2-D array in string creating erroneous output in C

Question

I am trying to do something as simple as printing the reverse of a string . EXAMPLE :

Hello World! This is me

Needed O/P:

me is This World! Hello

My code goes something like this:

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

int main(){
 char *arr[20] ;
 int i,j;
 int size;
 char *revarr[20];
 printf(" enter the number of words\n");
 scanf("%d",&size);
 for(i=0;i<size;i++)
 scanf("%s",&arr[i]);
 for(i=0;i<size;i++)
 {

    printf("%s\n",&arr[size-1-i]); //overwritten words
    revarr[i]=arr[size-1-i];
 }
 printf(" the reversed sentence is %s\n",(char *)revarr);
}

I except arr[0] , arr[1] etc to be separate entities but on printing and storing them they seem to be overlapping like this : i/p:

Hello World

o/p:

World
HellWorld
the reversed sentence is WorlHell@#$

I can't seem to figure out what is wrong! Thanks in advance!

EDIT : On printing

printf(&arr[0]);
printf(&arr[1]);

I get :

HellWorld
World

What I expected it to print is

Hello
World

Answer 1

You declared arr and revarr as array of char pointers. You need to dynamically allocate memory for their elements.
Also note that you do not need & in statements

scanf("%s",&arr[i]);  

and

printf("%s\n", &arr[size-1-i]);  
//             ^No need of &  

Here is the modified version of your code. Note that there is no need to use revarr to reverse the string.

#include<stdio.h>
#include<stdlib.h>

int main(){
    size_t i, size;
    printf("Enter the number of words\n");
    scanf("%d", &size);
    char *arr[size] ;  // Variable length array. Supported by C99 and latter

    for(i = 0; i < size; i++) 
    {
        arr[i] = malloc(20); // Assumimg words are no longer than 20 characters
        scanf("%s", arr[i]);
    }

    printf("The reversed sentence is:\n");  
    for(i = size-1; i >= 0; i--)  // Run loop in reverse order and print words
        printf("%s ", arr[i]); 
}

Answer 2

You haven't allocated memory for arr[0], arr[1], etc. before using them to read strings in

scanf("%s",&arr[i]);

That is cause for undefined behavior. You need something like:

int main(){
   char *arr[20] ;
   int i,j;
   int size;
   char *revarr[20];
   printf(" enter the number of words\n");
   scanf("%d",&size);
   for(i=0;i<size;i++)
   {
      // Allocate memory.
      // make it large enough to hold the input
      arr[i] = malloc(100);
      scanf("%s", arr[i]);
   }
   for(i=0;i<size;i++)
   {
      revarr[i]=arr[size-1-i];
   }

   printf(" the reversed sentence is: ");
   for(i=0;i<size;i++)
   {
       printf("%s ", revarr[i]);
   }
   printf("\n");


   // Deallocate the memory.
   for(i=0;i<size;i++)
   {
      free(arr[i]);
   }

   return 0;
}

Answer 3

This is a good approach for your problem :

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

int main(int argc, char *argv[])
{
     /* the string line will contain the line of the  input */
     char line[100];
     /* read the string with the function f gets */
     fgets(line,100,stdin);
   /* the tab will contain all the string of the variable  line */
     char *tab[20];
     /* the variable p will point to each string of the  line */
     char *p=NULL;
     /* we extract the strings of the line via the function strtok */
     p=strtok(line," ");
     int nb=-1;
     while (p!=NULL)
     {
         nb++;
         /* we allocate a space memory fo every str ing  */
            tab[nb]=malloc(sizeof(char)*100);
            strcpy(tab[nb],p);
            p=strtok(NULL," ");
     }
     /* there is an exception with the last string of the line we need to take care o f it */
     tab[nb][strlen(tab[nb])-1]='\0';
     int i;
     /* print the strings in reverse or der  */
     for (i=nb;i>=0;i--)
     {
         printf("%s ",tab[i]);
        /* dont forget to free the space memory at the end of the prog ram  */
         free(tab[i]);
     }
     printf("\n");

     return 0;
}

Answer 4

You need the following

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

int main(void) 
{
    size_t size;

    printf( "enter the number of words: " );
    scanf( "%zu", &size );

    char arr[size][20];
    char revarr[size][20];

    for ( size_t i = 0; i < size; i++ ) scanf( "%s", arr[i] );

    printf( "\n" );

    for ( size_t i = 0; i < size; i++ ) strcpy( revarr[i], arr[size-i-1] );

    printf( "the reversed sentence is"  );

    for ( size_t i = 0; i < size; i++ ) printf( " %s", revarr[i] );
    printf( "\n" );

    return 0;
}

If to enter

2
Hello World

then output will be

World Hello

Take into account that the code will be compiled only if your compiler supports C99. Otherwise you have to allocate memory dynamically for character arrays.

As for your code then it has undefined behaviour and in whole is invalid. You did not allocate memory for each element of arrays arr and revarr. You may not assign one array to another. Instead you have to use standard function strcpy and so on.