Repeat useful work with loops

Could the program print each day’s plan without a separate instruction for every day? Trace one trip around a loop, then find the condition that stops it.

Predict before running

A loop repeats a block of statements. Read this program without running it. Write the four output lines on paper. Then run the complete program in your practice project.

using System;

int day = 1;
int totalMinutes = 0;

while (day <= 3)
{
    totalMinutes += 20;
    Console.WriteLine($"Day {day}: {totalMinutes}");
    day++;
}

Console.WriteLine($"Total: {totalMinutes}");

Trace the changing state

An iteration means one pass through the body. The while condition is checked before every pass. The braces group the statements that belong to that pass. Indentation helps your eyes; the braces decide the group.

totalMinutes += 20 means add 20 to the current total and store the result. day++ increases day by one. The total and the counter do different jobs: one collects minutes, the other controls repetition.

Before writing a loop, name its start, its condition and its progress step. Here the start is 1, the condition is day <= 3, and the progress step is day++. If the day never changes, this condition stays true and the program keeps repeating. If you accidentally create such a loop while practising, stop the terminal program with Ctrl+C, then correct the progress step.

Put a counter in a for loop

When a counter controls the repetition, a for loop keeps its three parts together: start; condition; progress. This complete replacement prints the same three day numbers, without calculating a total.

using System;

for (int day = 1; day <= 3; day++)
{
    Console.WriteLine($"Day {day}");
}

Read the header in execution order: initialise day once, check the condition, run the body, increase day, then check again. Do not place an extra semicolon between the header and the opening brace: that would create an empty loop body.

Try it independently

Build a four-day plan that adds 25 minutes each day. Predict the total before running. Your final line should be Total: 100. Compare your attempt with this complete solution only after trying.

using System;

int totalMinutes = 0;
for (int day = 1; day <= 4; day++)
{
    totalMinutes += 25;
}
Console.WriteLine($"Total: {totalMinutes}");

Explain back

Point to the statement that makes each loop finish. Explain why changing <= to < changes the number of passes. Finally, change the plan to two days of 15 minutes and predict 30 before running.

Reference: Microsoft — iteration statements.