布尔语句 C++

Question

I am creating a program that compares all the values of an array and reads which values add up to the desired sum. If there aren't any values that add up to the desired sum, it should output a statement saying No matching sum was found, this is the part I'm having issues with. I should specify what it outputs. At the moment every time there isn't a match it prints "no matching sum was found" so It creates quite the messy output

using namespace std;

const int SIZE = 15;
int intArray[SIZE];

void readData(istream& inFile) {
    for (int i = 0; i < SIZE; i++){
        inFile >> intArray[i];
        cout << intArray[i] << " ";
    }
}

int main() {
    int sum;
    ifstream inFile;
    string inFileName = "inFile.txt";
    inFile.open(inFileName.c_str());

    readData(inFile);
    
    cout << "Please enter the number for which a matching sum is needed: ";
    cin >> sum;
    bool error = true;
    for(int i = 0; i <= SIZE; i++){
        for(int j = 1; j <= SIZE; j++){
            if((intArray[i] + intArray[j]) == sum){
                cout << "Item " << i << " (With value " << intArray[i] << ") " << "and item " << j << " (With value " << intArray[j] << ") add up to " << sum << endl;
                error = false;
            }
        }
    }
    if(error == true){
        cout << "No matching sum was found" << endl;
    }
    return 0;
}

Answer 1

The quick fix to your question is below.

for(int i = 0; i < SIZE; i++){
    for(int j = i+1; j < SIZE; j++){
        if((intArray[i] + intArray[j]) == sum){
            cout << "Item " << i << " (With value " << intArray[i] << ") " << "and item " << j << " (With value " << intArray[j] << ") add up to " << sum << endl;
            error = false;
        }
    }
}

Please be noted that:

1. You are reading an array of "SIZE" but looping across for SIZE+1 times, starting from 0 to SIZE
2. You are starting j loop from 1, which is an inefficient way. You can start the loop with i+1, as there is no possibility by which matching pairs would escape the notice of this loop.

Let me know if this fixes your issue. Happy Coding!

Answer 2

You have a minor problem in your function, where you iterate over the bounds of your array. In C++ arrays start at index 0. And therefore, the last element index is the array-size minus 1.

Example: Array with five elements

int array[5] = {10,20,30,40,50};

• array at index 0 (array[0]) = 10
• array at index 1 (array[1]) = 20
• array at index 2 (array[2]) = 30
• array at index 3 (array[3]) = 40
• array at index 4 (array[4]) = 50

And, if you want to iterate over the array in a loop, then, with the size 5, you need to write:

for (int i = 0; i < 5; ++i)

So, for this case, always < , not <= .

If you run your program you will notice that you get somehow double results, so, you may get index 3 and 5, and index 5 and 3. You may even get a double, same index, if the sum given by the user is double than a value in an array. Example: if you have the value 4 in you array, and are looking for the sum 8, your program will give you the value 4 with the same index.

So, you need to understand your requirements for the results

• If you want to get ordered pairs, then: For the sum 10, the pair 3,7 and 7,3 are different. In that case you need to start both loop indices from 0
• If you want to avoid one element being double, like for sum 8 and one element 4, then getting pair 4,4., you need to add an additional if -statement, like if (i != j) .
• If not-ordered pairs are desired, meaning, we do not want to see 3,7 and 7,3 then you need to start your second loop at index i+1.

Then, next, some optimization. You may notice that the term intArray[i] is invariant. It will never change. So, you can take it out of the inner loop and shift it to the outer loop. And additionally, you perform always the same addition. That is basically not necessary. You can rewrite:

intArray[i] + intArray[j] == sum      to
intArray[j]) == sum - intArray[i]

And if we replace the term sum- intArray[i] with delta , then we can optimize our loops to:

    bool error = true;
    for (int i = 0; i < SIZE; i++) {
        const int delta = sum - intArray[i];

        for (int j = i+1; j < SIZE; j++) {

            if (intArray[j] == delta) {
                cout << "Item " << i << " (With value " << intArray[i] << ") " << "and item " << j << " (With value " << intArray[j] << ") add up to " << sum << endl;
                error = false;
            }
        }
    }

We have now only one subtraction (instead of multiple additions), access the intArray[i] only once and make only comparisons in the inner loop.

Then we can generally improve the code and use more modern C++ elements. We need to add error checking for user input and define variables, when we need them and not before.

#include <iostream>
#include <fstream>
#include <vector>

int main() {

    // First open source file and check, if it could be opened
    // If we cannot open it then no need to do something else
    if (std::ifstream inFileStream{"r:\\inFile.txt"}; inFileStream) {

        // Instructions for the user
        std::cout << "\nPlease enter the number for which a matching sum is needed: ";

        // Get the required sum and check for a correct input
        if (int requiredSum{}; std::cin >> requiredSum) {

            // Now, the indicator that shows, if we could find a pair or not
            bool matchingPairFound{};
            // Container for data
            std::vector<int> data{};

            // Read next value from source file until end of file and store it
            for (int i{}, value{}; inFileStream >> value; i++) {

                data.push_back(value);

                // Delta value to find
                const int delta = requiredSum - value;

                for (int j = i - 1; j >= 0; --j) {
                    if (delta == data[j]) {

                        // We found a match
                        matchingPairFound = true;
                        // Show somedebug output
                        std::cout << "Item " << i << " (With value " << data[i] << ") " << "and item " << j << " (With value " << data[j] << ") add up to " << requiredSum << '\n';
                    }
                }
            }
            std::cout << "\nMatching sum was " << (matchingPairFound ? "found" : "not found") << '\n';
        }
        else std::cerr << "\n\n***Error: Could not read sum value\n\n";
    }
    else std::cerr << "\n\n***Error: Could not open input file\n\n";

    return 0;
}

That is not the end. If the requirement is that we just want to know if there is a pair adding up to a sum, we do not need to read all values. We will read value by value and then stop as soon as we have a result. The loops will then run backward

#include <iostream>
#include <fstream>
#include <vector>

int main() {

    // First open source file and check, if it could be opened
    // If we cannot open it then no need to do something else
    if (std::ifstream inFileStream{"r:\\inFile.txt"}; inFileStream) {

        // Instructions for the user
        std::cout << "\nPlease enter the number for which a matching sum is needed: ";

        // Get the required sum and check for a correct input
        if (int requiredSum{}; std::cin >> requiredSum) {

            // Now, the indicator that shows, if we could find a pair or not
            bool matchingPairFound{};
            // Container for data
            std::vector<int> data{};

            // Read next value from source file until end of file and store it
            for (int i{}, value{}; (inFileStream >> value) and not matchingPairFound; i++) {

                data.push_back(value);

                // Delta value to find
                const int delta = requiredSum - value;

                for (int j = i - 1; (j >= 0) and not matchingPairFound; --j) {
                    if (delta == data[j]) {

                        // We found a match
                        matchingPairFound = true;
                        // Show somedebug output
                        std::cout << "Item " << i << " (With value " << data[i] << ") " << "and item " << j << " (With value " << data[j] << ") add up to " << requiredSum << '\n';
                    }
                }
            }
            std::cout << "\nMatching sum was " << (matchingPairFound ? "found" : "not found") << '\n';
        }
        else std::cerr << "\n\n***Error: Could not read sum value\n\n";
    }
    else std::cerr << "\n\n***Error: Could not open input file\n\n";

    return 0;
}

And, if we want not to see content of the pair and not the indices, the program will be even more compact. We assumed unordered pairs as the result. And we do not care for duplicates in source input file. They will not add any value. There is a good container available for that purposes. It is the std::unordered_set which will only store unique values and use a fast hash algorithm to access the data.

The below code

• Reads any number of data from the input file
• Reads only as long as no decision could be made
• Stores only distinct values until a decision can be made
• Is with that size and speed optimized
• And is compact. Only 10 lines of code in function main

#include <iostream>
#include <fstream>
#include <vector>
#include <unordered_set>

int main() {

    // First open source file and check, if it could be opened
    // If we cannot open it then no need to do something else
    if (std::ifstream inFileStream{"r:\\inFile.txt"}; inFileStream) {

        // Instructions for the user
        std::cout << "\nPlease enter the number for which a matching sum is needed: ";

        // Get the required sum and check for a correct input
        if (int requiredSum{}; std::cin >> requiredSum) {

            // Now, the indicator that shows, if we could find a pair or not
            bool matchingPairFound{};

            // Container for data
            std::unordered_set<int> alreadySeenData{};

            // Read values from input file until End-Of-File or until a matching pair was found
            for (int value{}; (inFileStream >> value) and not matchingPairFound; alreadySeenData.insert(value)) {

                // Check, if matching pair was found
                matchingPairFound = (alreadySeenData.count(requiredSum - value));
            }
            // Show result
            std::cout << "\nMatching sum was " << (matchingPairFound ? "found" : "not found") << '\n';
        }
        else std::cerr << "\n\n***Error: Could not read sum value\n\n";
    }
    else std::cerr << "\n\n***Error: Could not open input file\n\n";
}