The statement coverage is also known as line coverage or segment coverage. The statement coverage covers only the true conditions.
- Through statement coverage we can identify the statements executed and where the code is not executed because of blockage.
- In this process each and every line of code needs to be checked and executed
Advantage of statement coverage:
- It verifies what the written code is expected to do and not to do
- It measures the quality of code written
- It checks the flow of different paths in the program and it also ensure that whether those path are tested or not.
Disadvantage of statement coverage:
- It cannot test the false conditions.
- It does not report that whether the loop reaches its termination condition.
- It does not understand the logical operators.
The statement coverage can be calculated as shown below:
To understand the statement coverage in a better way let us take an example which is basically a pseudo-code. It is not any specific programming language, but should be readable and understandable to you, even if you have not done any programming yourself.
Consider code sample 4.1 :
I F X>Y THEN Z = 0
Code sample 4.1
To achieve 100% statement coverage of this code segment just one test case is required, one which ensures that variable A contains a value that is greater than the value of variable Y, for example, X = 12 and Y = 10. Note that here we are doing structural test design first, since we are choosing our input values in order ensure statement coverage.
Now, let’s take another example where we will measure the coverage first. In order to simplify the example, we will regard each line as a statement. A statement may be on a single line, or it may be spread over several lines. One line may contain more than one statement, just one statement, or only part of a statement. Some statements can contain other statements inside them. In code sample 4.2, we have two read statements, one assignment statement, and then one IF statement on three lines, but the IF statement contains another statement (print) as part of it.
1 READ X
2 READ Y
3 Z =X + 2*Y
4 IF Z> 50 THEN
5 PRINT large Z
Code sample 4.2
Although it isn’t completely correct, we have numbered each line and will regard each line as a statement. Let’s analyze the coverage of a set of tests on our six-statement program:
TEST SET 1
Test 1_1: X= 2, Y = 3
Test 1_2: X =0, Y = 25
Test 1_3: X =47, Y = 1
Which statements have we covered?
- In Test 1_1, the value of Z will be 8, so we will cover the statements on lines 1 to 4 and line 6.
- In Test 1_2, the value of Z will be 50, so we will cover exactly the same statements as Test 1_1.
- In Test 1_3, the value of Z will be 49, so again we will cover the same statements.
Since we have covered five out of six statements, we have 83% statement coverage (with three tests). What test would we need in order to cover statement 5, the one statement that we haven’t exercised yet? How about this one:
Test 1_4: X = 20, Y = 25
This time the value of Z is 70, so we will print ‘Large Z’ and we will have exercised all six of the statements, so now statement coverage = 100%. Notice that we measured coverage first, and then designed a test to cover the statement that we had not yet covered.
Note that Test 1_4 on its own is more effective which helps in achieving 100% statement coverage, than the first three tests together. Just taking Test 1_4 on its own is also more efficient than the set of four tests, since it has used only one test instead of four. Being more effective and more efficient is the mark of a good test technique.