Syntax Error vs Runtime Error: The Difference

Two kinds of errors

When writing programs, you will encounter errors, and two of the most fundamental kinds are syntax errors and runtime errors. Though both are problems, they happen at different times and for different reasons. A syntax error prevents your code from running at all, while a runtime error occurs while the program is actually running. Understanding the difference between them is one of the first and most useful things to learn when programming, because it tells you where to look for the problem.

What a syntax error is

A syntax error is a mistake that breaks the grammar rules of the programming language, like a typo or misplaced symbol that the language does not understand. Just as a sentence with broken grammar may be incomprehensible, code with a syntax error cannot be understood by the computer. Because the code does not follow the language's rules, it cannot be run at all; the error must be fixed first. Syntax errors are usually caught before the program runs.

What a runtime error is

A runtime error occurs while the program is running, after it has started successfully. The code was valid enough to begin executing, but something goes wrong during execution, such as trying to divide by zero, accessing something that does not exist, or running out of memory. Unlike syntax errors, runtime errors are not about broken grammar; the code is well-formed, but it hits a problem while doing its work. These errors can cause the program to crash or behave unexpectedly.

When each is caught

The timing is a key difference. Syntax errors are typically caught early, before the program runs, because the language checks the code's grammar first and refuses to proceed if it finds a violation. Runtime errors, by contrast, only appear when the program is actually running and reaches the problematic situation. This means a program with no syntax errors can still fail at runtime, which is why testing a program by running it is so important.

How to handle them

Syntax errors are usually straightforward to fix: the tools point to where the grammar is broken, and you correct the typo or structure. Runtime errors can be trickier, since they depend on what happens as the program runs. Good practices help, such as anticipating problems and writing code that handles unexpected situations gracefully, and testing thoroughly to uncover runtime issues before users do. Debugging tools help track down where and why a runtime error occurs.

Why it matters

Understanding the difference between syntax errors and runtime errors is a foundational skill in programming. It helps you quickly understand what an error message is telling you and where to look, whether the problem is in how the code is written or in what happens when it runs. For anyone learning to program, grasping these two kinds of errors makes debugging far less frustrating and is genuinely useful knowledge.

Related on Skillo

See also: What is a bug? Software bugs and debugging explained, Compiler vs interpreter: what's the difference?.

Sources

• Runtime error - Wikipedia

Published date reflects the original event date (2023-05-30). This article is original Skillo editorial written from the sources above; facts were verified in September 2026.