Lecture #25: Arrays 3

December 3, 1999

Bob Pitha

bpitha@cs.ucsd.edu

http://www-cse.ucsd.edu/classes/fa99/cse5a

pointer subtraction

• Subtraction with pointers follows the same logic as addition, but the rules are necessarily different

– instead of moving forward in the array, as addition does, we’re moving backwards

– so given the declarations:

int a[8];

**int *p = a+4;**

– p now points to element a[4], the fifth element of a

– the expression p-2 evaluates to a pointer to a[2], the third element of a.

example

Text Box: int a[8];
int *p, *q;

p = a+4;
q = p-2;

example

Text Box: int a[8];
int *p, *q;

p = a+4;
q = p-2;

example

Text Box: int a[8];
int *p, *q;

p = a+4;
q = p-2;

pointer subtraction

• So just like with addition, the – operator can take a pointer and an integer as operands

– but you can only subtract an integer from a pointer, not vice versa

• You can go 2 lockers to the left of locker #583

• You can’t go locker #583 to the left of 2

• But unlike addition, you can subtract one pointer from another

– as long as the two pointers point to exactly the same type

pointer-pointer

• You can think of this algebraically:

– given the following declarations:

int a[8];

**int *p1 = a+2;**

**int *p2;**

if

p2 = p1 + 3

then

p1 == p2 – 3

and

p2 – p1 == 3

so pointer subtraction…

• … gives us the number of “things” between the two pointers.

so back to our example…

$Text Box: #include <stdio.h> int InputScores (double *); void PrintStats (double *, int); int main (void) { double scores[250]; int numScores; printf (“Scores: a program to compute grade statistics.\n”); printf (“Enter all scores, finish with –1:\n\n”); numScores = InputScores (scores); PrintStats (scores, numScores); }$

• Now we need to write the two functions

– InputScores

– PrintStats

what’s this supposed to do again?

• So let’s keep things simple: imagine there are only 5 students in my class

– they get the following scores on an exam: 57, 85, 97, 16, 82

– this is what I’d like my program to do:

Text Box: Scores: a program to compute grade statistics.
Enter all scores, finish with –1:

57 85 97 16 82 –1

Average: 61.4
Median: 82
Standard Deviation: 32.2

PrintStats()

• So the PrintStats function should:

– take the array and the number of scores as arguments

• remember that our array can hold up to 250 scores

• but that doesn’t mean that all 250 were entered

– it should compute the average, median, and standard deviation, and print them out

– the function declaration is this:

so let’s write the function

• First of all, we need to give names to our parameters

– the declaration doesn’t need names; all it needs are the data types

– but the actual function definition is declaring local variables to use within the function; these variables need names

• We also can put in the braces that will surround the code for the function

so what’s the function going to do?

• It must:

– compute the average

– compute the median

– compute the standard deviation

– print the statistics

• Let’s punt on the math for now…

– after all, this is a computer class, not a statistics class

– we’ll just use three more functions, which we can write later

our function so far

$Text Box: double ComputeAverage (double *, int); double ComputeMedian (double *, int); double ComputeStandardDev (double *, int); void PrintStats (double *scores, int num) { double average; double median; double stdDev; average = ComputeAverage (scores, num); median = ComputeMedian (scores, num); stdDev = ComputeStandardDev (scores, num); printf (“Average: %5.1f\n”, average); printf (“Median: %5.1f\n”, median); printf (“Standard deviation: %5.1f\n”, stdDev); }$

how would we do those functions?

Text Box: /*
* ComputeAverage: calculate and return the average
* of the numbers in an array
*/
double ComputeAverage (double *array, int size)
{
int i;
double sum = 0.0;

for (i = 0; i < size; ++i)
sum += array[i];
return sum/size;
}

next…

• Now we have to implement InputScores

• What this will do:

– keep reading numbers from the keyboard until the user enters a negative number

– store all the entered numbers in the array

– return the number of entries

• So as before, let’s set things up for the code

Text Box: int InputScores (double *scores){
...
}

function InputScores

• Now what?

– we want to do the following:

• read a number

• if it’s negative, return

• otherwise, store it in the array

• increment a running count of the number of entries

• and go back to read another one

– doesn’t this sound a lot like a loop?

• we’re repeating the same steps over and over

choosing a loop

• We have 3 main kinds of loops at our disposal

– for

– while

– do while

• How do we choose?

– in reality, we could use any of the three

InputScores using while

• Here’s one way to do it:

Text Box: int InputScores (double *scores)
{
int count = 0;
double score;

scanf (“%lf”, &score);
while (score >= 0.0) {
scores[count] = score;
count += 1;
scanf (“%lf”, &score);
}

return count;
}

InputScores using while

• Or slightly more compactly:

Text Box: int InputScores (double *scores)
{
int count = 0;
double score;

scanf (“%lf”, &score);
while (score >= 0.0) {
scores[count++] = score;
scanf (“%lf”, &score);
}

return count;
}

another InputScores

Text Box: int InputScores (double *scores)
{
int count = 0;
double score;

scanf (“%lf”, &score);
while (score >= 0.0) {
scores[count++] = score;
scanf (“%lf”, &score);
}

return count;
}