Section1: ADTs

Section materials curated by our Head TA Nick Bowman, drawing upon materials from previous quarters.

This week’s section exercises explore ADTs: Grids, Vectors, Stacks, Queues, Maps and other compound containers. Additionally, we included some exercises covering material from Week 1 lectures (functions, parameters, return, decomposition, strings). Since section doesn't meet in Week 1, these problems are provided as extra practice on those topics.

Each week, we will also be releasing a Qt Creator project containing starter code and testing infrastructure for that week's section problems. When a problem name is followed by the name of a .cpp file, that means you can practice writing the code for that problem in the named file of the Qt Creator project. Here is the zip of the section starter code:

📦 Starter code

1) Returning and Printing

Topics: Function call and return, return types

Below is a series of four printLyrics_v# functions, each of which has a blank where the return type should be. For each function, determine

  • what the return type of the function should be,
  • what value, if any, is returned, and
  • what output, if any, will be produced if that function is called.

Is it appropriate for each of these functions to be named printLyrics? Why or why not?

_____ printLyrics_v1() {
    cout << "Havana ooh na na" << endl;
}
_____ printLyrics_v2() {
    return "Havana ooh na na";
}
_____ printLyrics_v3() {
    return "H";
}
_____ printLyrics_v4() {
    return 'H';
}
Solution 
void printLyrics_v1() {
    cout << "Havana ooh na na" << endl;
}

string printLyrics_v2() {
    return "Havana ooh na na";
}

string printLyrics_v3() {
    return "H";
}

char printLyrics_v4() {
    return 'H';
}

Of these four functions, only printLyrics_v1 will print anything. Specifically, it prints out the string "Havana ooh na na.". The name “printLyrics” is inappropriate for the other functions, as those functions don’t actually print anything. 😃

The function printLyrics_v1 doesn’t return anything – it just sends information to the console. As a result, its return type should be void. The functions printLyrics_v2 and printLyrics_v3 each return strings, since C++ treats anything in double-quotes as a string. Finally, printLyrics_v4 returns a char, since C++ treats anything in single-quotes as a character.

2) References Available Upon Request

Topic: Reference parameters, range-based for loops

Reference parameters are an important part of C++ programming, but can take some getting used to if you’re not familiar with them. Trace through the following code. What does it print?

void printVector(const Vector<int>& values) {
    for (int elem: values) {
        cout << elem << " ";
    }
    cout << endl;
}

void maui(Vector<int> values) {
    for (int i = 0; i < values.size(); i++) {
        values[i] = 1258 * values[i] * (values[2] - values[0]);
    }
}

void moana(Vector<int>& values) {
    for (int elem: values) {
        elem *= 137;
    }
}

void heihei(Vector<int>& values) {
    for (int& elem: values) {
        elem++;
    }
}

Vector<int> teFiti(const Vector<int>& values) {
    Vector<int> result;
    for (int elem: values) {
        result += (elem * 137);
    }
    return result;
}

int main() {
    Vector<int> values = { 1, 3, 7 };
    maui(values);
    printVector(values);
    moana(values);
    printVector(values);
    heihei(values);
    printVector(values);
    teFiti(values);
    printVector(values);
    return 0;
}
Solution

Here’s the output from the program:

1 3 7
1 3 7
2 4 8
2 4 8

Here’s a breakdown of where this comes from:

  • The maui function takes its argument by value, so it’s making changes to a copy of the original vector, not the vector itself. That means that the values are unchanged back in main.
  • The moana function uses a range-based for loop to access the elements of the vector. This makes a copy of each element of the vector, so the changes made in the loop only change the temporary copy and not the elements of the vector. That makes that the values are unchanged back in main.
  • heihei, on the other hand, uses int& as its type for the range-based for loop, so in a sense it’s really iterating over the elements of the underlying vector. Therefore, its changes stick.
  • The teFiti function creates and returns a new vector with a bunch of updated values, but the return value isn’t captured back in main.

3) SumNumbers (sum.cpp)

Topics: Vectors, strings, file reading

The function sumNumbers reads a text file and sums the numbers found within the text. Here are some library functions that will be useful for this task:

In particular you will be asked to write the following function

int sumNumbers(string filename)

When given the following file, named numbers.txt, as input, your function should return 42.

42 is the Answer to the Ultimate Question of Life, the Universe, and Everything
This is a negative number: -9
Welcome to CS106B!
I want to own 9 cats.
Solution 
bool isNumber(string s)
{
    // strip negative sign off negative numbers
    if (s.length() > 0 && s[0] == '-'){
        s = s.substr(1);
    }
    for (char ch : s)
        if (!isdigit(ch)) return false;
    return s.length() > 0;
}

int sumNumbers(string filepath)
{
    ifstream in;
    Vector<string> lines;
    int sum = 0;

    if (!openFile(in, filepath))
        return 0;

    readEntireFile(in, lines);
    for (string line : lines) {
        Vector<string> tokens = stringSplit(line, " ");
        for (string t : tokens) {
            if (isNumber(t)) {
                sum += stringToInteger(t);
            }
        }
    }
    return sum;
}

4) Debugging Deduplicating (deduplicate.cpp)

Topics: Vector, strings, debugging

Consider the following incorrect C++ function, which accepts as input a Vector<string> and tries to modify it by removing adjacent duplicate elements:

⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️

void deduplicate(Vector<string> vec) {
    for (int i = 0; i < vec.size(); i++) {
        if (vec[i] == vec[i + 1]) { 
            vec.remove(i);
        }
    }
}

⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️⚠️

The intent behind this function is that we could do something like this:

Vector<string> hiddenFigures = {
    "Katherine Johnson",
    "Katherine Johnson",
    "Katherine Johnson",
    "Mary Jackson",
    "Dorothy Vaughan",
    "Dorothy Vaughan"
};

deduplicate(hiddenFigures);
// hiddenFigures = ["Katherine Johnson", "Mary Jackson", "Dorothy Vaughan”]

The problem is that the above implementation of deduplicate does not work correctly. In particular, it contains three bugs. Find those bugs, explain what the problems are, then fix those errors.

Solution

There are three errors here:

  1. Calling .remove() on the Vector while iterating over it doesn’t work particularly nicely. Specifically, if you remove the element at index i and then increment i in the for loop, you’ll skip over the element that shifted into the position you were previously in.
  2. There’s an off-by-one error here: when i = vec.size() - 1, the indexing vec[i + 1] reads off the end of the Vector.
  3. The Vector is passed in by value, not by reference, so none of the changes made to it will persist to the caller.

Here’s a corrected version of the code:

void deduplicate(Vector<string>& vec) {
    for (int i = 0; i < vec.size() - 1; ) {
        if (vec[i] == vec[i + 1]) {
            vec.remove(i);
        } else {
            i++;
        }
    }
}

5) Collection Mystery

Topics: Stacks and queues

void collectionMystery(Stack<int>& s) 
{ 
    Queue<int> q;
    Stack<int> s2;

    while (!s.isEmpty()) {
       if (s.peek() % 2 == 0) {
            q.enqueue(s.pop()); 
        } else {
            s2.push(s.pop());
        }
    }
    while (!q.isEmpty()) {
        s.push(q.dequeue()); 
    }
    while(!s2.isEmpty()) { 
        s.push(s2.pop());
    }
    cout<< s << endl;
}

Write the output produced by the above function when passed each of the following stacks. Note that stacks and queues are written in ​front to back order, with the oldest element on the left side of the queue/stack.

Stacks:

{1, 2, 3, 4, 5, 6}                ________________________________________
{42, 3, 12, 15, 9, 71, 88}        ________________________________________
{65, 30, 10, 20, 45, 55, 6, 1}    ________________________________________
Solution 
 {6, 4, 2, 1, 3, 5}
 {88, 12, 42, 3, 15, 9, 7}
 {6, 20, 10, 30, 65, 45, 55, 1}

6) Mirror (mirror.cpp)

Topic: Grids

Write a function ​mirror​ that accepts a reference to a ​Grid​ of integers as a parameter and flips the grid along its diagonal. You may assume the grid is square; in other words, that it has the same number of rows as columns. For example, the grid below that comes first would be altered to give it the new grid state shown afterwards:

Original state: 
{ { 6, 1, 9, 4},                
  {-2, 5, 8, 12},                  
  {14, 39, -6, 18},             
  {21, 55, 73, -3} }               

Mirrored state: 
 { {6, -2, 14, 21},
 {1, 5, 39, 55},
 {9, 8, -6, 73},
 {4, 12, 18, -3} }

Bonus: How would you solve this problem if the grid were not square?

Solution 
// solution
void mirror(Grid<int>& grid) {
    for (int r = 0;r < grid.numRows(); r++) {
        // start at r+1 rather than 0 to avoid double-swapping 
        for (int c = r + 1; c < grid.numCols(); c++) { 
            int temp = grid[r][c]; 
            grid[r][c] = grid[c][r]; 
            grid[c][r] = temp;
        } 
    }
}
// bonus 
void mirror(Grid<int>& grid) {
    Grid<int> result(grid.numCols(), grid.numRows());
    for (int r = 0; r < grid.numRows(); r++) {
        for (int c = 0; c < grid.numCols(); c++) {
            result[r][c] = grid[c][r];
        }
    }
    grid = result;
}

7) CrossSum (crosssum.cpp)

Topic: Grids

Write a function named crossSum that accepts three parameters - a constant reference to a Grid of integers, and two integers for a row and column - and returns the sum of all numbers in the row/column cross provided. For example, if a grid named g stores the following integers:

   col    0  1  2
row
 0     { {1, 2, 3},
 1       {4, 5, 6},
 2       {7, 8, 9} }

Then the call of crossSum(g, 1, 1) should return (4+5+6+2+8) or 25. You may assume that the row and column passed are within the bounds of the grid. Do not modify the grid that is passed in.

Solution 
int crossSum(const Grid<int>& grid, int row, int col) {
    int sum = 0;
    for (int c = 0; c < grid.numCols(); c++) {
        sum += grid[row][c];
    }
    for (int r = 0; r < grid.numRows(); r++) {
        sum += grid[r][col];
    }
    sum -= grid[row][col]; // subtract center because it was added twice
    return sum;
}

8) Stutter (stutter.cpp)

Topic: Queues

Write a function named ​stutter​ that accepts a reference to a Queue of integers as a parameter and replaces every element with two copies of itself. For example, if ​q​ stores ​{1, 2, 3}​, the call of ​stutter(q) should change it to store ​{1, 1, 2, 2, 3, 3}​.

Solution 
void stutter(Queue<int>& q) 
{
    int size = q.size();
    for (int i = 0; i < size; i++) {
        int n = q.dequeue(); 
        q.enqueue(n); 
        q.enqueue(n);
    }
}

9) Check Balance (balance.cpp)

Topic: Stacks

Write a function named checkBalance that accepts a string of source code and uses a Stack to check whether the braces/parentheses are balanced. Every ( or { must be closed by a } or ) in the opposite order. Return the index at which an imbalance occurs, or -1 if the string is balanced. If any ( or { are never closed, return the string's length.

Here are some example calls:

//   index    0123456789012345678901234567
checkBalance("if (a(4) > 9) { foo(a(2)); }") 
// returns -1 (balanced)

//   index    01234567890123456789012345678901
checkBalance("for (i=0;i<a;(3};i++) { foo{); )")
// returns 15 because } is out of order

//   index    0123456789012345678901234
checkBalance("while (true) foo(); }{ ()")
// returns 20 because } doesn't match any {

//   index    01234567
checkBalance("if (x) {")
// returns 8 because { is never closed
Solution 
int checkBalance(string code) {
    Stack<char> parens;
    for (int i = 0; i < (int) code.length(); i++) {
        char c = code[i];
        if (c == '(' || c == '{') {
        parens.push(c);
        } else if (c == ')' || c == '}') {
            if (parens.isEmpty()) {
                return i;
            }
            char top = parens.pop();
            if ((top == '(' && c != ')') || (top == '{' && c != '}')) {
                return i;
            }
        }
    }

    if (parens.isEmpty()) {
        return -1; // balanced
    } else {
        return code.length();
    }
}

10) Friend List (friendlist.cpp) [Covered Friday]

Topic: Maps

Write a function named friendList that takes in a file name and reads friend relationships from a file and writes them to a Map. friendList should return the populated Map. Friendships are bi-directional, so if Abby is friends with Barney, Barney is friends with Abby. The file contains one friend relationship per line, with names separated by a single space. You do not have to worry about malformed entries.

If an input file named buddies.txt looked like this:

Barney Abby
Abby Clyde

Then the call of friendList("buddies.txt") should return a resulting map that looks like this:

{"Abby":{"Barney", "Clyde"}, "Barney":{"Abby"}, "Clyde":{"Abby"}}

Here is the function prototype you should implement:

Map<string, Vector<string> > friendList(String filename)

Solution 
Map<string, Vector<string> > friendList(string filename) {
    ifstream in;
    Vector<string> lines;

    if (openFile(in, filepath)) {
        readEntireFile(in, lines);
    }

    Map<string, Vector<string> > friends;
    for (string line: lines) {
        Vector<string> people = stringSplit(line, " ");
        string s1 = people[0];
        string s2 = people[1];
        friends[s1] += s2;
        friends[s2] += s1;
    }
    return friends;
}

11) Twice (twice.cpp) [Covered Friday]

Topic: Sets

Write a function named twice that takes a vector of integers and returns a set containing all the numbers in the vector that appear exactly twice.

Example: passing {1, 3, 1, 4, 3, 7, -2, 0, 7, -2, -2, 1} returns {3, 7}.

Bonus: do the same thing, but you are not allowed to declare any kind of data structure other than sets.

Solution 
// solution
Set<int> twice(Vector<int>& v) {
    Map<int, int> counts;
    for (int i : v) {
        counts[i]++;
    }
    Set<int> twice;
    for (int i : counts) {
        if (counts[i] == 2) {
            twice += i;
        }
    }
    return twice;
}

// bonus
Set<int> twice(Vector<int>& v) {
    Set<int> once;
    Set<int> twice;
    Set<int> more;
    for (int i : v) {
        if (once.contains(i)) {
            once.remove(i);
            twice.add(i);
        } else if (twice.contains(i)) {
            twice.remove(i);
            more.add(i);
        } else if (!more.contains(i)) {
            once.add(i);
        }
    }
    return twice;
}