Object-oriented Software in Ada 95

1 Introduction to programming
1 Introduction to programming
A computer programming language is used by a programmer to express the solution to a problem in terms
that the computer system can understand. This chapter looks at how to solve a small problem using the
computer programming language Ada 95.
1.1 Computer programming
Solving a problem by implementing the solution using a computer programming language is a meticulous process.
In essence the problem is expressed in terms of a very stylized language in which every detail must be correct.
However, this is a rewarding process both in the sense of achievement when the program is completed, and
usually the eventual financial reward obtained for the effort.
Like the planet on which we live where there are many different natural languages, so the computer world also
has many different programming languages. The programming language Ada 95 is just one of the many computer
programming languages used today.
1.2 Programming languages
In the early days of computing circa 1950s, computer programs had to be written directly in the machine
instructions of the computer. Soon assembly languages were introduced that allowed the programmer to write
these instructions symbolically. An assembler program would then translate the programmer’s symbolic
instructions into the real machine code instructions of the computer. For example, to calculate the cost of a
quantity of apples using an assembly language the following style of symbolic instructions would be written by a
programmer:
LDA AMOUNT_OF_OF_APPLES ; Load into the accumulator # pounds
MLT PRICE_PER_POUND ; Multiply by cost per pound of apples
STA COST_OF_APPLES ; Save result
Note:
Each assembly language instruction corresponds to a machine code instruction.
In the period 1957—1958 the first versions of the high-level languages FORTRAN & COBOL were developed. In
these high-level programming languages programmers could express many ideas in terms of the problem rather
than in terms of the machine architecture. A compiler for the appropriate language would translate the
programmer’s high level statements into the specific machine code instructions of the target machine. Advantages
of the use of a compiler include:
• Gains in programmer productivity as the solution is expressed in terms of the problem rather
than in terms of the machine.
• If written correctly, programs may be compiled into the machine instructions of many
different machines. Hence, the program may be moved between machines without having to
be re-written.
For example, the same calculation to calculate the cost of apples is expressed in FORTRAN as:
COST = PRICE * AMOUNT
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