for(i=1; ;) {
Statements;
}
If there is only one statement inside the for loop, he curly brackets can be omitted as shown in the following example:
for(k = 0; k < 10; k++)Total = Total + Sum;
Nested for loops can also be used. In a nested for loop, the inner loop is executed for each iteration of the outer loop. In the following example the inner loop is executed five times and the outer loop is executed ten times. The total iteration count is fifty:
/* Example of nested for loops */
for(i = 0; i < 10; i++) {
for(j = 0; j < 5; j++) {
Statements;
}
}
In the following example, the sum of all the elements of a 3×4 matrix M is calculated and stored in a variable called Sum:
/* Add all elements of a 3x4 matrix */
Sum = 0;
for(i = 0; i < 3; i++) {
for(j = 0; j < 4; j++) {
Sum = Sum + M[i][j];
}
}
Since there is only one statement to be executed, the preceding example could also be written as:
/* Add all elements of a 3x4 matrix */
Sum = 0;
for(i = 0; i < 3; i++) {
for(j = 0; j < 4; j++) Sum = Sum + M[i][j];
}
while Statement The syntax of a while statement is:
while (condition) {
Statements;
}
Here, the statements are executed repeatedly until the condition becomes false, or the statements are executed repeatedly as long as the condition is true. If the condition is false on entry to the loop, then the loop will not be executed and the program will continue from the end of the while loop. It is important that the condition is changed inside the loop, otherwise an endless loop will be formed.
The following code shows how to set up a loop to execute 10 times, using the while statement:
/* A loop that executes 10 times */
k = 0;
while (k < 10) {
Statements;
k++;
}
At the beginning of the code, variable k is 0. Since k is less than 10, the while loop starts. Inside the loop the value of k is incremented by 1 after each iteration. The loop repeats as long as k 10 and is terminated when k = 10. At the end of the loop the value of k is 10.
Notice that an endless loop will be formed if k is not incremented inside the loop:
/* An endless loop */
k = 0;
while (k < 10) {
Statements;
}
An endless loop can also be formed by setting the condition to be always true:
/* An endless loop */
while (k == k) {
Statements;
}
Here is an example of calculating the sum of numbers from 1 to 10 and storing the result in a variable called sum:
/* Calculate the sum of numbers from 1 to 10 */
unsigned int k, sum;
k = 1;
sum = 0;
while(k <= 10) {
sum = sum + k;
k++;
}
It is possible to have a while statement with no body. Such a statement is useful, for example, if we are waiting for an input port to change its value. An example follows where the program will wait as long as bit 0 of PORTB (PORTB.0) is at logic 0. The program will continue when the port pin changes to logic 1:
while(PORTB.0 == 0); // Wait until PORTB.0 becomes 1
or
while(PORTB.0);
It is also possible to have nested while statements.
do Statement A do statement is similar to a while statement except that the loop executes until the condition becomes false, or, the loop executes as long as the condition is true. The condition is tested at the end of the loop. The syntax of a do statement is:
do {
Statements;
} while (condition);
The first iteration is always performed whether the condition is true or false. This is the main difference between a while statement and a do statement. The following code shows how to set up a loop to execute 10 times using the do statement:
/* Execute 10 times */
k = 0;
do {
Statements;
k++;
} while (k < 10);
The loop starts with k = 0, and the value of k is incremented inside the loop after each iteration. At the end of the loop k is tested, and if k is not less than 10, the loop terminates. In this example because k = 0 is at the beginning of the loop, the value of k is 10 at the end of the loop.
An endless loop will be formed if the condition is not modified inside the loop, as shown in the following example. Here k is always less than 10:
/* An endless loop */
k = 0;
do {
Statements;
} while (k < 10);
An endless loop can also be created if the condition is set to be true all the time:
/* An endless loop */
do {
Statements;
} while (k == k);
It is also possible to have nested do statements.
goto Statement A goto statement can be used to alter the normal flow of control in a program. It causes the program to jump to a specified label. A label can be any alphanumeric character set starting with a letter and terminating with the colon (“:”) character.
Although not recommended, a goto statement can be used together with an if statement to create iterations in a program. The following example shows how to set up a loop to execute 10 times using goto and if statements:
/* Execute 10 times */
k = 0;
Loop:
Statements;
k++;
if (k < 10) goto Loop;
The loop starts with label Loop and variable k = 0 at the beginning of the loop. Inside the loop the statements are executed and k is incremented by 1. The value of k is then compared with 10 and the program jumps back to label Loop if k 10. Thus, the loop is executed 10 times until the condition at the end becomes false. At the end of the loop the value of k is 10.
continue and break Statements continue and break statements can be used inside iterations to modify the flow of control. A continue statement is usually used with an if statement and causes the loop to skip an iteration. An example follows that calculates the sum of numbers from 1 to 10 except number 5: