Give small jobs a name with methods

The same minutes calculation is needed twice. Can you give it a name instead of copying it? Supply different inputs and predict the returned results.

Predict two calls

A method is a named piece of work that you can call. This lesson uses local functions written after the main statements in a top-level console program. We use the word method for the beginner idea; these particular helpers are local functions.

Read both calls first. Predict their output, then run the complete program.

using System;

int firstPlan = CalculateMinutes(20, 3);
int secondPlan = CalculateMinutes(15, 2);

Console.WriteLine(firstPlan);
Console.WriteLine(secondPlan);

static int CalculateMinutes(int minutesPerDay, int days)
{
    return minutesPerDay * days;
}

Follow input → work → result

CalculateMinutes(20, 3)
                 │  │
                 │  └── days = 3
                 └───── minutesPerDay = 20

              20 × 3
                 │
                 └── return 60 ──> firstPlan

The numbers supplied in a call are arguments. The names in the declaration are parameters. int before the helper name says the returned result is an integer. return sends the result back to the caller; the assignment stores it in firstPlan.

The body does not print anything. This is useful separation: CalculateMinutes calculates, while the main statements choose what to display. A returned value can be checked, stored or used in another calculation. A helper that only prints would be harder to reuse when you need the number itself.

static on this local function means it cannot secretly read local variables from the surrounding program. The needed values arrive through its parameters. You do not need classes or objects to use this pattern here.

Return a decision

A return type can also be bool. Here a helper reports whether the completed minutes meet the target. The same helper handles two different inputs.

using System;

Console.WriteLine(HasReachedGoal(30, 45));
Console.WriteLine(HasReachedGoal(45, 45));

static bool HasReachedGoal(int studiedMinutes, int targetMinutes)
{
    return studiedMinutes >= targetMinutes;
}

Try it independently

Write a helper named RemainingMinutes. It receives studied and target minutes. Return the difference when the target is larger; return zero when the target has already been met. For inputs 30 and 45 expect 15. For 50 and 45 expect 0. This small contract avoids a negative “remaining” amount.

Try before comparing with the complete solution below.

using System;

Console.WriteLine(RemainingMinutes(30, 45));
Console.WriteLine(RemainingMinutes(50, 45));

static int RemainingMinutes(int studiedMinutes, int targetMinutes)
{
    if (studiedMinutes >= targetMinutes)
    {
        return 0;
    }

    return targetMinutes - studiedMinutes;
}

Explain back

Describe one helper using only its name, inputs and output. Then change the first call to RemainingMinutes(10, 60) and predict 50. Explain why the helper can stay unchanged when the input changes. Keep names specific enough to describe the job; a name such as DoStuff gives the caller little help.

Reference: Microsoft — local functions and explicit inputs.