10.9. Activecode Exercises

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

Q1

Question

Fix the code below so that it creates a vector with 5 elements all initialized to 1, and changes the third element of that vector to a 2.

Example vectors_a1q

int make_vector () {
    vector<int> nums (5) = 1;
    return nums;
}

Answer

Below is one way to fix the program.

• Always include the <vector> header when using vectors.

• To initialize all vector elements to a certain value, you must include that as a second argument after the size.

• Finally, vectors are zero-indexed. The third element is at index '2'.

Example vectors_a1a

#include <iostream>
#include <vector>

std::vector<int> make_vector () {
    std::vector<int> nums (5,1);
    nums[3] = 2;
    return nums;
}

Q2

Fix the function below so that it returns how many even numbers are in nums.

Example vectors_a2

#include <vector>

int count_even (const vector<int>& nums) {
    for (int i = 0; i < nums.size(); i++) {
        if (i % 2 == 0) {
            count = count + 1;
        }
    }
    return count;
}

Q3

Question

Fix the function below so that it creates a vector of all of the words in words that end with the passed character.

Example vectors_a3q

#include <iostream>
#include <string>
#include <vector>

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

OK Answer

Below is one way to fix the function. However it is complicated and error prone.

We must initialize count to zero and declare 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.

Example vectors_a3a

#include <string>
#include <vector>

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

Better Answer

Below is a simpler way to fix the function. We still must initialize count to zero. However, it is better to avoid array indexing completely. In general, it is always better to avoid array indexing if it is not needed.

Example vectors_a3a2

#include <string>
#include <vector>

int ends_with (const std::vector<std::string>& words, char c) {
    int count = 0;
    for (const std::string& word: words) {
        if (word.back() == c) {
            ++count;
        }
    }
    return count;
}

Q4

Someone could have COVID 19 if their temperature is above 99.9 degrees Fahrenheit. Finish the code below so that it counts how many students in the class may have been exposed.

Example vectors_a4

int count_covid (const std::vector<double>& temps) {
}

Q5

Question

Finish the code below so that it creates removes elements from the end of the vector until it ends with "stop".

Example vectors_a5q

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

    while(          ) {

    }

}

Answer

Below is one way to finish the program. We just use the pop_back function until the last element of the vector is "stop".

Example vectors_a5a

#include <string>
#include <vector>

int main () {
    std::vector<std::string> words = {"roses", "are", "red", "violets", "stop", "are", "blue"};
    while (words.back() != "stop"){
        words.pop_back();
    }
}

Q6

Write the function ends_even that takes a vector and removes elements from the end of the vector until it ends with an even number.

No tests are provided for this question.

Example vectors_a6

#include <vector>

Q7

Question

Write a function called all_have_char that returns a bool if every string in the provided vector contains the test character. It should return true only if all strings contain the character.

No tests are provided for this question.

Example vectors_a7q

#include <vector>

Answer

Below is one way to finish the program. We loop through the vector, and search each string. If the string has the character, then it is added to count. We then check whether count is equal to the number of elements in vec and return a boolean.

Example vectors_a7a

#include <cstddef>
#include <vector>


bool all_have_char (const vector<string>& data, char c) {
    size_t count = 0;
    for (const string& tokens: data) {
       for (const char tok: tokens) {
            if (tok == c) {
                count++;
            }
        }
    }
    return count == data.size();
}

Q8

Question

Write a function mean which returns the average of a vector of numbers.

No tests are provided for this question.

Example vectors_a9q

// student code goes here.

Answer

Below is one way to finish the program. First we take the sum, then divide the sum by the number of elements in nums.

In the solution below, an empty vector results in a divide by zero error.

How could we guard against that?

Example vectors_a8a

#include <vector>

double mean (const vector<double> nums) {
    double sum = 0;
    for (const auto& value: nums) {
        sum += value;
    }
    return sum/nums.size();
}

Q9

Write the function hundy_bundy that returns a count of all numbers in a vector that are divisible by 100.

No tests are provided for this question.

Example vectors_a9

#include <vector>

Q10

Question

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! )

Example vectors_a11q

#include <vector>

Answer

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 we are not decrementing 0.

Example vectors_a11a

#include <vector>

bool is_even (int value) {
  return value % 2 == 0;
}

void make_odd ( std::vector<int>& nums) {
  for (int& value: nums) {
    if (is_even(value) && value > 0) {
      --value;
    }
  }
}

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 start with {1,2,3,4,5,6} we would print 3 2 1 4 5 6.

Example vectors_a11

// student code goes here.