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Foundations of Python Programming: Functions First

Section 3.5 Semantic Errors

The third type of error is the semantic error, also called a logic error. If there is a semantic error in your program, it will run successfully in the sense that the computer will not generate any error messages. However, your program will not do the right thing. It will do something else. Specifically, it will do what you told it to do, not what you wanted it to do.
The following program has a semantic error. Execute it to see what goes wrong:
This program runs and produces a result. However, the result is not what the programmer intended. It contains a semantic error. The error is that the program performs concatenation instead of addition, because the programmer failed to write the code necessary to convert the inputs to integers.
With semantic errors, the problem is that the program you wrote is not the program you wanted to write. The meaning of the program (its semantics) is wrong. The computer is faithfully carrying out the instructions you wrote, and its results are correct, given the instructions that you provided. However, because your instructions have a flaw in their design, the program does not behave as desired.
Identifying semantic errors can be tricky because no error message appears to make it obvious that the results are incorrect. The only way you can detect semantic errors is if you know in advance what the program should do for a given set of input. Then, you run the program with that input data and compare the output of the program with what you expect. If there is a discrepancy between the actual output and the expected output, you can conclude that there is either 1) a semantic error or 2) an error in your expected results.
Once you’ve determined that you have a semantic error, locating it can be tricky because you must work backward by looking at the output of the program and trying to figure out what it is doing.

Subsection 3.5.1 Test Cases

To detect a semantic error in your program, you need the help of something called a test case.

Note 3.5.1. Test Case.

A test case is a set of input values for the program, together with the output that you expect the program should produce when it is run with those particular inputs.
Here is an example of a test case for the program above:
Test Case
Input: 2, 3
Expected Output: 5
If you give this test case to someone and ask them to test the program, they can type in the inputs, observe the output, check it against the expected output, and determine whether a semantic error exists based on whether the actual output matches the expected output or not. The tester doesn’t even have to know what the program is supposed to do. For this reason, software companies often have separate quality assurance departments whose responsibility is to check that the programs written by the programmers perform as expected. The testers don’t have to be programmers; they just have to be able to operate the program and compare its results with the test cases they’re given.
In this case, the program is so simple that we don’t need to write down a test case at all; we can compute the expected output in our heads with very little effort. More complicated programs require effort to create the test case (since you shouldn’t use the program to compute the expected output; you have to do it with a calculator or by hand), but the effort pays off when the test case helps you to identify a semantic error that you didn’t know existed.
Semantic errors are the most dangerous of the three types of errors, because in some cases they are not noticed by either the programmers or the users who use the program. Syntax errors cannot go undetected (the program won’t run at all if they exist), and runtime errors are usually also obvious and typically detected by developers before a program is released for use (although it is possible for a runtime error to occur for some inputs and not for others, so these can sometimes remain undetected for a while). However, programs often go for years with undetected semantic errors; no one realizes that the program has been producing incorrect results. They just assume that because the results seem reasonable, they are correct. Sometimes, these errors are relatively harmless. But if they involve financial transactions or medical equipment, the results can be harmful or even deadly. For this reason, creating test cases is an important part of the work that programmers perform in order to help them produce programs that work correctly.
Check your understanding

Checkpoint 3.5.2.

    Which of the following is a semantic error?
  • Attempting to divide by 0.
  • A semantic error is an error in logic. In this case the program does not produce the correct output because the problem is not solved correctly. This would be considered a run-time error.
  • Forgetting the closing parenthesis ) on a print statement.
  • A semantic error is an error in logic. In this case the program does not produce the correct output because the code can not be processed by the compiler or interpreter. This would be considered a syntax error.
  • Forgetting to divide by 100 when printing a percentage amount.
  • This will produce the wrong answer because the programmer implemented the solution incorrectly. This is a semantic error.

Checkpoint 3.5.3.

    Who or what typically finds semantic errors?
  • The programmer.
  • You must fully understand the problem so the you can tell if your program properly solves it.
  • The compiler / interpreter.
  • The compiler and / or interpreter will only do what you instruct it to do. It does not understand what the problem is that you want to solve.
  • The computer.
  • The computer does not understand your problem. It just executes the instructions that it is given.
  • The teacher / instructor.
  • Your teacher and instructor may be able to find most of your semantic errors, but only because they have experience solving problems. However it is your responsibility to understand the problem so you can develop a correct solution.
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