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

Chapter 9 ■ Searching, Sorting, and Merging
We wish to determine if the number 61 is stored. In search terminology, 61 is called the search key
or, simply, the key. The search proceeds as follows:
• Compare 61 with the 1st number, num[0], which is 35; they do not match so
we move on to the next number.
• Compare 61 with the 2nd number, num[1], which is 17; they do not match so
we move on to the next number.
• Compare 61 with the 3rd number, num[2], which is 48; they do not match so
we move on to the next number.
• Compare 61 with the 4th number, num[3], which is 25; they do not match so
we move on to the next number.
• Compare 61 with the 5th number, num[4], which is 61; they match, so the
search stops and we conclude that the key is in position 4.
But what if we were looking for 32? In this case, we will compare 32 with all the numbers in the
array and none of them will match. We conclude that 32 is not in the array.
Assuming the array contains n numbers, we can express the above logic as follows:
for h = 0 to n - 1
if (key == num[h]) then key found, exit the loop
endfor
if h < n then key found in position h
else key not found
This is a situation where we may want to exit the loop before we have looked at all elements in the
array. On the other hand, we may have to look at all the elements before we can conclude that the
key is not there.
If we find the key, we exit the loop and h will be less than n. If we exit the loop because h
becomes n, then the key is not in the array.
Let us express this technique in a function search that, given an int array num, an integer key, and
two integers lo and hi, searches for key from num[lo] to num[hi]. If found, the function returns
the position in the array. If not found, it returns -1. For example, consider the statement:
n = search(num, 61, 0, 6);
This will search num[0] to num[6] for 61. It will find it in position 4 and return 4, which is then
stored in n. The call
search(num, 32, 0, 6)
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