Computer Science › Compilers & Languages
Syntax vs Semantics
What code looks like vs what it means.
Also known as: syntax, semantics, syntax error vs logic error
Syntax is the form of code: the rules about how it must be written. Semantics is its meaning: what it actually does when it runs.
x = 5 + # syntax error: the statement is incomplete
def average(a, b):
return a + b / 2 # valid syntax, but the meaning is wrong: operator precedenceThe second line is perfectly legal, and does the wrong thing: it computes a + (b / 2), not (a + b) / 2.
Two kinds of mistakes
| Syntax error | Semantic (logic) error | |
|---|---|---|
| What’s wrong | the code breaks the language’s grammar | the code is valid but doesn’t do what you intended |
| Who finds it | the compiler or interpreter, immediately | you, by testing and debugging |
| Message | “SyntaxError: unexpected token” | none: wrong output, or a runtime error later |
| Examples | missing bracket or colon, unclosed string | off-by-one, wrong operator, wrong variable |
for i in range(10) # SyntaxError: expected ':'
print(i)
if user.age > 18: # valid, but should it be >= 18? A semantic questionIn between: type and runtime errors
- Type errors (adding a string and a number) are detected by a compiler or type checker before running in some languages, and at run time in others (type systems).
- Runtime errors (dividing by zero, a missing key) occur while running (exceptions).
What to do
- Syntax errors: read the message and look at the line, and often the line before it, since a missing bracket is reported where the parser gets confused (reading error messages). Editors with highlighting and formatters catch most of them.
- Semantic errors: need tests, careful reading and debugging (debugging).
Learning syntax is the easy part. The hard part, and the real skill, is getting the meaning right.