or command. Low-level languages are imperative languages, and middle-level languages, such as C, are imperative and include another paradigm. While low-level languages can mimic the style of a structured language, these properties are not inherent in the language itself and must be imposed by the programmer as a practice. Assembly code can easily be written in a non-structured way.

Another type of programming, declarative programming, is a paradigm in which code dictates a desired outcome without specifying how that outcome is achieved. Declarative languages are an alternative to imperative languages. In a declarative language, the programmer declares the desired outcome, and it is the compiler’s job to create a series of imperative steps that obtains that outcome. For example, the Structured Query Language (SQL) used to query database systems makes it possible to specify what data should be retrieved from a database, without specifying how the database system should retrieve that data.

Functional languages are another alternative to imperative languages. Recall that a function is a mathematical object that defines how to convert an input into an output. For example, given x = 4, the function $f\left(x\right)=x+3$ converts the input 4 into the output 7. Functions can be defined in most programming languages and correspond to small sections of code that perform a specific task such as a calculation. Functions can be defined in most programming languages.

Functional programming is a programming paradigm in which algorithms are written as mathematical functions (Figure 4.10). In functional programming, practically every part of the program is written as a function. The programmer writes functions that convert inputs to outputs, and it is the compiler’s job to create imperative steps to evaluate the functions.

Declarative and functional languages are considered high-level languages because the compiler is creating these steps on behalf of the programmer. Functional languages are discussed in more detail in Chapter 7 High- Level Programming Languages.

In low-level languages and early high-level languages such as BASIC, some special statements called conditional statements (using “if/then”) and iteration (called “loops”) are programmed using an operation called GOTO, a non-structured operation that instructs a computer to jump to an entirely different part of the program. In large programs, these jumps from one spot to another interact in complex ways, so the flow of execution is difficult to understand when attempting to read the code. These sorts of programs are criticized for being messy “spaghetti code” (Figure 4.11).

Newer languages were developed to help avoid spaghetti code. In a structured programming language, control flow leverages conditionals (e.g., “if-then”) and iteration statements (e.g., “while” or “do while”) and never uses GOTO statements. For example, C is a structured language that includes the conditional statements “if” and “switch,” and the iteration statements “for,” “while,” and “do.” In proper C, all the code sections that involve conditionals and iteration are written with these statements, and GOTO should not be used. Note that the fact that GOTO is provided as a keyword in the C language relates to the fact that C is a low-level language and programmers at that level are given the choice of using unstructured programming if necessary, although it is not recommended.