The Journey Begins

A basic C program

    int main(int argc, char *argv[]) {
            return 0;
    }

Every C program has a main function like above. All function calls are made inside the main function (or as a result of some function that main calls).

When we run a C program, the computer will look for a function called main, and pass in two parameters… (argc and argv, more on that in a later week)

Types

The int and char in the code above are types. All variables have a type associated with it that defines what kind of data it is. The type lets the computer know how much memory is needed to store the data and how to interpret it.

Each variable’s type must be declared before we can store or read value from it. This is a declaration.

    int num;

After the declaration we can initialise the variable with a value and perform operations on it.

    int num;
    num = 1;
    num = num + 2;

Warning: Initialization is important because there is no default value for variables in C. The computer can allocate memory that has values left from before, so an uninitialised variable’s value may be entirely random.

Declaration and initialisation can be combined.

    int num = 1;

Basic types in C:

int = 3 - integer (whole number)

float = 3.14 - real number

double = 3.143141592653589793 - real number (a double can represent real numbers with more precision than a float because it has double the memory size of float)

char = 'c' - a character (ASCII)

Hello World

    #include <stdio.h>
    int main(int argc, char *argv[]) {
            printf("Hello World\n");
            return 0;
    }

We need to import the standard input & output library (stdio) to write our hello world program. Printf does not print newlines by default, so you will usually need to include \n at the end of your string.

' is different from "

In C, single quotes are used for characters and double quotes are used for strings. This may be counterintuitive to programmers who came from Python.

Input and Output

Printing just static texts is not fun. C’s printf can print numbers with some formating magic.

    printf("This is a number: %d\n", 3);
    printf("This is a double: %lf\n", 3.14);
    printf("This is a double with 1 decimal place: %.1lf\n", 3.14);

We can also read in input with scanf.

    double placeholder;
    printf("Give me a double: ");
    scanf("%lf", &placeholder);
    printf("You gave me: %.1lf\n", placeholder);

%lf tells scanf to expect a real number in the input. It stores the value into the double variable placeholder. & is a special operator in C, &placeholder evaluates to the memory location of placeholder, which scanf uses to place the value in the correct location. We will learn more about the & operator in later weeks.

More on printf and scanf: https://www.tutorialspoint.com/c_standard_library/c_function_printf.htm

https://www.tutorialspoint.com/c_standard_library/c_function_scanf.htm

Compiling

How do we run our C program? Before any human readable program can be executed by the computer, it needs to be translated into binary instructions that our CPU hardware can understand. Compilers are programs that translates human readable code into binary. Compilers are incredible pieces of technology, many can optimize your code during translation. If it sees a shortcut that still produces the same behaviour as your code, it will take it for you. GCC and Clang are examples of a compiler. In this subject, we will be using GCC.