3. Variables

3.1 Introduction to Variables

In maths at school, you learn algebra - substituting numbers for letters. In code, we do the same - we name our numbers to make it clearer for everyone to understand.

There are many types of variables:

• Integers: these are counting numbers that go into positives and negative.
  • E.g. the number of slices of pizza, the number of seconds past
• Characters: these are ASCII characters that you can type out on a keyboard.
  • 'A', 'z', '1'
• Floats: these are 'floating decimal numbers', which represents a number that can have decimal points.
  • E.g. pi (3.141592654...), 12.0

3.2 Creating a variable

Variables always have a type in C. To create one, you put in the type followed by the name. The convention for the name is camelCase - words are stringed together and all except the first word start with a capital.

int numberOfChickens;
char myFirstNameInitial;
float pi;

When created, you can assign it numbers.

int numberOfChickens;
numberOfChickens = 10;

You can also string both together - but be careful, as you can only do that once.

float pi = 3.14; // Set it to this at the start
pi = 3.14159; // Change it later

3.3 Printing variables

To print out variables, we need to add it into the printf statement. Let's modify our Hello World code to print out more than just text.

#include <stdio.h>

int main () {
    int numberOfChickens = 10;
    printf("I have %d chickens.\n", numberOfChickens);
    return 0;
}

Note the syntax - there are two parts of the printf function:

You'll get used to this syntax soon. When you run it, you should get the following output:

I have 10 chickens.

3.4 Errors

The worst errors are those that don't show up as errors.

When you code, you are bound to get errors. Errors you receive from the compiler are like a seatbelt – they help you a lot but the worst errors you get are those that don't show up as errors.

Let's try and change the code to make it wrong. Here, I have moved the quotation to the end of the printf( ... ) statement, and removed the return 0.

// THIS CODE DOES NOT WORK AND WILL PRODUCE AN ERROR
#include <stdio.h>

int main () {
    int numberOfChickens = 10;
    printf("I have %d chickens.\n, numberOfChickens");
}

When attempting to run this program, you will get the following error:

chickens.c: In function 'main':
chickens.c:5:2: warning: format '%d' expects a matching 'int' argument [-Wformat=]
printf("I have %d chickens.\n, numberOfChickens");
^
...

This error is very cryptic! The first thing you want to look at is chickens.c:5:2:: this means the error occurred on the 5th line.

It says that it's expecting a matching 'int' argument. This means that you have used a %d placeholder, but it cannot find the variable you want to put into that slot. That's because numberOfChickens is part of the string and not an argument (not on its own, after a comma).

3.5 Numbers and Rounding

Integers cannot have decimal points. If you do, the compiler will always round down for you.

#include <stdio.h>

int main () {
    int numberOfChickens = 3.5;
    printf("I got %d chickens.\n", numberOfChickens);

    return 0;
}

Running the program:

I have 3 chickens.

Remember we talked about floats above, which could be used to represent a number like PI? Try and make it print 3.5.

3.6 Exercises

1. Building: Write a program that prints out the following. It should use two variables, height (a float) and year (an integer).

The building is 3.513 metres high.
The year is 2018!