Cs50 lecture 3

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Cs50 lecture 3

In the system, your name and your airline class are stored in consecutive blocks of memory. To fix this, the system might reject any names that are too long. In the last example of vulnerable code with a function pointerwhat should the attacker input as their name to take over control of the program? Assume the code the attacker wants to run is more than 20 bytes long. What are the four parts of C memory?

What part of memory would a variable be stored if I define it: 1 outside any function, 2 as a local variable in a function, and 3 by calling malloc? From lowest address to highest: code, static, heap, stack. A variable defined outside any function would be stored in static memory.

A local variable defined in a function would be stored on the stack. A variable defined by calling malloc would be stored on the heap. Note: The 61C review lecture has a review of C memory layout if you want a refresher. What two values are saved on the stack when x86 calls a function?

cs50 lecture 3

Why do we need to save these values? When the function returns, it will restore the old saved value of EBP. Note: For more details on x86 function calls, consult the 61C review lecture.

CS50: Web Programming with Python and JavaScript

Suppose there is a vulnerable local variable that the attacker can overflow at address X on the stack, and the return address of this stack frame is at address Y.

What could the attacker input into the buffer to take over control of the program? First, the attacker writes Y-X bytes of garbage to fill up all the space between the buffer and the return address. Finally, the attacker writes their code. You should know that 0xffffffff a binary string of all 1s is a very large number when interpreted as an unsigned integer, and -1 when interpreted as a signed integer.

Q2 on Homework 2 has some practice on format string vulnerabilities. Some possible ideas: Use a memory-safe language, like Python, Java, or Go.An entry-level course taught by David J. Topics include abstraction, algorithms, data structures, encapsulation, resource management, security, software engineering, and web development. Problem sets inspired by real-world domains of biology, cryptography, finance, forensics, and gaming.

CS Introduction to Computer Science. An introduction to the intellectual enterprises of computer science and the art of programming. Take course on.

cs50 lecture 3

Open January 1, — December 31, Time commitment. Topic s. What you'll learn A broad and robust understanding of computer science and programming How to think algorithmically and solve programming problems efficiently Concepts like abstraction, algorithms, data structures, encapsulation, resource management, security, software engineering, and web development Familiarity in a number of languages, including C, PHP, and JavaScript plus SQL, CSS, and HTML How to engage with a vibrant community of like-minded learners from all levels of experience How to develop and present a final programming project to your peers.

David J. Rob Bowden. Zamyla Chan. Chemistry and Chemical Biology doctoral candidate Harvard University. Doug Lloyd. Associated Schools. Enroll now. You may also like. CS50's Introduction to Game Development. Learn about the development of 2D and 3D interactive games in this hands-on course, as you explore the design of games such as This course picks up where CS50 leaves off, diving more deeply into the design and implementation of web apps with Python, Learn about mobile app development with React Native, a popular framework maintained by Facebook that enables cross-platform Get updates on new courses.

Email address Subscribe.Notice that unlike integers and floats there is now a difference between asking the Python function print to output the variable and asking the Python interpreter directly for the value of the variable.

A string that starts with double quotes must end with double quotes, and therefore we can have single quotes inside. A string that starts with single quotes must end with single quotes and therefore we can have double quotes inside.

But, starting and ending a string """ or ''' tells Python to allow the string to cross multiple lines. Any character other than ''' or """if that is how the string started is allowed inside the string.

This is an escape characteras we will discuss next. This allows output in columns. Many expressions you might try to write involving strings and either ints or floats are illegal Python, including the following:.

For those that are not valid, what needs to be fixed? For those that are, what is the output when they are passed to the print function? Which of the following are legal? For those that are, show what Python outputs when these are typed directly into the interpreter. Python provides many operations for us to use in the form of functions.

We have already seen printbut now we are going to look at other functions that operate on strings. You can compute the length of a string with len. Function len is provided with the value of the string associated with variable s. You can convert an integer or float to a string with str. You can convert a string that is in the form of an integer to an integer using int. You can convert a string that is in the form of a float to a float using, not surprisingly, float.

We already know a bit about how to use printbut we can learn about more using help. We will talk about this more later. For now, we will focus on the sep and end and illustrate with examples. Python programs can ask the user for input using the function called input. We can also insert the string right into the input function call:.Lecture 3 Searching Big O Linear search Structs Sorting Selection sort Recursion Merge sort Searching Last time, we talked about memory in a computer, or RAM, and how our data can be stored as individual variables or as arrays of many items, or elements.

We can think of an array with a number of items as a row of lockers, where a computer can only open one locker to look at an item, one at a time. For example, if we want to check whether a number is in an array, with an algorithm that took in an array as input and produce a boolean as a result, we might: look in each locker, or at each element, one at a time, from the beginning to the end.

This is called binary searchsince we can divide our problem in two with each step, like what David did with the phone book in week 0. We might write pseudocode for linear search with: For i from 0 to n—1 If i'th element is 50 Return true Return false. For i from 0 to n—1 Find smallest item between i'th item and last item Swap smallest item with i'th item.This means that every single time we open that web page, it looks exactly the same.

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Many websites we visit every day, however, change every time we visit them. For large sites like those, it would be unreasonable for employees to have to manually edit a large HTML file every time a change is made, which is where dynamic websites can be extremely useful.

HTTP, or HyperText Transfer Protocol, is a widely-accepted protocol for how messages are transfered back and forth across the internet. Typically, information online is passed between a client user and a server. In this protocol, the client will send a request to the server, that might look something like the one below.

After receiving a request, a server will then send back an HTTP response, which might look something like the one below. Such a response will include the HTTP version, a status code means OKa description of the content, and then some additional information. The advantage to using a framework like Django is that a lot of code is already written for us that we can take advantage of. Now, when I start my application using python manage. Now, if we want to, we can change the index function within views.

We could even keep track of variables and do calculations within the function before eventually returning something. We can follow many of the same steps within our application to create pages that say hello to Brian and David.

Many sites are parameterized by items included in the URL. For example, going to www. You can even find your own public GitHub repositories by navigating to www.

Next, we have to create a more flexible path in urls. Now, we can try the site out with a few other URLs:. This is a great illustration of how any functionality we have in Python can be used in Django before being returned. For example, I could decide to return a blue header instead of just the text in our index function:. It would get very tedious to write an entire HTML page within views. It would also constitute bad design, as we want to keep separate parts of our project in separate files whenever possible.

cs50 lecture 3

Now, when we visit the main page of our application, we can see the header and title have been updated:. Notice that we passed a third argument into the render function here, one that is known as the context. In this context, we can provide information that we would like to have available within our HTML files.

This context takes the form of a Python dictionary. Now, we can create a greet. Now, when we try it out:. We may want to change what is displayed on our website depending on some conditions. For example, if you visit the site www. To get a sense for how this module works, we can look at the documentationand then test it outside of Django using the Python interpreter.

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If we cheat a little bit and make sure that our condition is always true, we see that the opposite case is filled:. In this file, we can add any styling we wish just as we did in the first lecture:.

CS50 Lecture

Now, if we restart the server, we can see that the styling changes were in fact applied:. Then, we can update our index function to render a template, and provide our newly-created list as context. Notice here that we are able to loop over our tasks using syntax similar to our conditionals from earlier, and also similar to a Python loop from Lecture 2. When we go to the tasks page now, we can see our list being rendered:.

Now that we can see all of our current tasks as a list, we may want to be able to add some new tasks.GitHub is home to over 50 million developers working together. Join them to grow your own development teams, manage permissions, and collaborate on projects. This is check50, a command-line program with which you can check the correctness of your programs.

Python 92 This is compare50, a fast and extensible plagiarism detection tool. Python 39 These are CS50 documents. HTML C This is CS50's library for Python.

Python This is submit50, CS50's command-line tool for submitting problems. Python 94 This is lib50, CS50's own internal library used in many of its tools. This is cs50 's theme for Jekyll.

Dockerfiles for the mailman suite. This is help50, a command-line tool that helps students understand error messages. This is CS50's command-line interface. Skip to content. Sign up. Pinned repositories. Type: All Select type. All Sources Forks Archived Mirrors.

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cs50 lecture 3

Python GPL CSS 34 9 12 1 Updated Jul 13, Python 55 18 2 5 Updated Jul 8, JavaScript 2 5 20 7 Updated Jul 7, Shell 7 2 1 0 Updated Jul 6, Ruby 15 0 1 Updated Jul 5, C 24 26 1 0 Updated Jun 11, This is CS50xHarvard University's introduction to the intellectual enterprises of computer science and the art of programming for majors and non-majors alike, with or without prior programming experience. An entry-level course taught by David J. Malan, CS50x teaches students how to think algorithmically and solve problems efficiently.

Topics include abstraction, algorithms, data structures, encapsulation, resource management, security, software engineering, and web development. Problem sets inspired by real-world domains of biology, cryptography, finance, forensics, and gaming.

Students who earn a satisfactory score on 9 problem sets i. This is a self-paced course—you may take CS50x on your own schedule.

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CS50 2019 - Lecture 3 - Algorithms

Purchase now Request Information. About this course Skip About this course. What you'll learn Skip What you'll learn. A broad and robust understanding of computer science and programming How to think algorithmically and solve programming problems efficiently Concepts like abstraction, algorithms, data structures, encapsulation, resource management, security, software engineering, and web development Familiarity in a number of languages, including C, Python, SQL, and JavaScript plus CSS and HTML How to engage with a vibrant community of like-minded learners from all levels of experience How to develop and present a final programming project to your peers.

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