Basic Programming Constructs

Overview of Programming in MATLAB

Programming in MATLAB means giving the computer a sequence of instructions that it can execute to solve a task. In this part of the course you focus on the basic building blocks of programs that you will reuse in many contexts. You will write sequences of commands, let MATLAB make decisions, repeat operations, and structure your code in a clear way. Later chapters go into specific constructs like if, for, and functions, so here the aim is to understand what a “program” looks like in MATLAB and how these constructs fit together.

MATLAB programs are mostly text files that contain MATLAB commands. When MATLAB runs a program, it reads those commands from top to bottom and executes them in order. Because the language is interpreted, you can test ideas directly in the Command Window and then move them into a script or function file once you are satisfied with them.

Sequential Execution of Commands

The simplest form of a MATLAB program is a sequence of commands. MATLAB starts at the first executable line and proceeds line by line. You can think of it as following a recipe. For example, the following code computes the length of the hypotenuse of a right triangle:

a = 3;
b = 4;
c = sqrt(a^2 + b^2);
disp(c)

Here the value of c depends on the values of a and b that were computed earlier. If you change the order of these lines, or try to use a variable before it is created, MATLAB either produces an error or uses an unexpected value. This idea of order is central. Even when you start to use decisions and loops, MATLAB still follows a clear left to right, top to bottom execution, except where control flow changes that straight line.

MATLAB also allows multiple commands on a single line separated by semicolons. However, long chains of commands on one line are harder to read and to debug. For beginners, it is usually clearer to keep one logical operation per line.

Expressions and Statements in Programs

When you build programs, you combine expressions and statements. An expression produces a value, such as 1+2, sin(pi/4), or x^2 + 3x + 1. A statement is usually a complete instruction, often an assignment, such as y = x^2 + 3x + 1;. In scripts and functions you mostly write statements. MATLAB evaluates any expressions on the right hand side of assignments, and then stores the result in variables.

Control structures, which you learn in detail in later chapters, are also statements. For example, if, for, and while open a block of code that MATLAB may execute conditionally or repeatedly. Inside those blocks you still write the same kinds of assignment and function call statements as in simple scripts.

Understanding that your program is a collection of statements executed in a particular order helps you read your own code and predict what it does. When you see an error, you can follow that order to see where values are coming from and how they change.

Introducing Control Flow

Sequential execution is not enough for most useful programs. Often you want MATLAB to choose between alternatives or to repeat some steps many times. Control flow refers to these patterns for controlling the path that execution takes through your code.

The basic types of control flow you use in MATLAB are conditional execution, iteration, and early termination of loops or functions. Conditional execution uses logic to decide whether to run certain statements or skip them. Iteration repeats a block of statements several times, which is useful when you need to process elements of an array or refine an approximation. Early termination means leaving a loop or a function before all of its statements would normally run, which is useful when you have already found what you need or detected a condition that makes further work unnecessary.

In MATLAB, conditional execution is typically written with if, elseif, and else. Iteration is written with for and while. Early termination is written with break, continue, and return. Separate chapters will show you their exact syntax. At this stage, it is important to recognize that these constructs let you change the linear flow of execution based on values and conditions in your program.

Logic and Decisions

Control flow is driven by logical expressions and relational operators. A logical expression is something that MATLAB can interpret as true or false. For example, the expression x > 0 is true if x is positive and false otherwise. Logical expressions control which branch of an if statement runs, or whether a while loop continues.

Relational operators compare values, for example == for equality, ~= for inequality, <, >, <=, and >= for order comparisons. Logical operators such as &&, ||, and ~ combine these comparisons into more complex conditions. When you write programs, you often use these operations to check the size of an array, test for special values, or decide whether your computation has converged.

Although you can type these expressions at the command line, they become especially useful inside programs, because they determine which code paths execute. Later, when you learn about debugging, you will often inspect logical conditions to understand why MATLAB followed a particular branch.

Using Variables in Program Flow

Variables are at the heart of programming. They store the intermediate results that your program uses to make decisions and control flow. For example, a loop counter variable might track how many times a loop has run, or an error variable might track the difference between a current estimate and a target value.

Ordinary assignment statements change the contents of variables as the program runs. For instance:

x = 0;
while x < 5
    x = x + 1;
end

Here the value of x changes on each iteration and the logical test x < 5 uses the current value. This back and forth between updating variables and testing them is a typical pattern in MATLAB code.

Since MATLAB uses the workspace to store variables created by scripts, it is important to be aware that running a script can read and overwrite existing variables. Functions have their own workspaces and keep variables local. That distinction becomes important as your programs grow, and it is one reason to move from quick scripts to more structured functions as soon as your code becomes nontrivial.

Building Simple Algorithms

An algorithm is a finite sequence of steps that solves a problem. MATLAB is well suited to expressing algorithms that work with vectors and matrices, but the logical structure of those algorithms always comes from the same basic programming constructs.

To turn a problem into a MATLAB program, you typically follow these stages. First, clarify the input and the desired output. Second, outline the high level steps in plain language. Third, translate those steps into a sequence of MATLAB statements, including any necessary decisions and loops. Finally, test your implementation with simple examples where you already know the answer.

For instance, if your task is to compute the approximate square root of a number by repeated guesses, your plan might involve starting with an initial guess, repeatedly improving the guess according to a formula, and stopping when the change between guesses becomes small. In MATLAB, this kind of plan naturally becomes a combination of variable assignments, a while loop that checks for convergence, and a logical condition that compares the change to a tolerance.

At this stage, what matters most is that you learn to think of your solutions as step by step processes that MATLAB can execute mechanically. The specific syntax of control structures is secondary to the way you structure your ideas into clear stages and decisions.

Structuring and Organizing Code

As your programs grow longer, clarity and organization become crucial. MATLAB offers both scripts and functions as program units. Scripts are collections of statements that run in the base workspace. Functions are files or local units with their own workspaces, inputs, and outputs. You will study their differences in detail in a later chapter, but already it is useful to think about how to keep your code readable and maintainable.

Good structure starts with logical grouping of related operations. Code that belongs together should be close together in the file. Long sections of code often benefit from being split into smaller functions, each of which performs a single well defined task. Descriptive names for variables and functions make the flow of your program easier to follow.

Indentation also plays an important role. MATLAB automatically indents lines inside if, for, and while blocks in the Editor. Proper indentation does not affect how MATLAB runs your code, but it greatly improves readability and helps you see where blocks begin and end. Clear formatting and spacing around operators and between sections of code can further improve readability.

Finally, comments explain the intention behind your code. You add comments in MATLAB with the % character. Comments do not change how the program runs, but they guide the human reader. At the beginning of a script or function, a short comment block that explains what the code does, what inputs it expects, and what outputs it produces is especially helpful.

From Interactive Commands to Reusable Code

When you begin with MATLAB, you often experiment by typing individual commands in the Command Window. This interactive style is useful for learning and quick exploration. However, as soon as you have a useful sequence of commands, it is better to save that sequence in a script or function file. This transforms a one time experiment into a reusable program.

Moving from interactive work to saved programs also encourages you to think more carefully about control flow, variables, and structure. Instead of manually retyping or editing commands each time, you can give MATLAB a clear sequence and let it follow the same steps every time. This reduces mistakes and makes your work reproducible.

Over time, you will rely less on ad hoc commands and more on well structured scripts and functions. The basic programming constructs introduced in this part of the course are the tools that let you make that shift.