CSS Computer Science (Optional) — Paper-I, Section-A, Topic II subtopic. The building blocks that define any programming language.

Core Concepts

• Syntax: the formal rules governing how valid statements are constructed (the grammar of the language). A compiler checks syntax first — a syntax error stops compilation entirely.
• Semantics: the meaning/behavior associated with syntactically valid statements. Code can be syntactically perfect but semantically wrong (logic errors).
• Tokens: the smallest meaningful units the compiler recognizes — every program is broken into tokens before parsing. Categories:
  • Keywords: reserved words with fixed meaning (int, if, return, class) — cannot be redefined or used as identifiers
  • Identifiers: programmer-defined names for variables, functions, classes, etc.
  • Literals/Constants: fixed values written directly (42, 3.14, 'x', "text", true)
  • Operators: symbols performing operations (arithmetic, relational, logical, bitwise, assignment)
  • Punctuators/Separators: symbols that structure code without performing computation (;, ,, { }, ( ), [ ])
• Comments: non-executable annotations for humans, stripped out before compilation — do not affect semantics at all

Syntax vs Semantics — Example

int x = 5 / 0;

• Syntactically valid — this line parses fine, follows all grammar rules
• Semantically problematic — division by zero is undefined behavior/runtime error, even though the syntax is correct

Contrast with:

int x = 5 0;

• This is a syntax error — missing an operator between 5 and 0; the compiler rejects it before it can even consider meaning

Grammar and Rules

• Every language defines its syntax formally, often using grammar notations like BNF (Backus-Naur Form), which the compiler's parser uses to validate structure
• A statement must be well-formed (follows the grammar) to compile; it must additionally be meaningful in context (e.g. correct types) to behave as intended

💻 Code Examples: C & C++

The syntax-vs-semantics example from above, made runnable, plus tokens labelled:

C:

#include <stdio.h>
int main(void) {
    int x = 5 / 2;       /* syntactically valid; semantics: integer division -> 2 */
    // int y = 5 0;       // would be a SYNTAX error -- missing operator, won't compile
    printf("%d\n", x);    // tokens: printf ( "%d\n" , x ) ;
    return 0;
}