Computer programming languages



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Computer Programming Languages Draft (incomplete) C. Herbert 9/15/2005

Computer programming languages


Low-level vs. high level languages

Machine code

Assembly language

Interpreters and compilers

internal vs. external storage

source code, object code, executable files

Development of high-level languages

Teaching languages vs. production languages

Scripting languages


Computer Programming Languages


A

Figure 2.1 – All information in modern computers is processed as base two binary numbers.


lmost all modern high-speed digital electronic computers are based on the binary numbering system. In the heart of the computer, inside the central processing unit (CPU), there are one or more arithmetic logic units (ALUs) that process information by performing binary arithmetic. All of the audio, video, word processing, Internet access and so on, which we associate with modern computers is processed with binary arithmetic inside the CPU by these relatively simple electronic circuits. All data to be handled by the computer, and all of its instructions, must be processed in the CPU as a stream of binary numbers.

The set of binary digits, or bits, that the CPU understands as its instructions to perform this binary arithmetic is called the computer’s “machine code.” Each CPU, or each family of CPUs, such as the Intel 8086 family, has its own machine code. So, there are as many machine codes as there are families of processing units. Eventually, everything that a computer does must be translated into its machine code.

W
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The same code in java, assembler, and machine code.
hen a new processing unit is first invented and manufactured, it only understands its machine code. Systems programmers work with these binary codes to create a new language called assembly language. They do this by using machine code to build an assembler, which is a program that translates assembly language into machine code. Assembly languages are made up of very primitive instructions, just like machine code, but they can be written using numbers in bases other than base two; mnemonics, or short words that sound like the instructions they represent, such as ADD for addition or SUB for subtraction; and symbolic names instead of numbers to refer to memory locations.

In the sample code on the left, …

Writing sophisticated software such as word processors and video games is still rather difficult and very time consuming in assembly language. Eventually, computer scientists and software engineers build translators that can handle high-level languages, which are closer to human languages. Java, JavaScript, Visual BASIC, C, C++, C#, and Python are all examples of modern high-level computer programming languages.

T


Figure 2.3 – All programs must be translated into machine code. Compilers, interpreters and assemblers perform this translation.


he translators that convert high-level languages into machine code fall into two categories: compilers and interpreters. Using a compiler, a programmer ends up with two stored copies of the program. The first, in the original high-level programming language, is called the “source code.” The second stored copy of the program, which is the same program after translation into a particular machine code, is called the object code. Even after translation into machine code, a program may still need to be processed so that it will run on a particular computer with a particular operating system. There is often another step necessary after compiling to mix the object code in with subroutines from the operating system. This step is sometimes called “linking and loading” or “making” an executable program. Sometime linking and loading happens when we try to run object code, and sometimes the compilers make and store an executable program as another step in the process of compiling. So, with a compiler, there are two stored copies of the program, the original source code and the object code, and sometimes a third copy called an executable program.

An interpreter is much simpler than a compiler. Rather than translating an entire source code program into object code at once, the compiler translates each instruction one at a time and then feeds it to the CPU to be processed before translating the next instruction. The only stored copy of the program is the original source code program. Often scripting languages, such as JavaScript or Visual BASIC for Applications (VBA), work this way. Scripting languages are simplified high-level programming languages that allow someone to program in a particular environment. JavaScript can be added to the HTML codes for Web pages to provide them with some primitive data processing capability. VBA allows someone to program features in Microsoft Office products such as Microsoft Word or PowerPoint.

Interpreters are also used for teaching languages. Serious computer programming languages such as Java and C# that are used by professional programmers are sometimes referred to as production languages. Teaching languages are languages that are not generally used in production environments, but are instead used to teach someone the logic of computer programming or the processes used in creating computer software before attempting to teach them to use production languages. The Alice programming language, which is included with this book, is an example of a teaching language. Its primary purpose is to be used as a tool to teach people to be better programmers.

The first high level language that ever existed was the FORTRAN language, created in 1957 by the U.S. Government in cooperation with IBM, which, at the time, was by far the world’s largest computer company. FORTRAN was intended to be used by scientists and engineers working on large, primitive main frame computers of the day – about 20 years before the first personal computers appeared – to program mathematical formulas and processes. For example, it is rather easy, assuming one knows the math, to write FORTRAN programs to perform matrix algebra on large sets of data or to perform the fast Fourier transformations that electrical engineers use in calculus-based applications. In fact, the name FORTRAN comes from the two words “formula translator.”

Before FORTRAN all software had to be created using assembly language and machine code. Once FORTRAN appeared, people began to use it for much more than science and engineering. The increasing use of FORTRAN to process commercial business data led to problems for financial accountants and auditors. A bank auditor, for example, needs to be able to read a computer’s instructions to see what it computer is doing with figures that represent banks deposits, account interest, and transaction fees. This was nearly impossible with FORTRAN, unless the auditor was also a trained computer programmer.

The solution to the problems of using FORTRAN in the business world were solved with appearance in 1960 of the COBOL language. Like the name FORTRAN, COBOL is an acronym that comes from the words “common business-oriented language.” COBOL was developed by a team of people working for the United States Navy under the direction of Grace Hopper, who rose to become an admiral before she retired nearly 40 years later. It is estimated that as of the year 2000 there were more lines of code written in COBOL than in any other computer programming language. COBOL has functions and instructions that are more suited to commercial data processing than FORTRAN, and is a wordier language, which makes it easier for financial auditors to understand without extensive training.

Yet COBOL, like FORTRAN, takes a while to master. For College students, this often meant that several semesters had to be spent learning programming before anything useful could be done with a computer. At the same time, computers were becoming smaller, less-expensive, and more accessible to the public. Personal computers were still some years away, but by the mid-1960’s many college campuses had computers that students could use. In 1965, in response to the promise of the computer on campus, and in order to make programming as accessible to students as the new “mini-computers” that had begun to appear, two professors at Dartmouth College in Hanover, New Hampshire, John Kemeny and Thomas Kurtz, invented the BASIC programming language. BASIC was an interpreter-based language rather than a compiled language like FORTRAN and COBOL, which was designed to be easy to learn and easy to use. It caught on quickly, and when personal computers began to appear in the late 1970’s every machine had to have a BASIC interpreter or people wouldn’t buy it, and more people learned BASIC than any other language.

Yet, as BASIC increased in popularity, one major problem with the language became evident. The BASIC language had a GOTO command, which is sometimes strangely referred to as an “unconditional branching” command. Each line in a BASIC program was numbered, and at any point in the program the GOTO instruction could suddenly re-direct the flow of control to a line number in another part of the program. The command was intended to let users set up branching ad looping command linked to IF…THEN statements, but it was so flexible to use that for more than just a simple straight line sequence of instructions, programmer often ended up with poorly designed logic that jumped repeatedly back and forth throughout the code. People other than the original programmer often had to spend hours trying to figure out how the program worked. People had to be trained to avoid creating what was referred to as “spaghetti code.”

The BASIC language was so easy to learn and so easy to use, that people often developed very bad programming habits, such as creating spaghetti code, before they were properly trained in how to design the logic of computer programs.

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Insert loop example

Pascal vs. BASIC here
n response to this problem, a computer scientist from the Netherlands named Nicholas Wirth, invented the Pascal programming language around 1970. He named the language after the 17th Century French Mathematician and Philosopher, Blaise Pascal, who 300 years earlier had been one of the first people to ever build a working mechanical calculator. From the beginning, Wirth’s Pascal programming language was intended to be used as a teaching language, and contained commands with built-in structured logic, which we will see in chapter xx.

Pascal was the first language to have built-in commands for looping and branching that forced the user to write programs according to good principles of structured design. In Pascal, it became natural for programmers to construct programs with a logical flow of instructions and almost impossible for them to end up with spaghetti code. Like BASIC, Pascal was a simple interpreter-based language and was easy to learn and easy to use.



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