Character array adds random characters at outputting

Question

cin.get(a, 256);
for(int i = 0; i < strlen(a); i++){
    if(strchr("aeiou", a[i])){
        s = 0;
        strcpy(substr, empty);
        isubstr = 0;
    }
    
    else{
        s++;
        substr[isubstr++] = a[i];
    
        if(s > maax || s == maax){
            maax = s;
            memset(show, 0, 256);
            strcpy(show, substr);
        }

    }
    
}
cout << show;

This is the code. It intends to find the longest substring with only consonants and if there are 2+ with the same lenght it outputs the farthest one (closer to the right)

Consider the following sequence:

jfoapwjfppawefjdsjkflwea

Splitted by vowels it wold look something like this:

jf |oa| pwjfpp |a| w |e| fjdsjkflw |ea|

Notice how "fjdsjkflw" is the largest substring without a vowel. This code outputs just that including some random numbers at the end:

fjdsjkflwê²a

Why does this happen? Why does it put NULL 3 characters beyond of what it's intended to?

Answer 1

For starters you should write a function that finds such a longest sequence of consonants.

You provided an incomplete code so it is difficult to analyze it. For example it is not seen where and how variables substr and empty used in this call

strcpy(substr, empty);

are defined and what are their meanings.

Also there are statements like this

memset(show, 0, 256);

that do not make a sense because for example after this statement there is the statement

strcpy(show, substr);

So the previous statement is just redundant.

Or for example it seems that one of these variables s and isubstr is also redundant.

I can suggest the following solution implemented as a function.

#include <iostream>
#include <utility>
#include <cstring>

std::pair<const char *, size_t> max_consonant_seq( const char *s )
{
    const char *vowels = "aeiouAEIOU";
    
    std::pair<const char *, size_t> p( nullptr, 0 );
    
    do
    {
        size_t n = std::strcspn( s, vowels );
        
        if ( n != 0 && !( n < p.second ) )
        {
            p.first = s;
            p.second = n;
        }
        
        s += n;
        
        s += std::strspn( s, vowels );
    } while ( *s );

    return p;
}

int main() 
{
    const char *s = "jfoapwjfppawefjdsjkflwea";
    
    auto p = max_consonant_seq( s );
    
    if ( p.second ) std::cout.write( p.first, p.second ) << '\n';
    
    return 0;
}

The program output is

fjdsjkflw

The function returns a pair of objects. The first one specifies the starting pointer of the maximum sequence of consonants in the passed string and the second object specifies the length of the sequence.

All what you need to understand how the function works is to read the description of the two C string functions strspn and strcspn.