| << Chapter < Page | Chapter >> Page > |
switch (day)
{case 1 :
case 7 : printf( "This is a weekend day");break;
case 2 :case 3 :
case 4 :case 5 :
case 6 : printf( "This is a weekday");break;
default : printf( "Not a legal day");break;
}
The syntax of while statement is simple:
while(expression)
statement
The statement is only executed if the expression is non-zero. After every execution of the statement, the expression is evaluated again and the process repeats if it is non-zero. What could be plainer than that? The only point to watch out for is that the statement may never be executed, and that if nothing in the statement affects the value of the expression then the while will either do nothing or loop for ever, depending on the initial value of the expression.
#include<stdio.h>#include<stdlib.h>main(){
int i;/* initialize */
i = 0;/* check */
while(i<= 10){
printf("%d\n", i);/* update */
i++;}
} It is occasionally desirable to guarantee at least one execution of the statement following the while, so an alternative form exists known as the do statement. It looks like this:
do
statementwhile(expression);
and you should pay close attention to that semicolon—it is not optional! The effect is that the statement part is executed before the controlling expression is evaluated, so this guarantees at least one trip around the loop. It was an unfortunate decision to use the keyword while for both purposes, but it doesn't seem to cause too many problems in practice.
A very common feature in programs is loops that are controlled by variables used as a counter. The counter doesn't always have to count consecutive values, but the usual arrangement is for it to be initialized outside the loop, checked every time around the loop to see when to finish and updated each time around the loop. There are three important places, then, where the loop control is concentrated: initialize, check and update. This example shows them.
As you will have noticed, the initialization and check parts of the loop are close together and their location is obvious because of the presence of the while keyword. What is harder to spot is the place where the update occurs, especially if the value of the controlling variable is used within the loop. In that case, which is by far the most common, the update has to be at the very end of the loop: far away from the initialize and check. Readability suffers because it is hard to work out how the loop is going to perform unless you read the whole body of the loop carefully. What is needed is some way of bringing the initialize, check and update parts into one place so that they can be read quickly and conveniently. That is exactly what the for statement is designed to do. Here it is.
for (expression1; expression2; expression3) statement
The first expression (expression1) is the initialize part; nearly always an assignment expression which is used to initialize the control variable. After the initialization, the check expression (expression2) is evaluated: if it is non-zero, the statement is executed, followed by evaluation of the update expression (expression3) which generally increments the control variable, then the sequence restarts at the check. The loop terminates as soon as the check evaluates to zero.
Notification Switch
Would you like to follow the 'Introduction to computer science' conversation and receive update notifications?
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|