Variables and Data Storage

Following the Program's Path

Every program is a series of instructions that a computer executes. The important question is not only what instructions exist, but also when each instruction runs. Some instructions run one after another in a fixed order. Some run only when a condition is true. Others run repeatedly, often with some variation, until a stopping condition is met. These three execution patterns work together whenever a program processes data, makes decisions, or automates a task.

A useful way to trace execution is to track both the current instruction and the data being processed. In the source scenario, the data is a list of student test scores. The program reads the scores, checks an average against 70, congratulates qualifying students, and then prints a summary.

Reading Instructions in Order

Sequential execution means that statements run one after another in the order they appear in the script. The program begins with the first instruction, completes it, and then moves to the next instruction. Unless another execution pattern changes the path, the order of the statements determines the order of the work.

Letting Conditions Choose

Conditional execution checks a condition and uses its result to decide what happens next. If the condition is satisfied, the program runs the associated block of statements. If it is not satisfied, the program skips that block. The condition therefore changes the path through the instructions without changing the fact that the program follows an ordered execution process.

In the student-score scenario, the condition asks whether the average score is above 70. A true result allows the program to continue to the repeated congratulations step. A false result means that step is skipped, and the program can continue toward the summary.

Returning to Repeated Work

Repeated execution runs a block of statements multiple times, usually with some variation, until a stopping condition is met. The repeated block is not treated as one single action. The program returns to that block, performs it again for another item or state, and eventually stops when the relevant work is complete.

In the source scenario, once the average is above 70, the program processes the students one by one. For each student whose score is above the average, it prints a congratulations message. The student being considered changes from one repetition to the next, and the repeated processing ends after the students have been handled.

Tracking Data Through Execution

When tracing a program, treat the data as part of the program's current state. The source scenario begins with student scores. Sequential execution reads them in order. The program then derives an average for its condition. If that condition permits the next stage, repeated execution examines students in turn and produces congratulations messages for the qualifying students. Finally, sequential execution reaches the summary.

This trace shows why data processing and execution control belong together. The scores provide the data being processed, the average provides the basis for a decision, and the individual student scores provide variation across repetitions. The source pack describes what the program reads, checks, processes, and prints; it does not specify a particular programming language or implementation for storing these items.

Mistakes in Program Tracing

• Treating every instruction as if it runs exactly once.

  Repeated execution runs a block multiple times, while sequential execution describes the order of statements.

  Fix: Mark which statements are sequential, which block is conditional, and which block repeats.

• Assuming a conditional block always runs.

  A condition can cause its block to run or be skipped.

  Fix: Evaluate the condition first, then follow only the branch allowed by its result.

• Forgetting that repeated execution has a stopping condition.

  Repeated execution continues until a stopping condition is met.

  Fix: Identify what marks the end of the repeated processing, such as finishing the students.

• Ignoring variation between repetitions.

  Repeated execution usually includes some variation; in the source scenario, the student being processed changes.

  Fix: Record what changes from one repetition to the next.

Practice the Full Trace

The Three Patterns Together

1. Sequential execution runs statements one after another in the order they appear.
2. Conditional execution checks a condition and runs or skips a block of statements.
3. Repeated execution runs a block multiple times, usually with variation, until a stopping condition is met.
4. Programs can combine all three patterns to process data, make decisions, and automate tasks.
5. Tracing both the execution path and the data being processed helps explain what a program does.