命令行参数中某些情况下的输出错误

Question

I'm required to come up with a C program that accepts up to 2 arguments. 1st argument is a sting, and 2nd argument is a character 'Q'. For example:

./piglatin Tuesday
uesdayTay
./piglatin Tuesday Q
uesdayTuh
./piglatin
You must enter 1-2 arguments.

I've come up with code that works about 80% of the time, except for when I just enter one argument, then there is additional garbage besides my intended output that's displayed on my screen. Also, if my second argument has more than 2 letters, the letters after the first gets printed out in the output. I can't find out what's wrong with my code, but here it is:

#include <stdio.h>
#include <string.h>

int main ( int argc, char* argv [] )
{
    char temp;
    int n;

    if ( argv [1] == NULL )
    {
        printf("You must enter 1-2 arguments\n");
        return 0;
    }

    else if ( argc == 2 && argv [2] == NULL )
    {
        n = strlen ( argv [1] );

        temp = argv [1][0];             
        for ( int j = 1; j <= n-1; j++)         
        {                                   
            argv [1][j-1] = argv [1][j];
        }

        argv [1][n-1] = temp;       
        argv [1][n] = 'a';              
        argv [1][n+1] = 'y';

        printf("%s\n", argv[1]);
        return 0;

    }

    else if ( argc == 3 && argv [3] == NULL )
    {
        if ( *argv [2] == 'Q' )
        {
            n = strlen ( argv [1] );

            temp = argv [1][0];             
            for ( int j = 1; j <= n-1; j++)         
            {                                   
                argv [1][j-1] = argv [1][j];
            }

            argv [1][n-1] = temp;       
            argv [1][n] = 'u';              
            argv [1][n+1] = 'h';

            printf("%s\n", argv[1]);
            return 0;
        }

        else
        {
            n = strlen ( argv [1] );

            temp = argv [1][0];             
            for ( int j = 1; j <= n-1; j++)         
            {                                   
                argv [1][j-1] = argv [1][j];
            }

            argv [1][n-1] = temp;       
            argv [1][n] = 'a';              
            argv [1][n+1] = 'y';

            printf("%s\n", argv[1]);
            return 0;
        }

    }

    else
    {
        printf("You must enter 1-2 arguments\n");
    }


    return 0;
}

Here are the execution traces that are wrong:

./piglatin Tuesday
uesdayTayANPATH=/soft/jdk1.7.0_76/man:/usr/man:/usr/share/man:/usr/local/man:/usr/X11R6/man

./piglatin R
RayANPATH=/soft/jdk1.7.0_76/man:/usr/man:/usr/share/man:/usr/local/man:/usr/X11R6/man

./piglatin Tuesday QR
uesdayTuhR

Answer 1

The problem, as I see it is with

    argv [1][n] = 'a';              
    argv [1][n+1] = 'y';

n-1 is the maximum allowable index to be used. Accessing out of bound memory causes undefined behavior .

Same goes for other cases, too.