C traps and defects Chapter 2 lexical "traps" 2.4 switch statements

switch sentence
C Linguistic switch The control flow of statements can pass through and execute each case part , This is C The difference of language . 
The two programs are used separately C Language and Pascal Statement writing ： 
switch(color) {
    case 1: printf("red");
    break;
    case 2: printf("yellow");
    break;
    case 3: printf("blue");
    break;
}
switch(color) {
    case 1: printf("red");
    case 2: printf("yellow");
    case 3: printf("blue");
}
The two pieces of code need to complete the same task . According to the variable color Value （1、2 or 3）, Print out separately red、yellow or blue. There is only one exception ： That's using Pascal Each of the program segments written in case Some do not C Linguistic break The corresponding part of the statement . The reason for this , Because C In language case Label as a real label , So the control flow of the program will go straight through case label , Without any impact . And in the Pascal in , Every case The label will implicitly end the previous case part .

use C Write the following program in  
switch(color) {
    case 1: printf("red");
    case 2: printf("yellow");
    case 3: printf("blue");
}
Further assume that variables color The value of is 2. Last , The program will print out
yellowblue
Because the control flow of the program is executing the second printf After the call of the function , It will naturally be executed in sequence , So the third one printf Function calls are also executed .

C In language switch This feature of the statement , Is its advantage , It is also one of its weaknesses . Say it's a weakness , It's because it's easy for programmers to miss everything case Part of the break sentence , Cause some difficult problems . Say it's the advantage , Because if the program wants to intentionally omit one break sentence , It can express some program control structures that are difficult to be easily realized in other ways . Especially for some big switch sentence , We often find that the processing of each branch is similar ： Make a slight change to a branch , The rest is exactly the same as in the case of another branch . 

The program has a switch sentence , Used to handle each different opcode . On this hypothetical computer , Just reverse the sign of the second operand , Subtraction and addition are essentially the same . 
case SUBTRACT:
    opnd2 = -opnd2;
    /* There is no break sentence */
case ADD:
    ...
Of course , Adding appropriate comments like the above example is a good practice . People who read this code can know that one is intentionally omitted here break sentence . 

Its function is to skip the white space characters in the program when the compiler looks for symbols . here , Space bar 、 The processing of tab and newline characters is the same , However, when encountering line breaks , The code line count of the program should be incremented ： 
case '\n':
    linecount++;
    /* There is no break sentence */
case '\t':
case ' ':
    ...