CODEWITHSUNDEEP
c++distance
user8305079
user8305079

Reputation:

Why does std::distance print out a `-`?

I wrote the following program to print out the occurrence of an alphabet starting with a=0, b=1 and so on. Could someone please point out why using std::distance is printing out a blank - and how I could get rid of it?

// Example program
#include <iostream>
#include <string>
#include <algorithm>
#include <vector>

using namespace std;

int main()
{
    string str;
    cin>>str;

    int n=str.size();
    std::vector<char> table(26);
    table = {
        'a',    'b',    'c',    'd',    'e',    'f',    'g',    'h',    'i',    'j',    'k',    'l',    'm',    'n',    'o',    'p',    'q',    'r',    's',    't',    'u',    'v',    'w',    'x',    'y',    'z'
    };

    int i=0;
    while(i<n) {
        char c=str[i];
        auto search = std::find(table.begin(), table.end(), c);
        if(search!=table.end()) {
            int dist = std::distance(search, table.begin());
            cout<<dist;     //prints out -0-1-2-3 instead of 0123, why?
        }
        i++;
    }

    return 0;
}

Working program is here

Upvotes: 2

Views: 2289

Answers (3)

Vlad from Moscow
Vlad from Moscow

Reputation: 311146

The standard function std::distance accepts a range of the kind [start, target]. So you need to write

auto dist = std::distance( table.begin(), search );

Or without using the function std::distance you can just write

auto dist = search - table.begin();

because the class template std::vector has random access iterators.

A few words about the program.

It is the case when it is better to use C strings instead of standard containers like std::vector and correspondingly to use standard C functions instead of C++ algorithms.

The program can look the following way

#include <iostream>
#include <string>
#include <cstring>
#include <cctype>

int main() 
{
    const char *table = "abcdefghijklmnopqrstuvwxyz";
    std::string s;

    std::getline( std::cin, s );

    for ( char c : s )
    {
        c = std::tolower( ( unsigned char )c );

        if ( const char *p = std::strchr( table, c ) )
        {
            std::cout << p - table << ' ';
        }
    }

    std::cout << std::endl;

    return 0;
}

If for example to enter 

Umedh Singh Bundela

then the output will look like

20 12 4 3 7 18 8 13 6 7 1 20 13 3 4 11 0

Instead of the expression in the statement

std::cout << p - table << ' ';

you can use a call of std::distance if you want provided that you'll include header <iterator>

std::cout << std::distance( table, p ) << ' ';

Upvotes: 0

eerorika
eerorika

Reputation: 238461

why using std::distance is printing out a blank -

- character is the minus sign. It is used to represent negative numbers. So, - is printed because you print negative numbers. The distances are negative because the first argument is latter than the second one.

how I could get rid of it?

If you calculate the distance from earlier position to the latter, the distance will be positive:

std::distance(table.begin(), search);

In this case you do know, but in case you don't know or care about the order and only want the absolute distance, you can get it using the function std::abs:

std::abs(std::distance(it1, it2));

PS. Passing latter iterator first is allowed only if the iterator is random access.

Upvotes: 3

cdhowie
cdhowie

Reputation: 169478

Because the distance between those two iterators is negative; you have them in the wrong order. The "lesser" iterator should be on the left, not the right.

You have: std::distance(search, table.begin())

You should have: std::distance(table.begin(), search)

Upvotes: 6

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