Programming Errors

Errors of different types

• Syntax errors – easiest to fix, found by compiler or interpreter• Semantic errors – logic errors, found by comparing results of test runs

to expected results• Runtime errors – errors caused by the environment, bad input, non-

existent files – found by the operating system usually

Syntax errors

• Syntax = rules of a language that state how to write statements in the language• You learned English syntax when you were in 3rd – 5th grades – punctuation,

spelling, sentence diagramming• Humans use English syntax but can manage if rules are broken• E.E. Cummings = poet – did not follow English syntax rules well

• Computer language translators follow syntax rules rigidly• Punctuation, order of statements, spelling of keywords, etc. must be correct• Give instant feedback when error is detected

• Syntax rules do not tell you WHAT to say, just HOW to say it

Semantic errors

• Errors in meaning• In English, there are lots of redundancies to prevent these kinds of errors

– “subject number must agree with verb number”, “tenses must agree”, “pronouns must have noun to refer to”• Computer languages – the semantics of a program is exactly “What does

this program do when it is executed?” What does it make the computer do? Does it put something on the screen? Make a noise? Change something in memory? So it is very precise – every language has a standard document which specifies the semantics for every keyword• The translators do NOT detect semantic errors!

How do you find semantic (logic) errors?• Testing! Lots of testing! • You develop a plan for testing which includes

• A description of the situation to be tested, e.g. “input file is empty”• The Input given to cause this situation to happen, e.g. “a file with nothing in it”• The expected output, e.g. “The program will stop with a message about no data”

• Then you run the program with those inputs and record what really happened “actual output” “The program crashed!”• The better your test cases, the more certain you can be that your program is

working correctly• Can you prove a program has NO errors? For any program more than a few

lines long, the answer is no

Runtime errors

• Usually caused by the environment• “file not found”• “Network connect is missing”• “Insufficient data in file”

• Generally these will cause the program to halt with ugly error messages• It is possible to program in such a way that these errors can be found

by the program while it’s running – it is not easy.• If found by the program, they are usually reported to the user and the

program terminated – sometimes default value can be substituted for the missing items, etc.