Introduction to C

Setup

From source code to a running program:
- The source code of a C program is a text usually stored in a file with .c extension
- The source code is compiled to an object file with .o extension
- The object file is linked to create the final executable
- The executable can be run under a debugger, which allows to execute the program step by step and monitor what is happening
You can use:
- Any text editor to write C programs
- Any of the available C compilers (which usually do also the linking)
- Any of the available debuggers
However, the best practice is to use an IDE (Integrated Development Environment)
IDE combines an editor, a compiler and a debugger in a single application
The editors in IDEs often recognize and highlight the syntax as you type
Additionally, advanced editors perform static analysis to provide hints about problems in the source code

VS Code

For this course, you can use VS Code or any other IDE / text editor
The guide below instructs how to configure VS Code

Ubuntu 20.04 installation

Run in the terminal:
```
sudo snap install code --classic
```
Install the compiler:
```
sudo apt update && sudo apt install gcc
```

Check if everything works fine by running:

gcc --version

You should see something like that:

gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Windows 10 installation

Run in the terminal (cmd.exe):

winget install Microsoft.VisualStudioCode

Download and run the latest MSYS2 installer from https://github.com/msys2/msys2-installer/releases (e.g. msys2-x86_64-20220128.exe)
Run the MSYS2 terminal and execute:
```
pacman -Syu
```
The terminal will be closed, so open a new MSYS2 terminal and execute:
```
pacman -Su
```

Next, install the compilers:

pacman -S --needed base-devel mingw-w64-x86_64-toolchain

Finally, configure the system to recognize compiler binaries:
- Search for Edit the system enviroment variables
- Click Environment variables button
- Double-click Path environment variable
- Click New and paste C:\msys64\mingw64\bin
  (this is a valid path if you install MSYS2 in the default location)
- Check if everything works fine by running a new terminal (cmd.exe) and executing:
```
gcc --version
```
  You should see something like that:
```
gcc (Rev9, Built by MSYS2 project) 11.2.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
```

Configuration

Start VS Code
Click the Extensions button on the left sidebar
Search for C/C++ and install it

Example program

Click File → Open Folder in the top menu
(this is important; you need to have a folder opened, not just a single file, to build programs)
Click New File and save it as main.c in the selected folder

Paste the following code into the editor:

#include <stdio.h>
int main() {
    printf("Hello World!\n");
    return 0;
}

Press CTRL+SHIFT+B (a shortcut to build the program) and select C/C++ from the drop-down menu
Press CTRL+SHIFT+` to open a new terminal
Run your code with ./main (Linux) or .\main.exe (Windows). You should see:
```
Hello World!
```
Let’s analyze the code line by line:
1. #include <stdio.h> tells the compiler that we want to use input/output functions from the standard library.
2. int main() { defines a function named main which returns an integer and takes no arguments.
3. printf("Hello World!\n"); prints in the terminal a string Hello World! and a new line character (the \n) – note that the line has to end with a semicolon ;.
4. return 0; ends the program with code zero meaning success (if you want to let know that something went wrong, you should return any non-zero value).
5. } a closing bracket of the main function.

Tasks

Write the following programs.

Ask for a name and print: Hello name
- To read a string of maximum 10 characters:
```
char name[11];
scanf("%s", name);
```
- In general, every string’s last byte is 0, so n characters require n+1 bytes to store
- The scanf function reads the input and converts it
- The %s means that it should treat the input as a string
Ask for an integer and print its absolute value
- To read an integer:
```
int value;
scanf("%d", &value);
```
- The %d means that the input contains a decimal number
- The & in front of value is required by scanf to know where to store the result
- To perform a conditional execution:
```
if (condition) {
    // do something if condition is true
} else {
    // do something else
}
```
Ask for an integer n and print out n times a string: Hello World!
- To execute operations in a loop:
```
for (int i = 0; i < n; i++) {
    // repeat something n times
}
```
Ask for an integer n and print an ASCII half-tree with asterisks
- For example, if n=4:
```
*
**
***
****
```
Ask for two numbers - hours and minutes - and print the total number of seconds they contain
Ask for a number of meters and print the number of miles
- Use real numbers instead of integers:
```
float real;
scanf("%f", &real);
```
- The number of miles in a kilometer is 0.621371192
Ask for hours, minutes and kilometers. Let’s say these describe the time you needed to run the given distance. Print the average speed [km/h]
Ask for a radius of a sphere and print its volume
- The formula is V = \frac{4}{3} \pi r^3
Ask for a number of book copies and print the total cost of buying and delivering them. The first ten books cost 40 zl each. Then the price is 35 zl per book. If customer orders 40 or more books, there is a 5% discount from the total book price. Delivery boxes contain up to twelve books. The cost of delivery per box is 12 zl. Customer pays for delivery of up to five boxes. The bookstore will pay for delivery of any additional box required to fulfill the delivery.
Ask for three numbers: a, b and c. Check if these three numbers can be interpreted as the lengths of sides of a triangle
- If any of the three lengths is greater than the sum of the other two, then the answer is no
Ask for a string and check if it is a palindrome (reads the same from the beginning and from the end)
Ask for two numbers: a and b. Check if a is a power of b
- a is a power of b if a is divisible by b and a/b is a power of b
- Solve it using recursion - what is the base case here?