7.x - 小结与测试 - 控制流和错误处理
The specific sequence of statements that the CPU executes in a program is called the program’s execution path. A straight-line program takes the same path every time it is run.
Control flow statements (also called Flow control statements) allow the programmer to change the normal path of execution. When a control flow statement causes the program to begin executing some non-sequential instruction sequence, this is called a branch.
A conditional statement is a statement that specifies whether some associated statement(s) should be executed or not.
If statements allow us to execute an associated statement based on whether some condition is true. Else statements execute if the associated condition is false. You can chain together multiple if and else statements.
A dangling else occurs when it is ambiguous which if statement an else statement is connected to. Dangling else statements are matched up with the last unmatched if statement in the same block. Thus, we trivially avoid dangling else statements by ensuring the body of an if statement is placed in a block.
A null statement is a statement that consists of just a semicolon. It does nothing, and is used when the language requires a statement to exist but the programmer does not need the statement to do anything.
Switch statements provide a cleaner and faster method for selecting between a number of matching items. Switch statements only work with integral types. Case labels are used to identify the values for the evaluated condition to match. The statements beneath a default label are executed if no matching case label can be found.
When execution flows from a statement underneath a label into statements underneath a subsequent label, this is called fallthrough. A break statement (or return statement) can be used to prevent fallthrough. The fallthrough attribute can be used to document intentional fallthrough.
Goto statements allow the program to jump to somewhere else in the code, either forward or backwards. These should generally be avoided, as they can create spaghetti code, which occurs when a program has a path of execution that resembles a bowl of spaghetti.
While loops allow the program to loop as long as a given condition evaluates to true. The condition is evaluated before the loop executes.
An infinite loop is a loop that has a condition that always evaluates to true. These loops will loop forever unless another control flow statement is used to stop them.
A loop variable (also called a counter) is an integer variable used to count how many times a loop has executed. Each execution of a loop is called an iteration.
Do while loops are similar to while loops, but the condition is evaluated after the loop executes instead of before.
For loops are the most used loop, and are ideal when you need to loop a specific number of times. An off-by-one error occurs when the loop iterates one too many or one too few times.
Break statements allow us to break out of a switch, while, do while, or for loop (also range-based for loops, which we haven’t covered yet). Continue statements allow us to move immediately to the next loop iteration.
Halts allow us to terminate our program. Normal termination means the program has exited in an expected way (and the status code will indicate whether it succeeded or not). std::exit() is automatically called at the end of main, or it can be called explicitly to terminate the program. It does some cleanup, but does not cleanup any local variables, or unwind the call stack.
Abnormal termination occurs when the program encountered some kind of unexpected error and had to be shut down. std::abort can be called for an abnormal termination.
Scope creep occurs when a project’s capabilities grow beyond what was originally intended at the start of the project or project phase.
Software verification is the process of testing whether or not the software works as expected in all cases. A unit test is a test designed to test a small portion of the code (typically a function or call) in isolation to ensure a particular behavior occurs as expected. Unit test frameworks can help you organize your unit tests. Integration testing tests the integration of a bunch of units together to ensure they work properly.
Code coverage refers to how much of the source code is executed while testing. Statement coverage refers to the percentage of statements in a program that have been exercised by testing routines. Branch coverage refers to the percentage of branches that have been executed by testing routines. Loop coverage (also called the 0, 1, 2 test) means that if you have a loop, you should ensure it works properly when it iterates 0 times, 1 time, and 2 times.
The happy path is the path of execution that occurs when there are no errors encountered. A sad path is one where an error or failure state occurs. A non-recoverable error (also called a fatal error) is an error that is severe enough that the program can’t continue running. A program that handles error cases well is robust.
A buffer is a piece of memory set aside for storing data temporarily while it is moved from one place to another.
The process of checking whether user input conforms to what the program is expecting is called input validation.
std::cerr is an output stream (like std::cout) designed to be used for error messages.
A precondition is any condition that must always be true prior to the execution of some segment of code. An invariant is a condition that must be true while some component is executing. A postcondition is any condition that must always be true after the execution of some code.
An assertion is an expression that will be true unless there is a bug in the program. In C++, runtime assertions are typically implemented using the assert preprocessor macro. Assertions are usually turned off in non-debug code. A static_assert is an assertion that is evaluated at compile-time.
Assertions should be used to document cases that should be logically impossible. Error handling should be used to handle cases that are possible.
An algorithm is a finite sequence of instructions that can be followed to solve some problem or produce some useful result. An algorithm is considered to be stateful if it retains some information across calls. Conversely, a stateless algorithm does not store any information (and must be given all the information it needs to work with when it is called). When applied to algorithms, the term state refers to the current values held in stateful variables.
An algorithm is considered deterministic if for a given input (the value provided for start) it will always produce the same output sequence.
A pseudo-random number generator (PRNG) is an algorithm that generates a sequence of numbers whose properties simulate a sequence of random numbers. When a PRNG is instantiated, an initial value (or set of values) called a random seed (or seed for short) can be provided to initialize the state of the PRNG. When a PRNG has been initialized with a seed, we say it has been seeded. The size of the seed value can be smaller than the size of the state of the PRNG. When this happens, we say the PRNG has been underseeded. The length of the sequence before a PRNG begins to repeat itself is known as the period.
A random number distribution converts the output of a PRNG into some other distribution of numbers. A uniform distribution is a random number distribution that produces outputs between two numbers X and Y (inclusive) with equal probability.