4.2 Introduction to Using Data Sets

1. What a data set is

A data set is a collection of specific pieces of information, or data 4.2.A.1. You meet data sets everywhere:

Everyday data set	The pieces of data
A week of temperatures	18.5, 21.0, 19.5, …
A class’s quiz scores	7, 9, 10, 6, …
A playlist	song titles
A step counter	steps per day

2. Processing values one at a time

You can manipulate and analyze a data set to solve a problem or answer a question 4.2.A.2. The key pattern: the values are accessed and used one at a time, and each one is processed according to the outcome you want 4.2.A.2.

Problem: “On how many days this week did a runner run more than 5 km?” Written in plain language, the algorithm is:

1. Set a counter to 0.
2. Look at the next day’s distance.
3. If that distance is greater than 5, add 1 to the counter.
4. Repeat steps 2 and 3 until every day has been looked at.
5. The counter is the answer.

This “visit each value and update a running result” pattern is called a traversal in code, and it is the heart of the rest of Unit 4.

3. Plan with a table or chart

Data can be represented in a diagram such as a chart or table, and that visual can be used to plan the algorithm 4.2.A.3. Here is the runner’s week as a table, with an extra row that traces the counter after each value is processed:

Day	Mon	Tue	Wed	Thu	Fri	Sat	Sun
Distance (km)	4.0	6.5	5.0	8.0	3.5	10.0	5.5
Greater than 5?	no	yes	no	yes	no	yes	yes
Counter after this day	0	1	1	2	2	3	4

The trace shows the answer is 4, and it also catches a common trap: 5.0 is not greater than 5, so Wednesday does not count.

A chart works too. A bar chart of the distances with a line drawn at 5 km lets you count the bars above the line, which is exactly what the algorithm does one bar at a time.

4. From plan to code (preview)

Once the table works on paper, each written step becomes a line of code. Topic 4.3 shows how to store the data in an array; this preview shows how closely the code follows the plan.

double[] distances = {4.0, 6.5, 5.0, 8.0, 3.5, 10.0, 5.5};
int count = 0;                          // step 1
for (double d : distances)              // steps 2 and 4: each value, one at a time
{
    if (d > 5)                          // step 3
    {
        count++;
    }
}
System.out.println("Days over 5 km: " + count);   // step 5

Output:

Days over 5 km: 4

Watch out: the trace table is also your test. If the code prints something different from the last cell of your trace, either the plan or the code has a mistake.