Back, Forward

1 - Introduction

This book will try to teach you the basics of the C programming language. It does not assume you have any programming experience. It is helpful to have taken enough math to know what a function is (https://en.wikipedia.org/wiki/Function_(mathematics)).

All the exercises in section 1 can be done with pen and paper. You can check your answers here. We'll start programming on the computer in Section 2.

Functions

Mathematical functions take some input and produce an output:

Below is a mathematical function.

It takes some input number, $x$, and returns some other number, $x+1$. Using the function is just a matter of doing the math: $f(5)= 5 + 1 =6$ and $f(-1)= -1 + 1 = 0$. $f$ is just a name, which could be anything we like. We could write the same function as

But it's still the same function.

Programming in C or any other language can mostly be thought of as giving the computer some input and having it calculate (compute) some output. The input can come from the user typing at their keyboard, using their mouse, or using reading some file on their computer or downloaded from the internet.

Exercise 1 (written):

1. Write a mathematical function which returns twice the input
2. Write a mathematical function which return the negative of its input

Answers

If we wanted to write the mathematical function above as a function in the C language it would look like this:

int PlusOne(int x)
{
    return x + 1;
}

This might look a like hieroglyphics at first so we are going to break it down one piece at a time.

In C we always tell the computer the type of data we are dealing with. int above stands for integer which means a positive or negative whole number. -1, 0, 1, and 1234567 are integers. 0.123 is not an integer. (We'll talk about other types later). In line 1 the first thing written is the type of data the function will output, which in this case is int.

Next we have at least one space, then the function's name; in this case PlusOne. The name can be almost anything but can't start with a number and can't include a few other special characters like spaces. The name does not have any effect on the function's behavior. I called this function PlusOne because that is a good description of what it does, but if I called it MultiplyBy100, or AAA the computer would still do the same calculations. It is a good idea to choose a name that gives some idea of what the function does; this will make using it later a lot easier.

After the name, in the ( ) parentheses we put the function inputs. There can be 0, 1 or more as shown below. This function has a single integer input.

Thats all for line 1!

Next we enclose the part of the function where the work happens with { } curly brackets. These brackets don't do anything except tell the computer where the start and end of the function is. Stuff enclodes by the {} is called a block

Inside the brackets are statements. The functions statment tell the computer how to produce the output from the inputs. Each statement ends with a ; (semicolon). The ; does not do anything. It just helps the computer to read (aka 'parse') your code. This function has a single return statement. The statement:

    return <whatever>;

means whatever follows return is the output.

Because we told the computer that PlusOne will output an int in line 1, that must be the type of data that follows the word return. If we tried to return x+0.1 that would be an error, because if x is an integer then x+0.1 is not an integer (not a whole number). If we forget a return statement that is also an error (NOTE: your program may run but it will give nonsense results). I'll explain how the computer will let you know you've made an error later.

You may also notice we put several spaces before the return statement (it is indented). These spaces are optional, but most people think it makes it easier to read. The placement of {} on lines by themselves is optional too. So, the function above could have been written:

int PlusOne(int x){return x+1;}

but most people find this harder to read. Almost all C code will indent a few spaces when inside a { }.

The technical term for using a function is calling the function. To call our function above we would write PlusOne(5). Any place you write PlusOne(5) in our program it is basically the same as inserting a 6.

Exercise 2: What value is output (returned) by PlusOne(-1)?

Answers

Function Inputs

Our first function above has a single integer as input. Some functions will have no inputs:

int ReturnFive()
{
    return 5;
}

Some functions will have more than one input

int Sum(int input1, int input2)
{
    return input1 + input2;
}

you just separate each input with a comma.

Some functions will have no outputs:

void DoNothing()
{

}

Why would we use a function with no outputs? We'll come back to this in Chapter3

Exercise 3:

1. Write a C function which takes a input a a single integer parameter returns twice the value.
2. Write a C function which returns takes a single integer input parameter returns the negative of it.
3. Write a C function which takes two integer parameters and returns the first minus the second

Answers

Variables

Let's look at another function

int AddThreeTimes(int x)
{
    return x + 5 + 5 + 5;
}

Hopefully it is clear what this function is doing. But what if we decide to revise our function later and wanted to change the number we adding from 5 to something else? We'd have to remember everywhere we wrote 5 in our program and change each one. We can write this function differently to avoid this hazard.

int AddThreeTimes(int x)
{
    int addMe = 5;
    return x + addMe +addMe + addMe;
}

Now the function has one extra statement on line 3 that defines a new variable called addMe. With this line we are telling the computer, I want to store an integer value in the name addMe, set it to 5 initally. Later if we want to use that value we can write addMe and it will be the same same result as putting the value there.

The rules for valid names of variables are basically are the same as functions. That is: names can contain all letters, some numbers and symbols, but can't start with a number. Just like with function names, variable names Names don't effect how they are used. Name things whatever you like but try to make the name meaningful.

You can change the value stored in a variable at any time in another statement as below:

int AddThreeTimes_v2(int x)
{
    int addMe = 5;
    addMe = 10;
    return x + addMe +addMe + addMe;
}

Exercise 4: What is the value returned by AddThreeTimes_v2(11)?

Note: If we only specify the type and name (e.g. int addMe; instead of int addMe = 5;) it is just called a declaration rather than a definition.

We must always remember to declare a variable before we try to use it, otherwise we'll get an error:

int Double()
{
    // BAD CODE!
    return x + x;
}

Herethe error is that the computer has no idea what we mean by x, so this code will not work

Types

Whether we store values in variables, or are defining function's inputs or outputs we always need to tell C what kind of data we are working with, this is called the data's type. Some simple types are described below, we'll cover more complex types later.

int

This is the one we have seen already. ints are positive or negative whole numbers. The largest and smallest numbers that can be stored in an int will depend on the computer you are working with, but a range of approximately $-2^{31}$ (min) to $2^{31}$ (max) is common.

float and double

To use decimals, we need something other than integers. Another numeric type is float. This stand for floating point, a fancy way of saying a number that might include a decimal point. 1,2,3 are floats, but so are 1.1 and 2.1 and 3.1 and 1,234.5. Floating point numbers are often written in exponential form in code like 334e6, which is an easy way of writing $334 * 10^6$.

We can only store up to a certain precision of number in a float. For example, you can't store the exact value of $\pi$ in a float because it would require it to store infinitely many digits and we'd run out of memory. (we'll learn more on computer memory in a future section)

More often than float you will see and use the type double which is just a float that uses double the memory but has double the precision.

Some examples of functions that use double:

double PlusOnePointFive(double x)
{
    return x + 1.5;
}

double Square(double x)
{
    return x * x;
}

double Divide(double x, double y)
{
    return x / y;
} 

Exercise 5: Write a function that takes a double named radius as input and returns a double which is the area of a circle with that radius. (hint: you can approximate $\pi$ as 3.1415)

char

char is short for character. Most of the letters, numbers, or symbols on your keyboard can be stored as a char.

char GiveMeAnE()
{
    return 'E';
}

Sets of characters can be used to display messages to the user. I'll show you how to do this in the next section.

Summary

• C functions take some data input and produce some output.
• You always tell C the type of data we are working with
• You the names of functions and variables will not affect the behavior
• Some common types are ints like 5, doubles like 3.1412 and chars like 'Z'

Index, To Section 2