Syntax Error vs Logic Error: What's the Difference?

A syntax error breaks the rules for writing code, so the program cannot run as written. A logic error means the code can run, but it does not do what the programmer intended.

Syntax errors

Syntax is the set of rules a programming language uses to decide whether code is written correctly. In Python, this line is missing a closing bracket:

print("Ready"

Python reports an error because it cannot interpret the incomplete instruction. Adding the closing bracket fixes the syntax:

print("Ready")

An error message often helps you find a syntax error. Check the line it points to, but also look at the line just before it: that may be where a bracket or quotation mark was left open.

Logic errors

A logic error is a mistake in the steps or conditions used to solve a problem. Suppose a program should print Eligible for anyone aged $18$ or over:

if age > 18:
    print("Eligible")

This code has valid syntax and runs. However, it excludes someone who is exactly $18$, so the result is wrong for that input. The condition should include $18$:

if age >= 18:
    print("Eligible")

To find a logic error, test the program with inputs where you know the expected result. For this example, checking ages just below, exactly at and just above $18$ helps reveal the mistake.

A trace is a step-by-step check of what a program does for a chosen input. For $age=18$, the original condition $age>18$ is false, so nothing is printed. The corrected condition $age\ge 18$ is true, so it prints Eligible. Comparing the actual result with the expected result makes the faulty condition easier to identify.

Exam mistake to avoid: Do not call every incorrect result a syntax error. If the program runs but produces the wrong result, look for a logic error.