4.9. Input with ReadLine

Console.WriteLine works fine for printing static (unchanging) messages to the screen. If we wanted to print a phrase greeting a specific user, then Console.WriteLine("Hello, Dave."); would be OK as long as Dave is the actual user.

What if we want to greet someone else? We could change the string inside the () to be 'Hello, Sarah' or 'Hello, Elastigirl' or any other name we need. However, this is inefficient. Also, what if we do not know the name of the user beforehand? We need to make our code more general and able to respond to different conditions.

It would be great if we could ask the user to enter a name, store that string in a variable, and then print a personalized greeting using Console.WriteLine. Variables to the rescue!

4.9.1. Requesting Data

To personalize the greeting, we have to get input from the user. This involves displaying a prompt on the screen (e.g. “Please enter a number: “), and then waiting for the user to respond. Whatever information the user enters gets stored for later use.

As we saw earlier, each programming language has its own way of accomplishing the same task. For example, the Python syntax is input("Please enter your name: ").

C# has a built-in module for collecting data from the user, called Console.ReadLine.

4.9.2. Syntax

Gathering input from the user requires the following setup:

Console.WriteLine("Enter your name");

string name = Console.ReadLine();

Console.WriteLine("Your name is " + name);

There is a lot going on here behind the scenes, but for now you should follow this bit of wisdom:

I turn the key, and it goes.

Most of us do not need to know all the details about how cars, phones, or microwave ovens work. We just know enough to interact with them in our day to day lives. Similarly, we do not need to understand how ReadLine works at this time. We just need to know enough to collect information from a user.

As you move through the course, you WILL learn about all of the pieces that fit together to make this process work. For now, here is a brief overview.

4.9.2.1. How to Prompt the User

In order to collect data from a user, you have to provide directions to ensure that the correct data is entered. This means we have to prompt the user.

To display a prompt and wait for a response, we use the following syntax:

Console.WriteLine("Question here...");

string input = Console.ReadLine();

When C# evaluates the expression, it follows the instructions:

1. Display Question text on the screen.

2. Wait for the user to respond.

3. Store the data in the variable input.

For our greeting program, we would code Console.WriteLine("What is your name?");. This prints the question for the user. We follow this question with string name = Console.ReadLine(). In the console, you will notice the cursor blinking in the line below. This is where the user enters a name and presses the Return or Enter key. When this happens, any text entered is collected by the ReadLine function and stored in name.

Try It

Let’s play around with the input statement. Open the repl.it link below and click the “Run” button.

Console.WriteLine("Enter your name: ");
string name = Console.ReadLine();

repl.it

Note that after entering a name, the program does not actually DO anything with the information. If we want to print the data as part of a message, we need to put name inside a Console.WriteLine statement.

After line 3, add Console.WriteLine("Hello, " + name + "!");, then run the code several times, trying different responses to the input prompt.

By storing the user’s name inside name, we gain the ability to hold onto the data and use it when and where we see fit.

Try adding another + name term inside the Console.WriteLine statement and see what happens. Next, add code to prompt the user for a second name. Store the response in otherName, then print both names using Console.WriteLine.

Try It

Update your code to request a user’s first and last name, then print an output that looks like:

First name: Elite
Last name: Coder
Last, First: Coder, Elite

4.9.3. Critical Input Detail

There is one very important quirk about the input function that we need to remember. Given Console.WriteLine(7 + 2);, the output would be 9.

Now explore the following code, which prompts the user for two numbers and then prints their sum:

Console.WriteLine("Enter a number");
string num1 = Console.ReadLine();


Console.WriteLine("Enter a number");
string num2 = Console.ReadLine();


Console.WriteLine(num1 + num2);

repl.it

Run the program, enter your choice of numbers, and examine the output. Do you see what you expected?

If we enter 7 and 2, we expect an output of 9. We do NOT expect 72, but that is the result printed. What gives?!?!?

The quirk with the ReadLine method is that it treats all entries as strings, so numbers get concatenated rather than added. Just like “Hello, ” + “World” outputs as Hello, World, “7” + “2” outputs as 72.

In C#, ReadLine entries must be strings!

If we want our program to perform math on the entered numbers, we must recast its type.

Try It

1. In line 3, Initialize a new int variable to hold your type conversion of num1.

2. To convert num1 into your new int varialbe, use Int32.Parse().

3. Run the program and examine the result.

4. Repeat step 2 for num2 starting in line 7.

5. What happens if a user enters Hi instead of a number?

6. If you want to work with doubles try it again using Double.Parse()

4.9.4. Check Your Understanding

Question

What is printed when the following program runs?

Console.WriteLine("Please enter your age: ");
string input = Console.ReadLine();

//The user enters 25.

Console.WriteLine(input);

1. System.String

2. System.Number

3. System.Int32

4. "25"

Question

What is printed when the following program runs?

Console.WriteLine("Please enter your age: ");
string input = Console.ReadLine();
int ageNum = Int32.Parse(input);

//The user enters 25

Console.WriteLine(ageNum.GetType());

1. System.String

2. System.Number

3. System.Int32

4. 25