Computer Science 9618/22 — May/June 2023

Background Concept

A constant is a named value that does not change while the program runs. In program design, constants are often used for fixed values such as tax rates, conversion factors or prices.

If a programmer writes a value like 3.75 directly inside statements, that value is called a hard-coded literal. Hard-coded literals work, but they are not usually the best design choice when the same value may appear in several places.

Using a named constant gives the value a meaningful name and stores it in one place.

Understanding the Question

The program uses postal charges such as 3.75 directly in the pseudocode. The question asks for a more appropriate way to represent those postal costs.

The key clue is that these are fixed charges that the programmer has built into the program. That points to constants rather than ordinary variables.

Approach

Look at the role of the value:

• it is a preset value
• it represents a known postal charge
• it is likely to be used in more than one place

That makes a named constant the best answer.

Step-by-Step Reasoning

In the example, the program contains:

• ItemPostalCost ← 3.75

The value 3.75 is a literal. Instead of repeating that literal, the programmer could declare a constant with a meaningful name, such as a constant representing the charge for that category of item.

Then the assignment would use the constant name rather than the number itself. This is more appropriate because the postal cost is a fixed programmed value, not a value entered by the user or calculated differently each time.

So the required answer is to represent the postal costs as named constants.

Key Takeaways

• Fixed values in a program should usually be stored as constants.
• A named constant is better than scattering literal numbers through the code.
• This improves the quality of the program design.

Common Mistakes

• Saying "variable" instead of "constant". A variable is for a value that can change during execution.
• Saying "array" without justification. An array might store several costs, but the direct improvement being tested here is replacing literals with constants.
• Repeating the example value only, rather than naming the programming feature.

Things to Be Careful About

• The question asks for a way of representing the postal costs, not for rewriting the whole program.
• The important word is more appropriate, so the answer should focus on software design quality.
• In Cambridge pseudocode, a constant should be clearly treated as fixed and given a meaningful identifier.

Background Concept

A good program is not judged only by whether it works. It should also be easy to read, maintain and update. Constants help with this.

When the same numeric value is written directly in many places, changing that value later becomes harder. If the programmer forgets one occurrence, the program becomes inconsistent. Named constants solve that problem because the value is defined once and then referred to by name wherever needed.

This is especially useful in real-world systems such as pricing, postage, tax or discount calculations, where fixed values may change over time.

Understanding the Question

Part (a)(ii) asks for the advantages of the answer to part (a)(i), with reference to this program. So it is not enough to say vague things like "it is better". The answer should connect directly to the postal cost program.

The program uses several possible postal costs. That means the same charge values may appear in multiple places. If those values are stored as named constants, there are clear benefits for this exact situation.

Approach

Think about what happens when a postage price changes.

• Would the programmer need to search through the whole program?
• Could one old value be left behind by mistake?
• Would a name be easier to understand than a bare number?

These lead naturally to the three strongest advantages:

• easier to update
• less chance of errors or inconsistency
• improved readability

Step-by-Step Reasoning

Suppose the cost 3.75 is used in several IF statements.

If the programmer writes 3.75 directly each time:

• every occurrence must be found and edited if the price changes
• one occurrence could easily be missed
• a reader has to guess what 3.75 represents

If instead the program uses a named constant:

• the programmer changes the value once, in the constant declaration
• every use of that constant automatically uses the new value
• the program is less likely to contain mixed old and new prices
• the constant name explains the meaning of the charge

For example, a name tells the reader that the value is a postal charge for a particular case, whereas 3.75 on its own tells the reader nothing about why that value is used.

That is why the best advantages are easier maintenance, fewer mistakes and improved clarity.

Key Takeaways

• Constants are useful when fixed values are reused.
• Named constants improve maintainability because one edit updates all uses.
• They also improve reliability and readability.

Common Mistakes

• Giving only one advantage when the question asks for several.
• Saying only "easier" without explaining why it is easier.
• Describing variables instead of constants.
• Forgetting to refer to this specific program about postal costs.

Things to Be Careful About

• The benefit is not that the value can change during execution; constants do not do that.
• The maintenance advantage matters because postal charges may be updated in future.
• Readability comes from the name, not just from storing the value elsewhere.
• Avoid repeating the same idea in different words; examiners usually want distinct points.