How can I remove the last comma from a loop in C++ in a simple way?

Question

This program is for printing prime numbers till the input given and separating every prime number with a comma.

void main(){

    int N, counter=0, isPrime;

    int k, j;

    cout << "Enter maximum range: ";

    cin >> N;

    for (j=2; j<=N; j++){

        isPrime = 0;
        k = 2;

        while (k<j){

            if (j%k==0){

                isPrime++;
            }
            k++;
        }
        if (isPrime==0){

            if (k==N){
                cout << j;
            }
            else{
                cout << j << ",";
            }
            counter++;
        }
    }
    cout << endl;
    system("pause");
}

It is only removing the last comma for prime number inputs, not for any other input. How can I fix this?

Input: 23
Output: 2,3,5,7,11,13,17,19,23
Input: 8
Output: 2,3,5,7,
Input: 9
Output: 2,3,5,7,

Add the comma to the previous iteration instead of to the current. Then check it's not the first iteration. — StoryTeller - Unslander Monica, Oct 10 '15 at 14:24
a little [helper class](http://coliru.stacked-crooked.com/a/c3ba6e1cf0b44dd3) if you do it often — sp2danny, Oct 10 '15 at 15:06

score 32 · Accepted Answer · edited Feb 10 '21 at 11:20

32

There is no need to if then else so much:

std::string delim = "";
for( auto&& item : vec )
{
   std::cout << delim << item;
   delim = ",";
}

No checking is needed for all cases, like the vector is empty or not.

If you accept an extra space in the beginning, just replace the string to char, and then the performance will be improved even more.

edited Feb 10 '21 at 11:20

Carlo Wood

5,648
2
35
47

answered Oct 10 '15 at 16:30

score 5 · Answer 2 · edited Apr 28 '18 at 13:58

5

Don't remove the last comma. Instead insert commas before each entry except the first.

edited Apr 28 '18 at 13:58

Peter Mortensen

30,738
21
105
131

answered Oct 10 '15 at 14:23

Ben

34,935
6
74
113

score 2 · Answer 3 · edited Jan 25 '18 at 14:04

2

To easily remove the last comma you can use the '\b' character.

for(auto item : vec)
    std::cout << item << ", " ;
std::cout << "\b\b " << std::endl;

edited Jan 25 '18 at 14:04

Community

1
1

answered Jan 24 '18 at 22:58

Erman

1,543
17
26

score 1 · Answer 4 · answered Oct 10 '15 at 14:28

1

Just decide from a pre condition:

bool first = true;
for(j=2;j<=N;j++){
   // ...
   if(k==N) {
   if(!first) {
       cout << ',';
   }
   else {
       first = false;
   }
   cout<<j;
}

answered Oct 10 '15 at 14:28

πάντα ῥεῖ

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score 0 · Answer 5 · edited Apr 28 '18 at 13:57

The easiest way is to output the first or last value manually:

#include <iostream>
#include <cmath>

using namespace std;

int main() {
    int N, counter = 0;
    bool isPrime;

    cout << "Enter maximum range: ";
    cin >> N;
    if (N>=2) {
        cout << "2";
    }
    for (int j = 3; j <= N; ++j) {
        isPrime = true;
        for (int k = 2; k < sqrt(j)+1; ++k) {
            if (j % k == 0) {
                isPrime = false;
                break;
            }
        }
        if (isPrime) {
            cout << ", " << j;
            counter++;
        }
    }
    cout << endl;
    system("pause");
}

SirKM · Answer 6 · 2018-04-29T12:39:02.327

With minimal changes to your existing code:

#include <iostream>
#include <string>

using namespace std;

void main()
{
    int N, counter = 0, isPrime;
    string separator = ""; // none at first
    int k, j;

    cout << "Enter maximum range: ";

    cin >> N;

    for(j = 2; j <= N; j++)
    {

        isPrime = 0;
        k = 2;

        while(k<j)
        {

            if(j%k == 0)
            {

                isPrime++;

                break; // exit while loop
            }
            k++;
        }
        if(isPrime == 0)
        {
            // if(k==N) not needed
            cout << separator << j;
            separator = ","; // comma after first
            counter++;
        }
    }
    cout << endl;
    system("pause");
}

Explanation
Basically, I added a separator string which is blank at the start, i.e. the empty string, but is set to a comma for each output after the first. As such the cout statement will not print a comma before the first number, but will do so for each subsequent number being printed.

You might want to explain what you've changed in the original code - this would make your answer more useful to others. Thanks. — rwp, Apr 28 '18 at 16:27
I added comments to each line I changed. I thought it was clear. — SirKM, Apr 29 '18 at 12:26

How can I remove the last comma from a loop in C++ in a simple way?

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