C ++循环-查找最小值和第二最小值

Question

I'm very very new to C++. Here I'm trying to write a program without any extra library. Using loops to find both the smallest value and the second smallest value from the user's inputs ( 0 is excluded and exits the program ).

Here is what I tried to do.

#include <iostream>
using namespace std;

int main()
{
    int value=0;
    int SmallestNumber=0;
    int SmallestNumber2=0;

    cout << "Enter number to find the smallest and second smallest(or 0 to exit): ";
    cin >> value;

    while (value != 0) {

        if (value< SmallestNumber && value != 0 )
        {
            SmallestNumber = value;
        }

        else if (value<SmallestNumber && SmallestNumber2 >SmallestNumber && value != 0)
        {
            SmallestNumber2 = value;
        }

        cout << "Enter number to find the smallest and second smallest(or 0 to quit): ";
        cin >> value;

    }

    cout << "Smallest number is: " << SmallestNumber << '\n' << endl;
    cout << "Second Smallest number is: " << SmallestNumber2 << '\n' << endl;

    return 0;
}

However, this program is not functioning properly. The smallest number finder works only if I input a negative value **, and the second smallest number value always outputs **0 .

Since I'm very new to C++, I tried many other solutions, but this is what I can really think of.

Can somebody please tell me what is wrong with the program, and how I can correct it?

A million thanks! Please help me :'(

---

Thanks for answering my question! I changed the initialization into this.

int value;
int SmallestNumber=0;
int SmallestNumber2=0;

but how do I initialize the smallest and the second smallest values..?

This is what I wanted my program to do

displaying the smallest and second smallest
    50
    1
    61
    93 
    -35
    38
    0

    -35 smallest
    1 second smallest

Answer 1

You start with a smallest value set to 0, so you will always get values only smaller than 0, that's why you have std::numeric_limits<int>::max() .

Then for your second smallest, you are never checking against the current second smallest value, you are just checking against the biggest, which you now is going to work. So change this:

if (value>SmallestNumber2 && value != 0)

You should probably check value != 0 outside the main if statements as well. And as @Caleb reminded me, what happens to the previous largest value if it gets replaced?

Answer 2

You have no position at which the former smallest value becomes the second smallest value, which can't work.

Consider this code:

int value;
int smallest = std::numeric_limits<int>::max()-1;
int second_smallest = std::numeric_limits<int>::max();

while(true)
{
    cin >> value;

    if(value == 0) break;
    if(value >= second_smallest) continue;
    if(value == smallest) continue; // assuming that a double value does not change anything

    if(value > smallest) // is between them
    {
        second_smallest = value;
        continue;
    }

    //now the case left is that the new value is the smallest
    second_smallest = smallest;
    smallest = value;
}

Basic idea: first of all, rule out things and from then on, assume that they do not hold. We begin with the break case (I prefer a while(true) in such cases to have manual control over breaking it inside). Then we rule out the case in which nothing happens. The two cases left are that we are between the old values and that we are below both, and we handle them accordingly.

In your code, your ifs get to bloated. Makes it hard to keep track of what is done and what is to be done.

One example of this is that you have several times && value != 0 in your code despite this always being true due to the condition of your while.

In general, you should really learn how to use a debugger, or at least how to use helpful messages for debugging. Your mistake of setting your variables to zero at the start would have been easy to detect.

Other minor things: You should decide for a style and stick to it. It is quite unusual to name variables with a major first letter. Camel case is fine though, smallestNumber would have been fine. Second, try to avoid using namespace std; . This can lead to collissions. Rather use single members of std, like using std::cout; . It is not that problematic in a source file (very problematic in a header) but I recommend to do it consistently to keep a good routine.

A thing left to do in the code would be to later catch if the variables are still at std::numeric_limits<int>::max() and that minus one, signalling that there was no user input, and printing a fitting message instead of those values.

Note that as you read in an integer, negative values are legal, which might not be what you want, given that you use zero to break. You might want to add a case

if(value < 0)
{
    cout << "Value was ignored due to being negative" << endl;
}

Answer 3

This is relatively simpel by using aa few different concepts, namely stdvector , stdstream and last stdsort

First off, the input part can be simplified to:

std::vector<int> numbers;

std::cout << "Enter multiple numbers, separated by spaces: ";
std::getline(std::cin, line);
std::istringstream stream(line);
while (stream >> number) {
    numbers.push_back(number);
}

Now, since you would like the 2 smallest numbers, my suggested method would be to simply sort the vector at this point:

std::sort(numbers.begin(), numbers.end());

Now the list of numbers are sorted in ascending order and it is matter of printing the 2 first values. I leave that as an exercise to you.

Answer 4

Also, if you want to keep the same concept and do the algorithm "yourself" there is few things to change.

First the initial value of SmallestNumber and SmallestNumber2 need to be as high as possible otherwise the numbers saved can only be the one lower than your initial value. Therefore you can use INT_MAX.

Second, the second smallest number, need to be set in 2 cases :

• when a new value is entered that is the second smallest
• when a new smallest value is set, the old smallest value become the new second smallest.

Third there are a lot of unnecessary code here. You check too many time if value is not null which you know from the while condition. And you have code duplication with the cout/cin statement. Which is prone to mistakes.

Here is a version of what it could look like :

    int value= INT_MAX;
    int SmallestNumber=INT_MAX;
    int SmallestNumber2=INT_MAX;


    while (value != 0) {
        if(value > SmallestNumber && value < SmallestNumber2)
        {
            SmallestNumber2 = value;
        }
        else if (value< SmallestNumber)
        {
            SmallestNumber2 = SmallestNumber;
            SmallestNumber = value;
        }
        cout << "Enter number to find the smallest and second smallest(or 0 to quit): ";
        cin >> value;
    }

    cout << "Smallest number is: " << SmallestNumber << '\n' << endl;
    cout << "Second Smallest number is: " << SmallestNumber2 << '\n' << endl;

    return 0;

ps : the version of @darune is a nicer solution.