Lecture 1 - CS50x 2024.pdf

This is CS50 CS50’s Introduction to Computer Science OpenCourseWare Donate  (https://cs50.harvard.edu/donate) David J. Malan (https://cs.harvard.edu/malan/) [email protected]  (https://www.facebook.com/dmalan)  (https://github.com/dmalan)  (https://www.instagram.com/davidjmalan/)  (https://w...

This is CS50 CS50’s Introduction to Computer Science OpenCourseWare Donate  (https://cs50.harvard.edu/donate) David J. Malan (https://cs.harvard.edu/malan/) [email protected]  (https://www.facebook.com/dmalan)  (https://github.com/dmalan)  (https://www.instagram.com/davidjmalan/)  (https://www.linkedin.com/in/malan/)  (https://www.reddit.com/user/davidjmalan)  (https://www.threads.net/@davidjmalan)  (https://twitter.com/davidjmalan) Lecture 1 Welcome! Hello World Functions Variables Conditionals Loops Operators and Abstraction Linux and the Command Line Mario Comments Types Summing Up Welcome!  In our previous session, we learned about Scratch, a visual programming language.  Indeed, all the essential programming concepts presented in Scratch will be utilized as you learn how to program any programming language.  Recall that machines only understand binary. Where humans write source code, a list of instructions for the computer that is human readable, machines only understand what we can now call machine code. This machine code is a pattern of ones and zeros that produces a desired effect.  It turns out that we can convert source code into machine code using a very special piece of software called a compiler. Today, we will be introducing you to a compiler that will allow you to convert source code in the programming language C into machine code.  Today, in addition to learning about how to code, you will be learning about how to write good code.  Code can be evaluated upon three axes. First, correctness refers to “does the code run as intended?” Second, design refers to “how well is the code designed?” Finally, style refers to “how aesthetically pleasing and consistent is the code?” Hello World  The integrated development environment (IDE) that is utilized for this course is Visual Studio Code, affectionately referred to as , which can be accessed via that same url, or simply as *VS Code.*  One of the most important reasons we utilize VS Code is that it has all the software required for the course already pre-loaded on it. This course and the instructions herein were designed with VS Code in mind.  Manually installing the necessary software for the course on your own computer is a cumbersome headache. Best always to utilize VS Code for assignments in this course.  You can open VS Code at cs50.dev (https://cs50.dev/).  The compiler can be divided into a number of regions:  Notice that there is a file explorer on the left side where you can find your files. Further, notice that there is a region in the middle called a text editor where you can edit your program. Finally, there is a command line interface , known as a CLI, command line, or terminal window where we can send commands to the computer in the cloud.  We will be using three commands to write, compile, and run our first program: code hello.c make hello./hello The first command, code hello.c creates a file and allows us to type instructions for this program. The second command, make hello , compiles the file from our instructions in C and creates an executable file called hello. The last command,./hello , runs the program called hello.  We can build your first program in C by typing code hello.c into the terminal window. Notice that we deliberately lowercased the entire filename and included the.c extension. Then, in the text editor that appears, write code as follows: #include int main(void) { printf("hello, world\n"); } Note that every single character above serves a purpose. If you type it incorrectly, the program will not run. printf is a function that can output a line of text. Notice the placement of the quotes and the semicolon. Further, notice that the \n creates a new line after the words hello, world.  Clicking back in the terminal window, you can compile your code by executing make hello. Notice that we are omitting.c. make is a compiler that will look for our hello.c file and turn it into a program called hello. If executing this command results in no errors, you can proceed. If not, double-check your code to ensure it matches the above.  Now, type./hello and your program will execute saying hello, world.  Now, open the file explorer on the left. You will notice that there is now both a file called hello.c and another file called hello. hello.c is able to be read by the compiler: It’s where your code is stored. hello is an executable file that you can run, but cannot be read by the compiler. Functions  In Scratch, we utilized the say block to display any text on the screen. Indeed, in C, we have a function called printf that does exactly this.  Notice our code already invokes this function: printf("hello, world\n"); Notice that the printf function is called. The argument passed to printf is ‘hello, world\n’. The statement of code is closed with a ;.  A common error in C programming is the omission of a semicolon. Modify your code as follows: #include int main(void) { printf("hello, world\n") }  Notice the semicolon is now gone.  In your terminal window, run make hello. You will now be met with numerous errors! Placing the semicolon back in the correct position and running make hello again, the errors go away.  Notice also the special symbol \n in your code. Try removing those characters and making your program again by executing make hello. Typing./hello in the terminal window, how did your program change? This \ character is called an escape character that tells the compiler that \n is a special instruction.  Restore your program to the following: #include int main(void) { printf("hello, world\n"); } Notice the semicolon and \n have been restored.  The statement at the start of the code #include is a very special command that tells the compile that you want to use the capabilities of a library called stdio.h , a header file. This allows you, among many other things, to utilize the printf function. You can read about all the capabilities of this library on the Manual Pages (https://manual.cs50.io). The Manual Pages provide a means by which to better understand what various commands do and how they function.  Libraries are collections of pre-written functions that others have written in the past that we can utilize in our code.  It turns out that CS50 has its own library called cs50.h. Let’s use this library in your program. Variables  Recall that in Scratch, we had the ability to ask the user “What’s your name?” and say “hello” with that name appended to it.  In C, we can do the same. Modify your code as follows: #include int main(void) { string answer = get_string("What's your name? "); printf("hello, %s\n", answer); } The get_string function is used to get a string from the user. Then, the variable answer is passed to the printf function. %s tells the printf function to prepare itself to receive a string.  answer is a special holding place we call a variable. answer is of type string and can hold any string within it. There are many data types, such as int , bool , char , and many others.  %s is a placeholder called a format code that tells the printf function to prepare to receive a string. answer is the string being passed to %s.  Running make hello again in the terminal window, notice that numerous errors appear.  Looking at the errors string and get_string are not recognized by the compiler. We have to teach the compiler these features by adding a library called cs50.h : #include #include int main(void) { string answer = get_string("What's your name? "); printf("hello, %s\n", answer); } Notice that #include has been added to the top of your code.  Now running make hello again in the terminal window, you can run your program by typing./hello. The program now asks for your name and then says hello with your name attached, as intended.  printf allows for many format codes. Here is a noncomprehensive list of ones you may utilize in this course: %c %f %i %li %s %s is used for string variables. %i is used for int or integer variables. You can find out more about this on the Manual Pages (https://manual.cs50.io) Conditionals  Another building block you utilized within Scratch was that of conditionals. For example, you might want to do one thing if x is greater than y. Further, you might want to do something else if that condition is not met.  We look at a few examples from Scratch.  In C, you can assign a value to an int or integer as follows: int counter = 0; Notice how a variable called counter of type int is assigned the value 0.  C can also be programmed to add one to counter as follows: counter = counter + 1; Notice how 1 is added to the value of counter.  This can be represented also as: counter = counter++; Notice how 1 is added to the value of counter. However the ++ is used instead of counter + 1.  You can also subtract one from counter as follows: counter = counter--; Notice how 1 is removed to the value of counter.  Using this new knowledge about how to assign values to variables, you can program your first conditional statement.  In the terminal window, type code compare.c and write code as follows: #include #include int main(void) { int x = get_int("What's x? "); int y = get_int("What's y? "); if (x < y) { printf("x is less than y\n"); } } Notice that we create two variables, an int or integer called x and another called y. The values of these are populated using the get_int function.  You can run your code by executing make compare in the terminal window, followed by./compare. If you get any error messages, check your code for errors.  Flow charts are a way by which you can examine how a computer program functions. Such charts can be used to examine the efficiency of our code.  Looking at a flow chart of the above code, we can notice numerous shortcomings.  We can improve your program by coding as follows: #include #include int main(void) { int x = get_int("What's x? "); int y = get_int("What's y? "); if (x < y) { printf("x is less than y\n"); } else if (x > y) { printf("x is greater than y\n"); } else { printf("x is equal to y\n"); } } Notice that all potential outcomes are now accounted for.  You can re-make and re-run your program and test it out.  Examining this program on a flow chart, you can see the efficiency of our code design decisions.  Considering another data type called a char we can start a new program by typing code agree.c into the terminal window.  Where a string is a series of characters, a char is a single character.  In the text editor, write code as follows: #include #include int main(void) { // Prompt user to agree char c = get_char("Do you agree? "); // Check whether agreed if (c == 'Y' || c == 'y') { printf("Agreed.\n"); } else if (c == 'N' || c == 'n') { printf("Not agreed.\n"); } }  Notice that single quotes are utilized for single characters. Further, notice that == ensure that something is equal to something else, where a single equal sign would have a very different function in C. Finally, notice that || effectively means or.  You can test your code by typing make agree into the terminal window, followed by./agree. Loops  We can also utilize the loops building block from Scratch in our C programs.  We look at a few examples from Scratch. Consider the following code: int counter = 3; while (counter > 0) { printf("meow\n"); counter = counter - 1; } Notice that his code assigns the value of 3 to the counter variable. Then, the while loop says meow and removes one from the counter for each iteration. Once the counter is not greater than zero, the loop ends.  In your terminal window, type code meow.c and write code as follows: #include int main(void) { printf("meow\n"); printf("meow\n"); printf("meow\n"); } Notice this does as intended but has an opportunity for better design.  We can improve our program by modifying your code as follows: #include int main(void) { int i = 3; while (i > 0) { printf("meow\n"); i--; } } Notice that we create an int called i and assign it the value 3. Then, we create a while loop that will run as long as i > 0. Then, the loop runs. Every time 1 is subtracted to i using the i-- statement.  Similarly, we can implement a count-up of sorts by modifying our code as follows: #include int main(void) { int i = 1; while (i