CIS3355: Business Data Structures
Fall, 2008
 

          Accessing Numeric Arrays through Pointers

If we were to declare variables one at a time, searching for a particular element in RAM would be cumbersome.

e.g. declaring the square roots of the numbers 1 to 10 would be:

float sqrt1=1.000,sqrt2=1.414, sqrt3=1.732, sqrt4=2.000,        sqrt5=2.236, sqrt 6=2.449, sqrt7=2.646, sqrt8=2.828, sqrt9=3.000, sqrt10=3.162

And searching for element 2.236 would be:

        If(sqrt1==2.236)then printf(“Stored in sqrt1\n”);

           Else if(sqrt2==2.236)then printf(“Stored in sqrt2\n”);

              Else if(sqrt3==2.236)then printf(“Stored in sqrt3\n”);

                Else if(sqrt4==2.236)then printf(“Stored in sqrt4\n”);

…..And so on until a match is found.

So we want to store the elements in an array and access them through the use of pointers to make the searching simpler.

We use the following C code to do this:

#include <stdio.h>

void main()

{float sqrt[10]={1.0,1.414,1.732,2.0,2.236,2.449,2.645,2.828,

3.0,3.162};    //requesting 40 contiguous bytes of storage starting at 15012 to    15015 (floats require4 bytes of storage)

float *sqrtptr;   //

sqrtptr=sqrt;   //set base address of sqrt (which is 15012)  equal to contents of sqrtptr

while (sqrtptr<=&sqrt[9])  //do the loop as long as the contents of location sqrtptr (which is    15012) is <= the address location of subscript 9 which is 15048

    {if (*sqrtptr==2.236)  

        printf(“%5.3f found at address%p\n”,*sqrtptr,sqrtptr);

     sqrtptr++;}}     //increments the contents of sqrtptr by 4

DON'T forget that array subscripts start at element 0.

DO use arrays instead of creating several variables that store the same thing. For example, if you want to store total sales for each month of the year, create an array with 12 elements to hold sales rather than creating a sales variable for each month.

Q Why is it better to use an array instead of individual variables?

A With arrays, you can group like values with a single name. In Listing 8.3, 1,000 values were stored. Creating 1,000 variable names and initializing each to a random number would have taken a tremendous amount of typing. By using an array, you made the task easy

Q What happens if I use a subscript on an array that is larger than the number of elements in the array?

A If you use a subscript that is out of bounds with the array declaration, the program will probably compile and even run. However, the results of such a mistake can be unpredictable. This can be a difficult error to find once it starts causing problems, so make sure you're careful when initializing and accessing array elements.

1. If an array is declared with 10 elements, what is the subscript of the first           element?

      Ans: zero

2.  What is the name of the array, and how many elements does it have?

   float expenses[12];

       Ans:  The array is named expenses, and it contains 12 elements.