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How To Think Like a Computer Scientist C++ Edition The Pretext Interactive Version

Barbara Ericson, Allen B. Downey, Jason L. Wright (Editor)

Exercises 10.18 Activecode Exercises

Answer the following Activecode questions to assess what you have learned in this chapter.
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1.

Fix the code below so that it creates a vector with 5 elements initialized to 1, and changes the third element of that vector to a 2.
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Solution.
Below is one way to fix the program. You must always include the <vector> header when dealing with vectors. Furthermore, to initialize a vector’s elements to a certain value, you must include that value as a second argument to the size. Finally, vectors are zero-indexed.
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#include <iostream>
#include <vector>
using namespace std;

int main () {
    vector<int> nums (5, 1);
    nums[2] = 2;
}
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2.

Fix the function below so that it returns how many even numbers are in nums. Check the hint below for help with the construction of the code.
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Hint.

Activity 10.18.1.

Fix the function below so that it returns how many even numbers are in nums. Use the lines to construct the code, then go back to complete the Activecode.
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3.

Fix the function below so that it creates a vector of all of the words in words that end with the passed character.
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Solution.
Below is one way to fix the function. You must initialize count to zero. You also must initialize last as an integer. To access a string inside of vec, we use vec[i]. To get the last character, we must index the string to the last index, which is one less than the length of the string.
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#include <iostream>
#include <string>
#include <vector>
using namespace std;

int endsWith(const vector<string>& vec, char c) {
    int count = 0;
    for (size_t i = 0; i < vec.size(); i++) {
        size_t  last = vec[i].size() - 1;
        if (vec[i][last] == c) {
            count++;
        }
    }
    return count;
}
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4.

Someone could have COVID-19 if their temperature is above 99.9 degrees Fahrenheit. Finish the code below so that it counts and prints how many students in the class may have been exposed. Check the hint below for help with the construction of the code.
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Hint.

Activity 10.18.2.

Someone could have COVID-19 if their temperature is above 99.9 degrees Fahrenheit. Finish the code below so that it counts and prints how many students in the class may have been exposed. Use the lines to construct the code, then go back to complete the Activecode.
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5.

Finish the code below so that it creates removes elements from the end of the vector until it ends with "stop".
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Solution.
Below is one way to finish the program. We just use the pop_back function until the last element of the vector is "stop".
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#include <iostream>
#include <vector>

using namespace std;

int main () {
    vector<string> words = {"roses", "are", "red", "violets", "stop", "are", "blue"};

    while (words[words.size() - 1] != "stop"){
        words.pop_back();
    }
}
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6.

Write the function endsEven that takes a vector and removes elements from the end of the vector until it ends with an even number. Check the hint below for help with the construction of the code.
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Hint.

Activity 10.18.3.

Write the function endsEven that takes a vector and removes elements from the end of the vector until it ends with an even number. Use the lines to construct the code, then go back to complete the Activecode.
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7.

Write a function called has_char that returns a boolean of whether every string in the vector vec contains the character let. It should return true if all strings contain the let.
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Solution.
Below is one way to finish the program. We loop through the vector, and we loop through each string inside it. If the string has the character, it is added to count. We then check whether count is equal to the number of elements in vec and return a boolean.
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#include <iostream>
#include <vector>
using namespace std;


int has_char(const vector<string>& vec, char let) {
    size_t count = 0;
    for (size_t i = 0; i < vec.size(); i++) {
        for (size_t c = 0; c < vec[i].size(); c++) {
            if (vec[i][c] == let) {
                count++;
            }
        }
    }
    if (count == vec.size()) {
        return true;
    }
    return false;
}
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8.

Write the function randomNums that takes two integers: num which is the number of random numbers you wish to generate, and max, which is the maximum value of random number you wish to generate. Your function should return a vector of num integers that are between 1 and max, inclusive. Check the hint below for help with the construction of the code.
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Hint.

Activity 10.18.4.

Write the function randomNums that takes two integers: num which is the number of random numbers you wish to generate, and max, which is the maximum value of random number you wish to generate. Your function should return a vector of num integers that are between 1 and max, inclusive. Use the lines to construct the code, then go back to complete the Activecode.
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9.

Write the function mean which returns the average of a vector of numbers.
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Solution.
Below is one way to finish the program. First we take the sum, then divide the sum by the number of elements in nums.
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#include <iostream>
#include <vector>
using namespace std;

double mean (const vector<double> nums) {
    double sum = 0;
    for (size_t i = 0; i < nums.size(); ++i) {
        sum = sum + nums[i];
    }
    return sum/nums.size();
}
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10.

Write the function hundyBundy that returns a count of all numbers in the passed vector vec that are divisible by 100. Check the hint below for help with the construction of the code.
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Hint.

Activity 10.18.5.

Write the function hundyBundy that returns a count of all numbers in the passed vector vec that are divisible by 100. Use the lines to construct the code, then go back to complete the Activecode.
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11.

Write the function make_odd which subtracts 1 from every even number in a vector of integers. We don’t want any negative values so don’t subtract 1 from 0. ( remember to take in the vector by reference to make changes to the actual vector! )
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Solution.
Below is one way to finish the program. We us the modulus operator to check for even numbers and decrement them. we keep an extra check for 0 to make sure wew are not decrementing 0.
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#include <iostream>
#include <vector>
using namespace std;

void make_odd ( vector<int> &nums) {
    for (size_t i = 0; i < nums.size(); ++i) {

        if((nums[i] % 2 == 0) && (nums[i] != 0)){
            nums[i]--;
        }

    }
}
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12.

Write the function weird_print that prints the first half of a vector of integers in reverse order and then prints the second half in the order present in the vector. If we had vec = {1,2,3,4,5,6} we would print 3 2 1 4 5 6. You can assume the size of the vector will always be even. Check the hint below for help with the construction of the code.
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Hint.

Activity 10.18.6.

Write the function weird_print that prints the first half of a vector of integers in reverse order and then prints the second half in the order present in the vector. If we had vec = {1,2,3,4,5,6} we would print 3 2 1 4 5 6. You can assume the size of the vector will always be even. Use the lines to construct the code, then go back to complete the Activecode.
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