C Handout 7 – Loops, #define and arrays
(10/24/00)
Loops
Often
you will want to execute a section of code more than once. The mechanism for doing this is called a
loop. We will discuss only tow of the
loops available in C: the while loop
and the for loop.
while loops
The while loop is used to execute a sequence of operations more than once. The syntax is:
while (logical-expression) C-statement-block
In this construct the C-statement-block gets repeatedly executed as long as the logical expression is TRUE. You’ve already seen this construct in action. The program you wrote last week had the form:
void main() {
while (1) {
.
. /* the bulk of your code would go here */
.
}
}
In this example the code following the while loop gets executed forever, since the logical expression, (1), always evaluates to TRUE.
If we wanted the program to stop when you hit the stop button we could use the following:
void main() {
while (!stop_button()) {
.
. /* the bulk of your code would go here */
.
}
}
Recall that stop_button() returns TRUE if the stop button is pressed, and FALSE if not. So this code will loop, as long as the stop button is not pressed.
for loops
There is another loop construction that is quite useful for cycling through a series of values:
for (initialization; logical-expression; action)
C-statement-block
This construct is a bit more complicated, and is probably best expressed with an example. Assume an integer variable, speed, has been declared elsewhere in the program, along with the function SetSpeed().
/*Accelerate slowly
(so wheels don’t spin*/
for (Speed=0;
Speed<=100; Speed=Speed+10;) {
SetSpeed(Speed, Speed);
sleep(0.1);
}
The for loop is really just a shorthand for the while loop. The code above is equivalent to this:
/*Accelerate slowly
(so wheels don’t spin*/
Speed=0;
/* initialization*/
while (Speed<=100){
/* logical expression*/
SetSpeed(Speed, Speed);
sleep(0.1);
Speed=Speed+10;
/* action*/
}
Note that the initialization statement comes before the loop, the logical expression is checked by the while loop, and the action is taken at the end of the loop.
The C pre-processor
(#define)
Any
program can be difficult to read. To
make C code a little easier to read, you can use the pre-processor to read you
can use the C pre-processor to define constants to make your code easier to
read. The pre-processor directives
(i.e., #define’s) must go at the top of your file.
For
example if we put the following lines at the top of our file.
#define FALSE 0
#define TRUE !FALSE
#define LEFTMOTOR 0
#define RIGHTMOTOR 1
#define FWD_FULL 100
#define RVS_FULL -100
#define STOP_MOTOR 0
#define LEFT_BUMPER !digital(7)
#define RIGHT_BUMPER !digital(8)
When the C compiler comes across the defined term, it replaces it with the expression on the left. The defined term does not need to be in all caps, but this is a convention to distinguish these constants from variables. This allows us to write more easily understood code. This is probably best seen with some code snippets.
To control the motor, we could use command such as:
motor(LEFTMOTOR,FWD_FULL);
/*Set left motor to max spedd*/
motor(RIGHTMOTOR,STOP_MOTOR);
/*Stop the right motor*/
If we use the previous example with the while loop, we could write:
void main() {
while (stop_button()==FALSE)
{
.
. /* the bulk of your code went here */
.
}
}
Important: #define’s are not variables, they cannot be changed.
The following code would produce an error.
FWD_FULL=95; /*Error,
#defined quantities cannot be changed!!!*/
Advanced Directives: Feel free to skip this section if you want to:
Pre-processor
directives can have arguments (this example is from the interactive C help
page).
#define RIGHT_MOTOR 0
#define LEFT_MOTOR 1
#define GO_RIGHT(power)
(motor(RIGHT_MOTOR,(power)))
#define GO_LEFT(power) (motor(LEFT_MOTOR,(power)))
#define GO(left,right)
{GO_LEFT(left); GO_RIGHT(right);}
void main()
{
GO(0,0);
}
Arrays
Often
it is convenient to be able to have multiple data stored, without using a
separate variable for each. The
structure used to implement this in C is called an array.
For
example, you might want to store the readings from the four photo-cells on your
robot in a single entity. You could do
this with an array.
void main() {
int i; /*loop counter*/
int cds[4]; /*Array declaration*/
/*Sample all 4 CdS cells*/
for (i=0;i<4;i++) {
cds[i]=analog(i);
}
printf(“\nCds Cell 3 reads
%d.”,cds[3]);
}
The array has 4 elements, c[0], c[1], c[2], and c[3]. Note that the array index starts at 0, and so only goes to N-1, where N is the length of the array.
Initializing
arrays.
Often you will want to initialize an array to specific values. As an example, we might want to define 4 threshold values for the light sensors, to distinguish light from dark. We could define an array, and then use 4 separate lines to assign values, or we could do it with a single line:
int threshold[]={120, 131,
119, 110};
This code fragment define 4 threshold values for the light sensors, to distinguish light from dark, one for each of the four sensors. You could compare these 4 values against the four readings to determine which is sensing light vs dark.
A short example that the
concepts introduced.
#define FALSE 0
#define TRUE !FALSE
/*
Use #define to define
constants.
#define's are
implemented as a simple text substitution.
These need two go at the
top of the file
/*
/*
Define an integer
globally.
done is a flag to tell
main program when to end.
*/
int done;
/*
This function will be
used as a separate process.
It plays 3 notes,
defined in the array c[].
*/
void PlayNotes() {
int i;
/*loop counter*/
float MyNotes[]={250.0, 250.0,
100.0};
for(i=0;i<3;i++) { /*loop trough notes*/
tone(MyNotes[i],0.5); /* Play note*/
sleep(0.1); /* delay*/
}
done=stop_button(); /*Done will be TRUE if stop is pressed.*/
}
void main() {
int i=0; /*Declare a variable and initialize
it.*/
done=FALSE; /*This is a
global variable*/
while (!done) {
PlayNotes();
sleep(1.0);
i++;
}
printf(“\nPlayed notes %d
times.”,i);
}
Note
that in this example, the main routine stops running while the notes are being
played. There is an example later in
this handout where the main routine continues to run while the music plays.
Review Sheet – Loops, #define and arrays
(Use both sides – problems 1 through 4)
Due 10/26 when you come to lab.
Name:______________________________________
You may work in groups, but each
person should turn in a sheet.
1) What
is the sequence of numbers printed by the following?
#define MAXN 10
void main() {
int i;
for (i=0; i<MAXN; i+=2) {
while (i==6) i--;
printf(“\n %d”,i);
}
}
2) Write
a snippet of code that performs the following.
Given the array x[], compute a new array y[] that is the average of two
consecutive values of x[]. Note that
there is one less element in y[] than in x[].
Start with
void main() {
float x[]={1.0, 2.5, 3.0, 2.0, 2.5};
float
y[4];
/*add your code here*/
}
3) Write code that performs the following. Given a function represented by the dotted
line, the array x[] and the array t[]represent sampled values of the function (i.e.,
the pairs (t[],x[]) represent the coordinates shown as “x” on the plot).
