Statements and Control Flow

12 min read

Statements

A statement in a programming language is a syntactic unit that expresses an action to be carried out by the computer. It is the smallest independent element of a program, functioning much like a complete sentence in a natural language. When a program runs, the execution flow moves from one statement to the next, commanding the system to perform specific operations.

Statements are generally categorized based on the type of action they perform. Assignment statements allocate data to variables, while control flow statements—such as loops and conditionals—determine the order in which other statements execute. There are also declaration statements that introduce new variables or functions to the program, and expression statements that evaluate a value and may trigger side effects.

totalCost = price + salesTax          # Assignment statement
if age >= 18 then print("Access Granted")  # Conditional statement
for i in 1 to 10 do print(i)          # Loop statement
sqrt(16)                              # Expression statement

Expressions

An expression in a programming language is a combination of values, variables, operators, and function calls that the language interprets and evaluates to produce a single value. Unlike a statement, which executes an action or a command, an expression is fundamentally centered around computation. Every expression must result in a value, which can then be assigned to a variable, passed to a function, or used within a larger structure.

Expressions are built using operands, which are the data objects being manipulated, and operators, which dictate the manipulation itself. For instance, combining two numbers with an addition operator forms an arithmetic expression. Expressions can also be relational, comparing two values to yield a boolean result, or logical, combining multiple conditions to determine an ultimate truth value.

price + salesTax  # Arithmetic expression evaluating to a value
age >= 18         # Relational expression evaluating to a boolean value

Conditional Statements

A conditional statement is a programming construct that allows a program to make decisions and execute different blocks of code based on whether a specific condition is true or false. It acts as a gatekeeper for program flow, ensuring that certain actions only occur under the right circumstances. The core of any conditional statement is a boolean expression, which the computer evaluates to determine the path of execution.

The most common form of this construct is the if-then-else structure. If the initial condition evaluates to true, the program executes the code immediately following the conditional check. If the condition evaluates to false, the program skips that code entirely and either moves forward or executes an alternative block of code defined by an else clause. Developers can also chain multiple conditions together using else-if clauses to handle complex, multi-layered scenarios.

score = 85
if score >= 90 then
    print("A")
else if score >= 80 then
    print("B")
else if score >= 70 then
    print("C")
else
    print("F")
end if

Another widespread type of conditional statement is the switch or case statement, which compares a single variable against a list of pre-defined values. This structure is often used as a cleaner, more readable alternative to a long sequence of individual if-else checks when testing a single expression for multiple distinct outcomes.

day = 2
switch day:
    case 1:
        output "Monday"
    case 2:
        output "Tuesday"
    case 3:
        output "Wednesday"
    case 4:
        output "Thursday"
    case 5:
        output "Friday"
    case 6:
        output "Saturday"
    case 7:
        output "Sunday"
    default:
        output "Invalid day"
end switch

Loop Statements

A loop statement is a programming construct that repeatedly executes a specific block of code as long as a specified condition remains true. Instead of writing the same lines of code multiple times, programmers use loops to automate repetitive tasks efficiently. Each repetition of the loop is known as an iteration, and the loop continues until its controlling condition evaluates to false, at which point the program moves on to the next instruction.

There are several primary types of loop statements used across programming languages:

• while loop checks a boolean condition before executing its code block, meaning the block might not run at all if the initial condition is false.
• do-while loop executes the code block first and then checks the condition, guaranteeing that the code runs at least once.
• for loop is typically used when the exact number of iterations is known beforehand, as it neatly bundles initialization, condition testing, and incrementation into a single line.

# while loop
count = 0
while count < 5:
    print(count)
    count = count + 1
end while
# Output: 0, 1, 2, 3, 4

# do-while loop
count = 0
do:
    print(count)
    count = count + 1
while count < 5
# Output: 0, 1, 2, 3, 4

# for loop
for count in 0 to 4:
    print(count)
end for
# Output: 0, 1, 2, 3, 4

Programmers often utilize specific control words, like break and continue, to alter loop behavior on the fly. A break statement terminates the loop entirely and exits immediately, while a continue statement skips the remainder of the current iteration and jumps directly to the next condition check.

for count in 0 to 5:
    if count == 3:
        break  # Exit the loop when count is 3
    print(count)
end for
# Output: 0, 1, 2

for count in 0 to 5:
    if count == 3:
        continue  # Skip the rest of the current iteration when count is 3
    print(count)
end for
# Output: 0, 1, 2, 4, 5

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