Learn to Program with C_ Learn to Program using the Popular C Programming Language ( PDFDrive )

Chapter 8 ■ Arrays
Points to note about Program P8.1:
• Using #define, we set the symbolic constant MaxNum to 100; we use it to
declare the array and in the prompt for numbers. This makes the program
easy to modify if we change our mind and wish to cater for a different
amount of numbers.
• We enter the while loop when the number read is not 0. Inside the loop, we
add it to the sum, store it in the array, and count it. Each time we reach the
end of the loop, the value of n is the amount of numbers stored in the array
so far.
• On exit from the while loop, we test n. If it is still 0, then no numbers were
supplied and there’s nothing else to do. The program does not make the
mistake of trying to divide by n if it is 0. If n is positive, we confidently divide
the sum by it to find the average.
• The for loop ‘steps through’ the array, printing the numbers and their
differences from the average. Here, n is the number of elements of the array
that were actually used, not necessarily the entire array. The elements used
are num[0] to num[n-1].
• The program works out the sum of the numbers as they are read. If we need
to find the sum of the first n elements after they have been stored in the
array, we can do this with the following:
sum = 0;
for(int h = 0; h < n; h++) sum += num[h];
Program P8.1 does the basics. But what if the user entered more than 100 numbers? Recall
that, as declared, the elements of num range from num[0] to num[99].
Now suppose that n is 100, meaning that 100 numbers have already been stored in the array.
If another one is entered, and it is not 0, the program will enter the while loop and attempt to
execute the statement
num[n++] = a;
Since n is 100, this is now the same as
num[100] = a;
But there is no element num[100] – you will get an “array subscript” error. When you start
working with arrays, you must be very careful that your program logic does not take you outside
the range of subscripts. If it does, your program will crash.
To cater for this possibility, we could write the while condition as
while (a != 0 && n < MaxNum) { ...
If n is equal to MaxNum (100), it means we have already stored 100 values in the array and
there is no room for any more. In this case, the loop condition will be false, the loop will not be
entered, and the program will not try to store another value in the array.
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