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4.8 Types of Errors
Various types of errors may be generated while creating or executing a program. The process of identifying and
removing such errors is known as debugging. The errors are called bugs. The errors in a program broadly fall in three
categories:
● Syntax Error
● Logical Errors
● Runtime Errors
4.8.1 Syntax Errors
Every programming language is described by a set of rules, collectively called the syntax of the language. If a program
violates a syntax rule, the interpreter/compiler will flash an error. Such errors are known as syntax errors. Thus, a Python
interpreter will execute only those statements that are syntactically correct. When a syntax error is encountered, the
execution of the program stops, and the error message is displayed. The execution of the program resumes only after
the error is rectified. For example,
>>> print("hello)
SyntaxError: unterminated string literal (detected at line 1)
In the statement print("hello), the closing quotation marks are missing, leading to a syntax error. The Python
interpreter noted the beginning of string, but could not find the closing quote marks. A syntax error must be removed
before the Python interpreter can proceed.
Python Interpreter gives a brief description of each error, as seen in the above example.
4.8.2 Logical Errors
A logical error does not give the desired output, even though there is no syntax error in the program. Such errors are
sometimes difficult to identify as all the statements in the program are executed successfully. When a logical error is
encountered, often it helps to work backwards by examining the output produced by the program and looking for the
cause of the error.
For example, to calculate the percentage of marks obtained by a student, the expression should be
percentage = marksObtained/maxMarks * 100
However, if the programmer writes the formula as
percentage = maxMarks/marksObtained * 100
The statement being syntactically correct, will be executed but it will not give the correct output.
4.8.3 Runtime errors
As the name suggests, a runtime error occurs during the execution of the program. The statement is syntactically
correct but cannot be executed by the interpreter. For example, while executing the statement
result = numerator/denominator
if the value of denominator is 0 (zero), it will result in a runtime error (division by zero). Similarly, a runtime error will
be generated, if the user enters a string instead of a number. Program 4.2 below illustrates the runtime error.
Data Types and Operators 87

