CompTIA Tech+ (FC0-U71): Software Development Concepts — Languages, Data Types and Program Logic

A programming language category describes what a language is for and how its code reaches the processor. A processor only executes machine code — binary instructions for its own instruction set — so everything a person writes must be translated, either all at once before the program runs (compiled), step by step while it runs (interpreted), or not at all because it is not a program (markup, which describes documents). Tech+ objective 4.1 names six categories, and the exam asks you to tell them apart from a short description or a single example.

Categories overlap: Python is both interpreted and a popular scripting language, and JavaScript is interpreted (in practice, modern engines also compile it on the fly) and scripts web pages. Answer from the property the scenario describes — when translation happens, what the language is used for, or how close it is to the hardware.

The difference is when translation happens. A compiler translates the whole program into machine code once, ahead of time, producing an executable file. An interpreter, in the classic model, reads the source code each time the program runs and translates and executes it one statement at a time. (Real implementations blur the line: the main Python implementation first compiles source to bytecode and then interprets that, and JavaScript engines compile hot code on the fly, but for Tech+ the classic model is what is tested.)

Match the property a scenario stresses to a row of the table. Execution speed and a standalone executable are strengths of compiled code; portability, quick edit-and-test cycles and errors that surface only at run time are characteristics of interpreted code. Neither is better in general — the requirement decides.

Scripting languages automate tasks, markup languages describe the structure of documents and data, query languages read and change data in a database, and assembly language is a human-readable form of a processor's machine instructions.

• Scripting languages automate tasks: creating user accounts in bulk, backing up folders on a schedule, clearing out old log files, making a web page interactive. Scripts are short, quick to write and usually run by an interpreter. PowerShell automates Windows administration, Bash automates Linux and other Unix-like shells, and Python and JavaScript are also widely used for scripting.
• Markup languages place tags around content to describe its structure or meaning; they contain no program logic. HTML uses a fixed set of predefined tags that browsers understand (<h1> for a heading, <p> for a paragraph, <a> for a link). XML lets the author name their own tags (<isbn>, <author>) to describe data for storage and exchange.
• Query languages ask a database for data and change it. SQL (Structured Query Language) is the standard for relational databases; a statement such as SELECT title FROM books WHERE year > 2020 returns the matching rows. Vendors add dialects, such as Microsoft's Transact-SQL for SQL Server. A script can send a query, but the query itself is written in a query language.
• Assembly language is the lowest-level human-readable language. Each mnemonic, such as MOV (move data) or ADD, corresponds almost one to one to a machine instruction, so assembly is tied to one processor family and an assembler converts it to machine code. It gives precise control and speed but is slow to write, which is why today it is used mostly for small, hardware-specific parts of a system — boot code, interrupt handlers and performance-critical routines — while most firmware is written in a higher-level language such as C.

A data type tells the program what kind of value a piece of data is, which decides how much memory it needs and which operations make sense on it. Objective 4.2 names five types; choose the one that fits the value exactly.

"Numbers" is the umbrella for the numeric types. Use an integer for things that are counted (seats on a bus, pages in a book, goals in a match); there is never half a seat. Use a float when the fraction matters (a body temperature, a distance in kilometres, an exchange rate) — storing 2.75 in an integer would lose the .75. A Boolean fits anything with exactly two states — whether a door sensor reports open, whether a user has verified their email address — and also the result of a comparison: temperature > 30 evaluates to true or false, so the stored result is a Boolean even though the temperature itself is a number.

A char is the best fit for a value that is always exactly one character, such as a T-shirt size code (S, M or L) or a middle initial; a string is for text of any length, such as a street address with spaces and digits in it. A string could hold a single letter, and a string or integer could imitate a two-state value, but the best type is the one that allows only valid values.

Why type matters: strings, numbers and concatenation

The same operator can do different things depending on type. With numbers, + adds; with strings, + in many languages concatenates — joins the text end to end. So 12 + 5 gives 17, but "12" + "5" gives "125", and in languages such as JavaScript a string plus a number also joins them as text.

This is a common bug. Anything a user types into a form field, or a program reads from a file or the command line, arrives as a string, even when it looks like a number. Unless the program converts it to an integer or float first, "adding" to it produces joined text rather than a sum — so when a calculation shows the digits side by side instead of a total, the value was still text.

These are the ways a program names and stores data.

• An identifier is the name given to something in code — a variable, constant, function or object — such as totalPrice or calculateTax. It is how code refers to the thing, not the thing itself. Good identifiers describe what they name.
• A variable is a named storage location whose value can change while the program runs: a countdown timer, a running total, the number of login attempts so far.
• A constant is a named value that cannot change while the program runs, such as the number of minutes in an hour or the value of pi. Declaring it constant means an accidental change is caught as an error, which a variable would allow.
• An array stores many values of the same kind under one identifier, each reached by its position number, the index. Most programming languages count from zero (zero-based indexing): in an array days holding Mon, Tue, Wed, Thu and Fri, days[0] is Mon and days[4] is Fri — the last index is always one less than the number of elements. Asking for an index outside that range, such as days[5], is an out-of-range error.

A function is a named, reusable block of code that performs one task. You write it once and call it from anywhere in the program, so the same logic (converting a temperature, validating an email address) is not copied in several places, and a fix in the function fixes every caller.

Values passed into a function when it is called are its parameters (the values supplied at the call are also called arguments). The value the function sends back to the code that called it is its return value.

FUNCTION average(first, second)
    RETURN (first + second) / 2
END FUNCTION

mean = average(8, 12)    // mean is 10

Here first and second are parameters, 8 and 12 are the arguments supplied at the call, (first + second) / 2 produces the return value, and average is the function's identifier. Functions hold code; variables, constants and arrays hold data.

Object-oriented programming models a program as objects — things that bundle data with the actions that work on it. A class is the template that defines what every object of that kind has and can do; an object is one instance created from the class, with its own values.

Properties and attributes both describe an object's data (some languages draw a fine line between the two words, but at this level they mean the same thing). The quick test: if it describes the object, it is a property or attribute; if the object does it, it is a method. A balance describes an account, so it is a property; depositing is something the account does, so deposit() is a method. And each account you open — savings, checking — is a separate object with its own balance, all built from the one BankAccount class.

Object-oriented methods also organise code: related data and behaviour live together in one class (encapsulation), and a new class can reuse and extend an existing one (inheritance), so a large program is built from well-defined, reusable pieces.

Organisational techniques help people plan, read and maintain a program; none of them change what the program does.

• Pseudocode writes a program's logic as plain, English-like statements (IF temperature > 30 THEN show heat warning) before any real code exists. It follows no language's exact rules and no computer can run it; it is a planning step that any programmer can then turn into real code.
• A flowchart plans the same logic graphically, with standard shapes joined by arrows that show the direction of flow.
• Comments are notes written inside the source code that explain what a section does or why. The computer ignores them when the program runs (they start with markers such as // or #).
• Documentation is written material kept separate from the code — user manuals, setup guides, API references, release notes, design notes — explaining how the program is set up, operated or maintained.

Near-twins: pseudocode and flowcharts both plan logic (text vs shapes); comments and documentation both explain code to people (inside the code vs a separate guide).

Every program is built from three control structures: sequence, branching and looping. Sequence is the default: statements run one after another, in order, each once.

Branching (selection) tests a condition and chooses which path to take, using IF … THEN … ELSE. In an IF / ELSE IF / ELSE chain, conditions are checked from the top; the first one that is true runs its block and the rest of the chain is skipped. ELSE runs only if none of the conditions was true.

Conditions are combined with logic (Boolean) operators:

Comparisons such as >, <, >=, <=, = (equal) and != or <> (not equal) each produce a Boolean, which is what the IF tests.

Looping (iteration) repeats a block of code while a condition holds or for a set number of times. A loop checks a condition like a branch does, but where a branch picks a path once, a loop goes back and runs the block again.

• A FOR loop runs a known number of times, stepping a counter: FOR i = 1 TO 3 runs with i = 1, 2 and 3.
• A WHILE loop repeats as long as its condition is true and checks the condition before each pass, so it stops at the first check that is false. If the condition never becomes false, the loop never ends (an infinite loop).

To find what a loop produces, trace it: write down each variable's value after every pass.

count = 0
FOR i = 1 TO 5
    IF i MOD 2 = 0 THEN
        count = count + 1
    END IF
NEXT i
PRINT count

Here i takes the values 1 to 5; only 2 and 4 are even (MOD gives the remainder of a division), so count becomes 1 and then 2, and the program prints 2. For a WHILE loop, remember the check happens before the update: the value left in the variable afterwards is the first one that made the condition false, not the last one that passed it.

n = 2
WHILE n < 30
    n = n * 2
END WHILE
PRINT n    // n goes 2, 4, 8, 16, 32; 32 < 30 is false, so 32 prints

Typical tracing mistakes are ignoring an IF inside the loop, stopping one pass early, and overwriting a total instead of adding to it.