CODEWITHSUNDEEP
creverse
Alex Niu
Alex Niu

Reputation: 3

C not copying characters into new array

I wrote the following code to reverse a string:

#include <stdio.h>
#include <strings.h>
void reverse(char* str)
{
    int size = strlen(str)+1;
    printf("%d\n",size);
    char* rev = malloc(size*sizeof(char));
    int i,j;
    printf("Contents of rev as it is being filled: ");
    for(i=size, j=0 ; i>=0 ; i--, j++)
    {
        //printf("%c\n", str[i]);
        rev[j] = str[i];
        printf("%c", rev[j]);
    };
    printf("\n");
    printf("%s\n",str);
    printf("Result of trying to print rev after it has been made : %s\n",rev);
}

int main()
{
    reverse("Hello, World");

    return 0;
}

When the contents of the reversed string are printed, they are correct. However, when I try to print rev after the loop I get nothing. When is this happening?

Upvotes: 0

Views: 79

Answers (4)

Ian
Ian

Reputation: 30813

Here is the modified code which I tested as working

void reverse(char* str)
{
    int size = strlen(str); //read 1. remove +1 here
    printf("%d\n",size);
    char* rev = malloc(size*sizeof(char) + 1); //read 2. add +1 here
    int i,j;
    printf("Contents of rev as it is being filled: ");
    for(i=size-1, j=0 ; i>=0 ; i--, j++) //read 3. gives -1 here
    {
        //printf("%c\n", str[i]);
        rev[j] = str[i];
        printf("%c", rev[j]);
    };
    rev[j] = '\0'; //read 4. gives '\0' at the end
    printf("\n");
    printf("%s\n",str);
    printf("Result of trying to print rev after it has been made : %s\n",rev);
}

There are some things to note:

  1. strlen(str) (without having +1) should be enough to declare the size of your string.
  2. rev size, since you want to print it using %s, should be increased by 1, this is to add '\0' character, which is the indicator of termination of printing string. This is just how printf using %s works.
  3. what you want is starting from str[size-1]; because str size is size, the offset should be one less than the size.
  4. Only done for printing's sake. If you do not print, this (as well as 2.) is not needed. Edit: for completeness' sake, please read Jonathan Leffler's comment on this, I myself find it helpful.

Result in my PC:

enter image description here

Upvotes: 3

Imran
Imran

Reputation: 4750

In your program you have copied the last '\0' as the first character of rev; as a result when you print rev it shows nothing. Besides, you have to add the null character '\0' at the end of rev. That is the solution of your problem. 

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void reverse(char* str)
{
    int size = strlen(str);
    printf("%d\n",size);
    char* rev = (char*)malloc(size*sizeof(char));
    int i,j;
    printf("Contents of rev as it is being filled: ");
    for(i=size-1, j=0 ; i>=0 ; i--, j++)
    {
        rev[j] = str[i];
        printf("%c", rev[j]);
    };
    rev[j] = '\0';
    printf("\n");
    printf("%s\n",str);
    printf("Result of trying to print rev after it has been made : %s\n",rev);
}

Upvotes: -1

haccks
haccks

Reputation: 106082

i = size is out of bound array access. Change it to i = size-2 to skip the '\0' character at the end of the string and you need to null terminate the reversed string. 

for(i=size-2, j=0 ; i>=0 ; i--, j++)
{
      //printf("%c\n", str[i]);
      rev[j] = str[i];
      printf("%c", rev[j]);
}
rev[j] = '\0';

Upvotes: 3

Kevin
Kevin

Reputation: 7324

You are reversing the range [0, size] which includes the null terminator. So rev[0] is the null terminator, making it print nothing. You need to reverse the range [0, size-1] and then add the null terminator to the end. size in this case is strlen(str) rather than strlen(str)+1.

Upvotes: 2

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