Lecture 4/13: Lecture 4 Q&A
Lecture 4 Questions and Answers (Exported from Zoom Q&A log)
Q: Will anyone be sticking around after the lecture to answer questions in person?
A1: yes chris will be in this call and thereās also the post-lecture zoom meeting that Iāll be manning
Q: if the OH are not possible due to timezones, can we still get a ādebugging sessionā ?
A1: Can you attend any of my/Chris/Nick office hours?
A2: weāve also added a 9-11am lair time
Q: where do we find the post lecture meeting
A1: zoom link is in the canvas event for lecture, same place where the webinar link is
Q: Can you also use āunsigned intā before i to fix the warning issue?
A1: Yes!
Q: why do we have āint i=0;i<(int)plainText.length();i++ā (why is int in bracket?)
A1: It is a ātypecastā converting the expression to the named type. In this case, it is taking the size_t return from s.length() and converting to plain int
Q: Why is there an (int) in the code being explained
A1: It is a ātypecastā converting the expression to the named type. In this case, it is taking the size_t return from s.length() and converting to plain int
Q: In the for loop
A1: live answered
Q: why does the ceaser cipher for loop has int i=0 instead of size_t i=0?
A1: You could do either, earlier Chris showed code where it was written using size_t, this loop is using int
Q: how is the number converted to a char?
A1: The ascii table assigns each character a number
Q: Can you explain whatās going on in that else statement and after?
A1: live answered
Q: can we use functions from Stanford libraries in our assignments?
A1: Yes
Q: so for strings we would opt for toUpperCase?
A1: Yes
Q: so i can only print out char (as an actual letter), and char is a special data type that i can add numbers to and that adds to the equivalent ascii value?
A1: Yes, exactly
Q: could the else statement been moved down one line and put aligned with the if statement??
A1: This particular else goes with the outer if, not the inner.
Q: would the isdigit function work on types like double,long, float?
A1: isdigit works on a single character not a string of characters
Q: if operators compare ascii values how would you compare lengths of strings
A1: if (s.length() < t.length()) ā¦
Q: does it compare every letter or just the first one?
A1: it compares as many characters as needed to establish ordering, this will generally be up through the first character that differs (or if it runs off the end of one string)
Q: is it better to modify strings or create new strings?
A1: Both are reasonable, depends on which better suits the situation
Q: When comparing strings would you first need to check if they are the same length?
A1: No, if string a is a complete prefix of string b, ithen a is less than b
Q: Does he mean s2 instead of s3?
A1: I believe there are three strings declared in his example, s1, s2, and s3
Q: Can we refer to indexes ithat donāt exist in the case of other data structures such as dictionaries in C++ just as we can do in python?
A1: It depends on the data structure what the effect will be. For example, attempting to access the 9th member of. Vector that has only 4 elements will raise an error. Trying to access a non-existent key in a Map will generate create a new empty entry for it
Q: how do hackers exploit a buffer overflow?
A1: thereās a whole class on this, cs155! Itās a little too complicated to explain concisely unfortunately
A2: Come to my office hours and Iāll tell you in person! (but only if you promise not to do evil)
Q: does that compare length or ASCII value
A1: Can you identify what you are referring to by āthatā?
Q: Can you use s.find() to look for a ch instead of string?
A1: yes, the function is overloaded, two versions one that looks for single character, another that looks for substring
Q: could we get an example of s.compare?
A1: string s = āappleā, t = ābananaā; int result = s.compare(t)
result will be negative, zero, or positive depending on ordering of s to t (less, equal, greater)
Q: yes sorry the compare function Chris just went over
A1: string s = āappleā, t = ābananaā; int result = s.compare(t)
result will be negative, zero, or positive depending on ordering of s to t (less, equal, greater)
Q: that returns -1, 0 or 1
A1: live answered
Q: Where is the other video on strings?
A1: posted in the same place the lecture videos are posted! Canvas > course Videos
Q: where is the video on strings - I only saw a video on references?
A1: posted in the same place the lecture videos are posted! Canvas > course Videos
Q: Does C++ work like Java where strings with the same characters can still be false when evaluating equality?
A1: No, that is a quirk particular to Java (about whether equivalent strings evalute as true ==), C++ string equality works as you would expect
Q: when/why do we use references?
A1: To pass a variable with a function and allow the function to make a persistent modification to it
Q: What is the difference between taking in a string by value versus by reference?
A1: If the function changes a string that was passed by reference, the change will be persistentand seen after function completes If the parameter passed by value, any change only exists inside the function
Q: thereās 3 tās in b ??
A1: live answered
Q: why isnāt it TreeFOOt?
A1: live answered
Q: isnt b just TreeFOOt?
A1: live answered
Q: Isnt there an extra e in the new b value
A1: live answered
Q: does the insert put what you are inserting at the index (thus replacing the previous char at that index), or after that index?
A1: first character of the inserted string will be placed at index
Q: Where did the extra 'e' come from?
A1: live answered
Q: Why is there an e after FOO?
A1: live answered
Q: Shouldn't that be trefooet?
A1: live answered
Q: was there an extra e added by mistake?
A1: live answered
Q: Is there an extra āeā?
A1: live answered
Q: how did he get an extra e at the end of Tree?
A1: live answered
Q: I think it should be TreFOOet, not TreeFOOet
A1: live answered
Q: why are there 3 āeās?
A1: live answered
Q: There should only be one "e" before foot, right?
A1: live answered
Q: Why is the final string TreeFooet and not TreeFoot? Where does the third e come from?
A1: live answered
Q: how can you refer to string a if itās defined in a seperate function?
A1: You generally are not able to access variables outside of the current function.
Q: why is it et at the end?
A1: live answered
Q: Wait is it TreFOOet or TreeFOOet? Does the insert put the FOO in before or after the referenced character?
A1: live answered
Q: cool yeah thx for catching
A1:
Q: Thank you!
A1:
Q: So insert(3,ā¦) will insert the string before the current character at index 3?
A1: Correct.
Q: Does the world at large also have access to the Stanford library?
A1: We publish Stanford library, but it not in common use outside of academia
Q: In the .insert function, is the FOO inserted before or after the third indexed character?
A1: the first characer of the inserted string is place at the index you insert at (in this case index 3)
Q: how do ascii values work for strings? is the ascii value of a string the sum of the ascii values of its characters? or the ascii values put next to one another?
A1: Ascii is defined for single characters, there is not an equivalent for an entire string.
Q: sorry if this has been answered / is obvious, but if I have a helper function that returns a string can I then access that string in say, a main function or something?
A1: Save the return value of function call by assignig to a string variable:
int main() { string result = myFunction(args);
Q: where can we find the reference video?
A1: The Course videos page of Canvas
Q: Are the two videos (extra preview and mini reference) both mandatory and cover material relevant to the assignment due this week?
A1: live answered
Q: are hashmaps collections
A1: Yes
Q: this doesnāt directly relate to the current lecture but is it considered improper form/syntax to subtract from the index within a for loop? as in for(int i = 0, i<x. i++) and then having an i- - for certain cases within the loop
A1: It is unusual and makes the code a little harder to follow, but works just fine
Q: should we watch the additional videos? is it supplemental information or like lecture?
A1: live answered
Q: are vectors arrays?
A1: Vectors are not arrays - there are some key differences between the two that weāll talk about later in the class!
Q: what's the meaning of collection? what is it used for?
A1: Collection means an aggregate type, does not store a single value but many values gathered into one
Q: Do we have the Stanford library by default in Qt?
A1: All of the assignment projects will have the Stanford library avaialble by default
Q: is vec part of the syntax or is that the name of your vector?
A1: It is the variable name
Q: Can we use vectors in both parts or assignment one?
A1: Yes, you will need Vectors in both perfect and soundex
Q: so when we initialize a vector vec, we dont need to set it equal to = {}? we can just type vec; ?
A1: Yes. The default initialization for a Vector creates an empty vector
Q: in terms of memory or efficiency, are there any advantages of a vector of characters versus a string? (of course, we wouldnāt have access to all of the built in functions for string)
A1: string is generally more efficient and richer operations/functionality
Q: How do you actually import a Stanford library outside of just assignments for class?
A1: Copy one of our provided project as a starting point
Q: Where do we initialize a vector? is it at the top of our program like in Java?
A1: You can initialize the vector where you declare it
Q: Is there ever a reason to create a vector of chars instead of a string?
A1: Not really
Q: so is collection just like array that in another language (don't remember which tho)?
A1: Yes, arrays, lists, vectors are all the same abstraction
Q: why do you not loop through magic.size() - 1?
A1: The last iteration of the loop is size -1, correct
Q: what does the function size actually do for a vector?
A1: returns the number of elements in the vector
Q: How do str.substr() work when you want to split a string from the other end of a string?
A1: substr somewhat like python āsliceā, can to pick off prefix, suffix, etc. by how you choose start and stop arguments
Q: do we need to do capital V Vector
A1: The typename is capital V Vector in all places where you use the type
Q: Can the data type in the vector only be primitive data types?
A1: You can store any type in Vector, primitives, classes, structs, custom user types, and so on
Q: Wait I may have missed this, but do you need to watch the extra videos to do assignment 1?
A1: live answered
Q: can a vector have characters instead of numbers, and if so then is it still vector
A1: yes it can! if you wanted to store characters that would be Vector
Q: like what if I want it to be a list of strings?
A1: Vectors can store alomost any type! So you would declare a Vector
Q: Is there a way to print the vector whole? What would this output look like?
A1: cout Ā«Ā vec Ā«Ā endl;
Q: is āvecā in the examples like vec[i] a stand in for the actual name of the vector?
A1: The variable was named vec so that is how we later refer to it
Q: for vec[i], where shall we specify the string of concern?
Q: is vec.add() equivalent to append or extend in Python?
A1: Yes
Q: can we do arithmetic with vectors? Like sum, multiply, divide with another vector of same size?
A1: No
Q: Does vec.sort() change the original vector?
A1: yes! it modifies the vector in place and does not return anything
Q: Is there a function or possibly a variable for putting a placeholder inside a vector (eg in matlab, NaN can be used as placeholders inside a vector) ?
A1: No, there is not the notion of a placeholder that fills empty slots
Q: Can you call vec.size() on an empty vector?
A1: Yes, it will return 0
Q: where do we find these other vidoes?
A1: Course videos tab on Canvas
Q: Where can we find the lecture slides again?
A1: course webiste under the lectures tab
Q: Will there always be videos or is that just because weāre running behind this week?
A1: live answered
Q: since a string is an array of characters, can i add a character to a string?
A1: Yes