Lecture Materials

Class Announcements

  1. Section sign ups: Sections start this week. Section assignments will be emailed out to you by Tuesday. If you didn't sign up for a section, or need to change your assignment, a late signup form will open on Tuesday evening.
  2. Early assignment start: We have covered everything you need for assignment #1. It is due on Friday. Don't leave submission to the last second.
  3. LaIR is open: See the Office Hours link on the main page for hours.


Learning Goals

The goal of today's class is for you to be able to create, use and modify basic variables. You will also be able to think through how expressions evaluate.

Questions & Answers


Q: When will assignment submission open?

A1:  Once you get assigned a section leader which will be early this week.


Q: How much does it actually matter that our functions have the same pre and post condition? I have a function that mismatches the two bc it works in my code and while loops. If I wanted to mismatch them I’d have to add a function after every time I call this function

A1:  There are nod rules. If its a beautiful way of breaking down the problem, its good times :)


Q: What can I do if I’m very lost with downloading programs and navigating the first Karel assignment?

A1:  Get to the LAIR http://web.stanford.edu/class/cs106a/


Q: Stylistically, would it be better to write turn_left() three times, or write for i in range(3)?

A1:  I like the for loop option!


Q: Is looking up syntax for a python feauture, not code for a specific program, considered “looking up code” which is against the honor code

A1:  You can look up python syntax. But you can also find any syntax you will need on the course slides!


Q: Will pycharm automatically save your code after logging out?

A1:  You should hit file -> save all or cmd+s before closing pycharm.


Q: Where do we find out what sections we are in (i.e. what time and the Zoom links?)

A1:  Tuesday around 5pm PT.


Q: To clarify, it’s OK if pre and post conditions don’t match?

A1:  its ok


Q: If there are only two times during the sections that work for me, what will happen if I don't get into either of the two sections that I signed up for?

A1:  If you can’t make it to your assigned section for a given week, you can attend a different section!

A2:  We often are able to get everyone in a time that works


Q: I need the help from LAIR but it says no queues are open; what should I do?

A1:  You can start signing up for LaIR starting at 4:30pm PT Sundays-Thursdays.

A2:  Its not open at the moment. It starts in the evening


Q: Will not completing the optional MidpointKarel project impact our grade at all for the overall assignment? Just asking in case I do not have time to complete that specific project.

A1:  No! Its optional :)


Q: Should we put the comment inside the function or above

A1:  Often inside, but just having the comment is what matters :)


Q: In a previous example, there was a general description AND pre and post conditions before every function. Should we just pick one style or do both these comments?

A1:  Both is always the safest. But they are two (good) ways to describe the functions


Q: Should my newly defined functions be in any particular order?

A1:  No hard rule. I like to go from higher level (like main, jump_hurdle) and get more low level (like turn right) as you go down


Q: Should the comments be written before the def function() line or is it ok for it to be inside of the function?

A1:  Inside the function is preferred, but just having the comments is what matters.


Q: If we create a function turn_right() for example, do we need to make a comment to explain it or is it unnecessary? If so, should we put pre-and post- conditions in the comment?

A1:  At the start, comment more than you think is necessary. In the future you can pair back when you have more experience


Q: Is it ok to have while loops or if/else statements in the main function?

A1:  Yes! Try to decompose but if it makes sense to have these in main you can do that.


Q: Do the pre and post conditions of loops need to align as well as the pre and post conditions of functions?

A1:  Pre and post conditions for loops should align. Pre and post condiitons of functions often do not align.


Q: So the comment for your main function, all it has to do is describe and say what your code does right? It doesn't need a pre or post condition necessarily correct

A1:  correct


Q: Is it preferred that we comment our pre and post condition in our code?

A1:  yes!


Q: can we begin our program with a control flow? for example a while loop would follow def_main():

A1:  allowed!


Q: should we define our functions before or after our main function? is there a stylistic preference for one or the other?

A1:  I like to put main first and all of my function defintions below it. You. can also put all of your helper functions above main. I just recommend that you are consistent with how you choose to order your functions.


Q: Is there a reason why we cannot submit our assignments yet for Karel?

A1:  Yes! Once you have been assigned a section leader you will be able to submit.


Q: Is it possible to have too many lines on the first assignment? (should we decompose as far as possible or no?)

A1:  There is no line limit. Try to decompose as much as you can!


Q: What is a corner vs an avenue?

A1:  A corner is where a given street and given avenue intersect.


Q: What is the maximum length you suggest a comment for a function should be?

A1:  Several lines. Comments should be descriptive but concise.


Q: In the Karel assignment, can we change the name “main”

A1:  You should not change the name ‘main’


Q: what does it mean "print the "hello world" " to the screen?

A1:  In python, you can use print(“print this message”) to allow your program to output text.


Q: What’s the point of main? I haven’t used it before… I’d just place the functions and code and let it run.

A1:  When you run your program, main is the function that gets called.


Q: Why are there underscores before and after “name” and main” at then end?

A1:  We will discuss that later this quarter! Don’t worry about it for now.


Q: Is there any specific reason why Mehran runs his program through the comman line rather than clicking “Run” or “Run without debugging”?

A1:  You can do either!


Q: If I added a descriptive comment, should I still add a description of pre and post conditions?

A1:  Yes please.


Q: do we need a comment for every new function? for example when we define turn_right as turn_left turn_left turn_left, do we need to comment something like “karel needs to turn right” ?

A1:  Yes! Each function you define should have a comment.


Q: Do you not need eval(input(“”)) for integers?

A1:  You can just use input()


Q: what if you put in a non-number? How would it be converted?

A1:  When the program runs you will get an error at the int() line that says “invalid literal for int()…”


Q: for which functions do we comment the pre and post conditions?

A1:  All functions in Karel!


Q: can you breakdown which parts of num1 = input does what? For example, which part makes the text print, which part makes the cursor apear, etc.

A1:  Input(“Question: “) prints “Question: ” and then waits for a user to type input. Once the user types in their input, that value is sotred in num1


Q: do user inputs always initally go in as strings?

A1:  Yes


Q: How does python know how to add?

A1:  The inventors of python kindly wrote this feature for us to use. Come to office hours if you want to talk about how this happens on a lower level.


Q: So what you print cannot be in a form besides text (like an integer)?

A1:  You can print an integer!


Q: Do we need comments that explain what a function does or can we just have pre/post conditions?

A1:  Both would be best.


Q: Can't we make the function only accept integers for its input?

A1:  By the end of the q you will know how to do this!


Q: is there a space after the colon?

A1:  Yes! That way there will be a space before the user types their input.


Q: Do we always need text inside input()? Or can it be left blank

A1:  It can be left blank but that could be confusing for users because they would not know what to input.


Q: how does the program run the input command if it is changed to a number in the next line. num = input(x) then reassigned to num = 1, wouldn’t this just remeber the 1.

A1:  In this case, whatever the user typed in would be lost and num would be 1.


Q: Is the midpoint problem extra credit or just optional?

A1:  Extra credit!


Q: Are we allowed to use variables if it's applicable to the p-set for friday, or should we limit it to what we learned the first week?

A1:  You cannot use variables in Karel.


Q: In this class, are there partner assignnments or is it all individual?

A1:  All individual.


Q: are there shortcuts like ++ or -- in Python?

A1:  '++ and — do not work in python. You can use += 1 or -= 1.


Q: Can you create variables that are accessable to an entire program/ multiple functions?

A1:  Yes this is possible in python but you should not do this in 106A.


Q: What if a function is called in another function—are the variables moved as well?

A1:  Great question! They are not moved. Later in the course you will learn how to move these variables to other functions.


Q: So Python doesn’t require us to declare variables (string, int, double) before we intialize them?

A1:  Correct! How awesome is that?


Q: If you use the same character for different variables doesn't that make it semantically challenging for other people to read the code?

A1:  There are stylistic reasons to do use the same character for variables in different functions but for the most part you are correct!


Q: Can variables in python be used for multiple types of values, for instance x=int then later x= “string” ?

A1:  Yes!


Q: Do you have to assign variable types in Python?

A1:  Nope!


Q: Would you have to use the int() function to convert the num_in_class = 500 and num_students from an integer to a string before performing operations (i.e. subtraction)?

A1:  Yes!


Q: Could you store something like a fraction in a variable but not as a strong as numbers? Like something with an operator?

A1:  there isn’t a “native” variable type for this — but i have seen many programmers make their own. At the end of cs106a you will learn how to make your own variable types!


Q: are strings variables or objects? or values?

A1:  Great question! Strings are objects. A variable can store a string. A string can be the value assigned to a variable.


Q: can we put y = “10” with the double quotes

A1:  Yes. That would assign y to be the string “10”


Q: What is the number type called Double?

A1:  in other languages they distinguish between “floats” and “floats that get more memory” aka doubles… python gives all floats a lot of memory


Q: Why did the total(26) have to be converted to a string value in the last print statement in the program earlier?

A1:  you aren’t allowed to add a string and a number! so first convert the number to a string…


Q: if you divide two int’s and the quotient is not an int does python convert it to a float? ex. 7 / 2 == 3.5 or 4 ?

A1:  converts to a float!


Q: Would the pi (3.14…) be considered to be a float?

A1:  Yes!


Q: Do you need to use a separate line to define num1 as an int or can you just type int before num1 when you’re equating it to the input function?

A1:  You can do it in one step like: num1 = int(input(“Enter first number: “))


Q: Could we also print: print('The total is', total, ''.") or is there a specific reason he uses str()?

A1:  You can print that way as well. Mehran is creating one string rather than printing 3 separate things.


Q: what would happen if the input was text, and i tried to convert it into an integer

A1:  You would get an error when you try to convert it to an int that the input cannot be converted.


Q: could you type num1 = int(input(“Enter number here: “)) ? One line?

A1:  yes!


Q: so if you do num1 = float(num 1), will it be a real number then>?

A1:  Yes


Q: what is the difference between a number in a string (“9”) and an integer number (9)? in what situation would you use one over the other

A1:  If you have a number as a string, you cannot do any arithmetic operations.


Q: If you do print(“The total is “, total, “ .”), that would give you the same result without converting the int to a string right?

A1:  Correct.


Q: If the input number was a float would this program round it to the nearest integer?

A1:  Yes. The int line would do that.


Q: so what happens to the old “string” luggage? does it disappear

A1:  Great question! Come to office hours to learn more about this. For now you can think of it as disappearing.


Q: will assignments be given when the previous assignment is due or will we get them before?

A1:  The next assignment will come out the day the current assignment is due.


Q: so would a float be float(num2) ?

A1:  Yes.


Q: What would happen if you added 9 and 17 but they were strings and not integers

A1:  you would get “917”


Q: why is it string instead of int or float for the sum?

A1:  We convert total to be a string so that we can print it out as one string.


Q: Why did 26 have to be a string? can it not be typed as an integer in the terminal?

A1:  When you type into the terminal, it is stored as text.


Q: what if we continue with int version of total

A1:  If you do not covert total to a string, you will get an error telling you that you cannot concatenate a string and an int.


Q: If you left total as an integer value in the final print statement, would it result in an error?

A1:  Yes! It would tell you that you cannot concatenate a string and an int.


Q: What if you did int(4.5)? How does the floart get converted?

A1:  it becomes 4

A2:  It removes the decimal. So this would covert that to 4.


Q: Total’s variable type is unchanged because it is converted to a string within the print function?

A1:  Yes!


Q: can we use f-strings instead of concatenation?

A1:  Try to use concatenation in 106A.


Q: Do we always have to convert a number from integer to string in order for print to work?

A1:  no. only if you want to “concatenate” strings and ints


Q: is it impossible to print an integer directly?

A1:  yes. lets say x is an int… you can type print(x)


Q: Can you not print the total as an int within the printed string?

A1:  No because you cannot concatenate ints and strings. So this would result in an error.


Q: if you want to print just a variable, without adding it to any text, do you still have to convert it to a string? or can you just type the variable in the parantheses without the quotes?

A1:  no!


Q: If you then change str1 str2 or str3 will str4 update or does it have to be defined again as being str1 + str2 + str3

A1:  if you update str1, str2, or str3, after you declared str4, str4 will not update to have these new values.


Q: does the size of floats ever factor in to using integers over them if you have thousands of float variables, or is the difference always negligible?

A1:  sometimes it matters. Never in cs106a. But more often in applications like “ray tracing” or “deep learning” where you have trillions of variables


Q: How do I get to Office Hours? When I go through the website it is saying I don’t have the proper authentication

A1:  You have to log in with your sunet


Q: Why was the last one “x” + str() and this one just “x=“, x? Do we not always need the plus sign?

A1:  You can do either print(“x” + str()) which will concatenate “x” and str() into one string. You can also do print(“x=“, x) which will first print “x=“ then will print x.


Q: Do the commas automatically convert the following variable into a string?

A1:  it does!


Q: is it better style to use commas or + in print statements?

A1:  I like commas because it puts whitespace automatically


Q: why did we make total a string in the previous problem? Couldn't we could leave total as an int?

A1:  crashes! You can’t use the “+” operator with a string and an integer


Q: so the only difference between using + vs a comma in a print function is that there’s a space added if you use a comma?

A1:  yes


Q: does it matter if its 1 concatenated string vs 3 different print ?

A1:  three different print statements will have “new lines” in between them


Q: in the print part, when putting the total, why does it have to be str(total)? What happens if just “total” is entered?

A1:  You will get an error saying that you cannot concatenate ints and strings.


Q: Can you add floats and integers together?

A1:  yes!


Q: What is meant by base 10? Is that in contrast to binary?

A1:  Yes!


Q: Are we going to be assigned another assignment before Friday or is Assignment 2 not going to be assigned until next week

A1:  Assignment 2 will be released on Friday.


Q: should I email the cs190 or wait till tuesday to sign up ?

A1:  Wait until Tuesday please.


Q: Was Karel the only textbook for the class?

A1:  no, we have a new textbook, written by nick parlante. Just added to the website


Q: Can we use or in Karel

A1:  Yes!


Q: chris do you have office hours right now?

A1:  live answered


Q: Are we required to read/buy the textbook?

A1:  To read yes! It is free on the cs106a class website.


Q: can we use the same commands that we learned in Karel in python (eg. for i in range etc..

A1:  Yes! You can use for loops in python!


Q: When does assignment 2 come out?

A1:  Friday!


Q: can you define a string again?

A1:  A sequence of characters surrounded by quotes.


Q: are the 4 types we learned today the only types in python?

A1:  There are lots of types in python.


Q: how do we have to submit the homework?

A1:  There is a submitting assingments handout. You will be able to submit later this week.