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#include<stdio.h>#include<stdlib.h>main(){
int i;for(i = 0; i<10000; i++){
if(getchar() == 's')break;
printf("%d\n", i);}
}
It reads a single character from the program's input before printing the next in a sequence of numbers. If an ‘s’ is typed, the break causes an exit from the loop.
If you want to exit from more than one level of loop, the break is the wrong thing to use. The goto is the only easy way, but since it can't be mentioned in polite company, we'll leave it till last.
This statement has only a limited number of uses. The rules for its use are the same as for break, with the exception that it doesn't apply to switch statements. Executing a continue starts the next iteration of the smallest enclosing do, while or for statement immediately. The use of continue is largely restricted to the top of loops, where a decision has to be made whether or not to execute the rest of the body of the loop. In this example it ensures that division by zero (which gives undefined behavior) doesn't happen
#include<stdio.h>#include<stdlib.h>main(){
int i;for(i = -10; i<10; i++){
if(i == 0)continue;
printf("%f\n", 15.0/i);/*
* Lots of other statements .....*/
}}
Of course the continue can be used in other parts of a loop, too, where it may occasionally help to simplify the logic of the code and improve readability. It deserves to be used sparingly.
Do remember that continue has no special meaning to a switch statement, where break does have. Inside a switch, continue is only valid if there is a loop that encloses the switch, in which case the next iteration of the loop will be started.
There is an important difference between loops written with while and for. In a while, a continue will go immediately to the test of the controlling expression. The same thing in a for will do two things: first the update expression is evaluated, then the controlling expression is evaluated.
Everybody knows that the goto statement is a ‘bad thing’. Used without care it is a great way of making programs hard to follow and of obscuring any structure in their flow. Dijkstra wrote a famous paper in 1968 called ‘Goto Statement Considered Harmful’, which everybody refers to and almost nobody has read.
What's especially annoying is that there are times when it is the most appropriate thing to use in the circumstances! In C, it is used to escape from multiple nested loops, or to go to an error handling exit at the end of a function. You will need a label when you use a goto; this example shows both.
goto L1;
/* whatever you like here */L1: /* anything else */
A label is an identifier followed by a colon. Labels have their own ‘name space’ so they can't clash with the names of variables or functions. The name space only exists for the function containing the label, so label names can be re-used in different functions. The label can be used before it is declared, too, simply by mentioning it in a goto statement.
Labels must be part of a full statement, even if it's an empty one. This usually only matters when you're trying to put a label at the end of a compound statement—like this.
label_at_end: ; /* empty statement */
}
The goto works in an obvious way, jumping to the labeled statements. Because the name of the label is only visible inside its own function, you can't jump from one function to another one.
It's hard to give rigid rules about the use of gotos, but, as with the do, continue and the break (except in switch statements), over-use should be avoided. Think carefully every time you feel like using one, and convince yourself that the structure of the program demands it. More than one goto every 3–5 functions is a symptom that should be viewed with deep suspicion.
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