Values and variables: give your program something to remember

Asha changes her plan from 30 to 45 minutes. Will a reminder copied earlier change too? Predict both numbers before running the program.

Predict a tiny plan

In the previous lesson, the messages were fixed text. A variable gives a value a name that later instructions can use. Read the program below and predict the three output lines. In the middle line, the slash is just printed text separating two numbers.

using System;

string learner = "Asha";
int plannedMinutes = 30;
int reminderMinutes = plannedMinutes;
bool finished = false;

plannedMinutes = 45;

Console.WriteLine(learner);
Console.WriteLine($"{plannedMinutes}/{reminderMinutes}");
Console.WriteLine(finished);

Three types you can use today

The type describes what kind of value a variable can hold. In this program, string holds text, int holds a whole number within its supported range, and bool holds true or false. The words true and false are C# values, so they have no quotes. Writing "false" would create text instead. When WriteLine displays a bool, its normal text is True or False with a capital first letter.

The equals sign means assign the value on the right to the variable on the left. First calculate or read the right-hand side, then store the result. This is why assigning plannedMinutes = plannedMinutes + 10 is useful: it reads the old value, adds 10, then replaces that variable’s value.

Put values inside a message

The dollar sign immediately before the opening quote makes an interpolated string. A name inside braces is evaluated and its value is inserted into the message. The surrounding words and punctuation remain text. Without the dollar sign, a normal string prints the braces and variable name literally.

Choose names that explain the value and its unit: plannedMinutes is clearer than x. Keep the declared type in mind when editing. Assigning a text name to an int variable is a type error; changing a variable’s value does not change its declared type. You can update a variable many times, but each update happens only when that instruction executes.

Build a different plan

Start in a blank Program.cs. Create a learner named Ravi, start with 20 planned minutes, add 10 minutes, and mark finished as true. Print the learner, duration and finished state. Predict before running; your result should match this complete solution:

using System;

string learner = "Ravi";
int plannedMinutes = 20;
plannedMinutes = plannedMinutes + 10;
bool finished = true;

Console.WriteLine($"{learner}: {plannedMinutes} minutes");
Console.WriteLine($"Finished: {finished}");

Try changing only the initial duration to 35. The duration line should now contain 45. If your result differs, trace the assignments rather than changing several lines at once.

Explain back: Why does reminderMinutes stay at 30 in the first program? What does the dollar sign change about a string?

References: Microsoft — built-in type keywords, string interpolation.