science

Increment and decrement operators

Increment and decrement operators is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Increment and decrement operators rather than just read about it. In short: Increment and decrement operators are unary operators that increase or decrease their operand by one. They are commonly found in imperative programming languages.

Key takeaways

Increment and decrement operators belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Increment and decrement operators to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Increment and decrement operators from memory before moving on to harder problems.

Reference excerpt

Increment and decrement operators are unary operators that increase or decrease their operand by one. They are commonly found in imperative programming languages. C-like languages feature two versions (pre- and post-) of each operator with slightly different semantics. In languages syntactically derived from B (including C and its various derivatives), the increment operator is written as ++ and the decrement operator is written as --. Several other languages use inc(x) and dec(x) functions. The increment operator increases, and the decrement operator decreases, the value of its operand by 1. The operand must have an arithmetic or pointer data type, and must refer to a modifiable data object. Pointers values are increased (or decreased) by an amount that makes them point to the next (or previous) element adjacent in memory. In languages that support both versions of the operators:

The pre-increment and pre-decrement operators increment (or decrement) their operand by 1, and the value of the expression is the resulting incremented (or decremented) value. The post-increment and post-decrement operators increase (or decrease) the value of their operand by 1, but the value of the expression is the operand's value prior to the increment (or decrement) operation. In languages where increment/decrement is not an expression (e.g., Go), only one version is needed (in the case of Go, post operators only). Since the increment/decrement operator modifies its operand, use of such an operand more than once within the same expression can produce undefined results depending on the language. In C for example, in expressions such as x - ++x, it is not clear in what sequence the subtraction and increment operations should be performed. There, such expressions invoke undefined behavior, and should be avoided. In languages with typed pointers like C, the increment operator steps the pointer to the next item of that type -- increasing the value of the pointer by the size of that type. When a pointer (of the right type) points to any item in an array, incrementing (or decrementing) makes the pointer point to the "next" (or "previous") item of that array. Thus, incrementing a pointer to an integer makes it point to the next integer (typically increasing the pointer value by 4); incrementing a pointer to a structure of size 106 bytes makes it point to the next structure by increasing the pointer value by 106.

Examples The following C code fragment illustrates the difference between the pre and post increment and decrement operators:

In languages lacking these operators, equivalent results require an extra line of code:

The post-increment operator is commonly used with array subscripts. For example:

The post-increment operator is also commonly used with pointers:

These examples also work in other C-like languages, such as C++, Java, and C#.

Increment operator can be demonstrated by an example: Output:

Supporting languages The following list, though not complete or all-inclusive, lists some of the major programming languages that support the increment and decrement operators.

Apple's Swift once supported these operators, but they have been depreciated since version 2.2 and removed as of version 3.0. Pascal, Delphi, Modula-2, and Oberon uses functions (inc(x) and dec(x)) instead of operators. Tcl uses the incr command. Notably Python, Ruby and Rust do not support these operators.

History The concept was introduced in the B programming language circa 1969 by Ken Thompson.

Thompson went a step further by inventing the ++ and -- operators, which increment or decrement; their prefix or postfix position determines whether the alteration occurs before or after noting the value of the operand. They were not in the earliest versions of B, but appeared along the way. People often guess that they were created to use the auto-increment and auto-decrement address modes provided by the DEC PDP-11 on which C and Unix first became popular. This is historically impossible, since there was no PDP-11 when B was developed. The PDP-7, however, did have a few 'auto-increment' memory cells, with the property that an indirect memory reference through them incremented the cell. This feature probably suggested such operators to Thompson; the generalization to make them both prefix and postfix was his own. Indeed, the auto-increment cells were not used directly in implementation of the operators, and a stronger motivation for the innovation was probably his observation that the translation of ++x was smaller than that of x=x+1.

See also Augmented assignment – for += and -= operators PDP-7 PDP-11 Successor function

References

Worked examples

Example 1 — a first encounter with Increment and decrement operators

Start with the simplest possible case. Write down what Increment and decrement operators claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Increment and decrement operators before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Increment and decrement operators ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Increment and decrement operators

In research: Increment and decrement operators appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Increment and decrement operators in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Increment and decrement operators is common in secondary-school and first-year university syllabi. It links to neighbouring topics Operators (programming), Unary operations, so understanding it makes those chapters shorter.
In everyday life: Look for Increment and decrement operators outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Increment and decrement operators in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Increment and decrement operators means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Increment and decrement operators out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Increment and decrement operators in simple terms?

Increment and decrement operators are unary operators that increase or decrease their operand by one. They are commonly found in imperative programming languages.

Why does Increment and decrement operators matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Increment and decrement operators?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Increment and decrement operators.