Section 9.18 Worked Example: Arrays  Find Value
Subsection 9.18.1 Subgoals for Evaluating Arrays
Set up array from 0 to size1
Evaluate data type of statements against array

Trace statements, updating slots as you go
Remember assignment subgoals
You can watch this video or read through the content below it.
Problem: Assume that the integer array alpha
has been properly declared and is full of data values, and that the variable target
is an int with a value in it. Evaluate these statements and determine the value of loc
. If any error occurs, give the reason.
int loc = 1;
boolean found = false;
for (int i = 0; i < alpha.length && !found; i++) {
if (alpha[i] == target) {
loc = i; found = true;
}
Subsection 9.18.2 SG1: Set up array from 0 to size1
alpha is an array of ints and has values, but we donβt know what those values are
however, we can still diagram a representation of this array
notice that the largest index is size  1
Subsection 9.18.3 SG2: Evaluate data type of statements against array
References to the array are in the loop and selection statements:
for (int i = 0; i < alpha.length && !found; i++) {
if (alpha[i] == target) {
loc = i; found = true;
}
This loop has index i go from 0 to size  1 (<length) by increments of 1.
Then the value at alpha[i] is compared to the int value of target.
If the value at alpha[i] is equal to target, then the value i is copied into loc.
All indexes into the array are valid, and all assignments are valid.
Subsection 9.18.4 SG3: Trace statements, updating slots as you go
Let us trace with a sample array and assume the value of target
is 15.
The first statement, int loc = 1;
gives loc
a value that is not a valid index for any array.
Then a forloop is used to traverse the array and compare each element to target
. The chart below uses one line to represent the memory and comparisons during each iteration of the loop, starting when i has a value of zero.
When we find the target value in the array, we store the index (location) of where it is in the array.
Some questions to consider:
What would happen if the
target
value is not in the array? Then the selection statement is never true, andloc
is never changed from its initial value of 1.Why is 1 a good initial value for
loc
? It is not a valid index for any array. You or another programmer using this algorithm could check the value ofloc
to make a decision (selection!) for how the program will behave when thetarget
value is found or not found at a valid array index.What would happen if there were 2 occurrences of the target value in the array? The loop does not end when the
target
value is found, so additional occurences would overwrite the value ofloc
with the last occurence.
The more general answer to the original question is: βloc
contains the index of the last occurrence of target
in the array alpha
or 1 if target
is not in the array.β