Reverse String function in C without library functions

Question

This is my task:

Realize function that reverses null terminated string. The prototype of the function is void Reverse(char *ptr);. Do not use standard library functions.

This is my code for now:

void Reverse(char *ptr) {
    char *newString;
    char ch = *ptr;
    unsigned int size = 0;

    for (int i = 1; ch != '\0'; i++) {
        ch = *(ptr + i);
        size++;
    }

    newString = (char*)malloc(size);

    for (int left = 0, right = size - 1; left < size; left++, right--) {
        *(newString + left) = *(ptr + right);
    }

    printf("%s", newString);
    printf("\n");
}

It reverses the string and saves it in the newString

My first problem is that when I print the newString to see if the functions works the string is reversed, but after it there are some symbols.

For example: If I have char *str = "hello"; and Reverse(str); in the main method the result of printf("%s", newString) will be olleh****.

But if change the newString = (char*)malloc(size); to newString = (char*)malloc(1); it works fine.

My second problem is that I don't know how to save the newString into the given one. I am using a new String because the given one can't be changed.

Is the intent to return a reversed string, or to simply print in reverse? — Peter
– Peter, Commented Jul 15, 2016 at 12:36
Peter I can't understand too if i have to print it or return it. — Stanislav Fotev
– Stanislav Fotev, Commented Jul 15, 2016 at 12:51
Consider that you can reverse a string in-place by successive swaps. For example, the string "james". If you swap the first and last characters, you get "samej". And if you swap the second and next-to-last, you get "semaj", which is the reversed string. I'll leave you the task of writing the loop to do that. — Jim Mischel
– Jim Mischel, Commented Jul 15, 2016 at 15:22

Vlad from Moscow · Accepted Answer · 2016-07-15 13:26:20Z

4

For starters it is better to declare the function like

char * Reverse( char *ptr );
^^^^^^

because standard C string functions usually return pointers to destination strings.

The function should reverse the original string. It may not create a dynamic string because the caller of the function will be unable to free it provided that the function has return type void.

The function can look as it is shown in the following demonstrative program.

#include <stdio.h>

char * Reverse( char *ptr )
{
    char *first = ptr, *last = ptr;

    while ( *last ) ++last;

    if ( first < last )
    {
        for ( ; first < --last; ++first  )
        {
            char c = *first;
            *first = *last;
            *last  = c;
        }
    }

    return ptr;
}

int main( void ) 
{
    char s[] = "Hello World!";

    puts( s );
    puts( Reverse( s ) );

    return 0;
}

Its output is

Hello World!
!dlroW olleH

Take into account that you may not call the function like

puts( Reverse( "Hello World!" ) );

because string literals are immutable in C.

If you are going to declare the function like

void Reverse( char *ptr );

then just remove the return statement in the shown function. For example

#include <stdio.h>

void Reverse( char *ptr )
{
    char *first = ptr, *last = ptr;

    while ( *last ) ++last;

    if ( first < last )
    {
        for ( ; first < --last; ++first  )
        {
            char c = *first;
            *first = *last;
            *last  = c;
        }
    }
}

int main( void ) 
{
    char s[] = "Hello World!";

    puts( s );
    Reverse( s )
    puts( s );

    return 0;
}

If to use your approach with indices then the function can look like

#include <stdio.h>

void Reverse( char *ptr ) 
{
    size_t size = 0;

    while ( *( ptr + size ) ) ++size;

    if ( size != 0 )
    {
        for ( size_t left = 0, right = size - 1; left < right; left++, right-- ) 
        {
            char c     = ptr[left];    // or char c = *( ptr + left ); and so on
            ptr[left]  = ptr[right];
            ptr[right] = c;
        }           
    }
}

int main( void ) 
{
    char s[] = "Hello World!";

    puts( s );
    Reverse( s );
    puts( s );

    return 0;
}

edited Jul 15, 2016 at 13:26

answered Jul 15, 2016 at 12:32

Vlad from Moscow

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13 Comments

Peter Over a year ago

Some standard C string functions return pointers. Not all do.

Vlad from Moscow Over a year ago

@Peter What standard C string functions have return type void?

Stanislav Fotev Over a year ago

I can't change the prototype.

Vlad from Moscow Over a year ago

@StanislavFotev No problem. Just remove the return statement in the function as I already pointed out in my post.:) In this case the demonstrative program will have statements like puts( s ); Reverse( s ); puts( s );

chux Over a year ago

Hmmm, Suspect your nifty (UV) Reverse( char *ptr ) can be simplified without if(). Will ponder that.

|

davek · Accepted Answer · 2016-07-15 12:25:19Z

1

In your loop to get the size, you're not counting the null terminator. So your newString is missing the null terminator as well. make sure to do newString = malloc(size + 1);, and to place the null terminator onto the end of newString.

answered Jul 15, 2016 at 12:25

davek

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3 Comments

LordWilmore Over a year ago

And remember to make your loop termination case left < size-1

Stanislav Fotev Over a year ago

Thanks for the answer I managed my 1st problem but i don't know how to return the string without changing the prototype.

davek Over a year ago

In that case they probably want the original string input to be reversed, so use Vlad's answer and just leave the prototype the same & remove the return at the end of it.

chqrlie · Accepted Answer · 2016-07-15 13:13:21Z

You have several problems in your code:

you do not allocate enough space for the resulting string, you must allocate size+1 bytes and set the \0 terminator at the end of the string.
you only copy half the characters from ptr to newString.
you are not even supposed to allocate memory, since you cannot call any library functions. You should instead reverse the string in place.
Your test fails because modifying a string literal invokes undefined behavior. You should pass an initialized array as shown below.

Here is an improved version:

#include <stdio.h>

void Reverse(char *ptr) {
    unsigned int left, right;

    for (right = 0; *(ptr + right) != '\0'; right++) {
         continue;
    }

    for (left = 0; left < right; left++, right--) {
        char ch = ptr[left];
        ptr[left] = ptr[right - 1];
        ptr[right - 1] = ch;
    }
}

int main(void) {
    char buf[] = "Hello world";
    Reverse(buf);
    printf("%s\n", buf);
    return 0;
}

It should print dlrow olleH.

Tavij · Accepted Answer · 2016-09-03 13:57:18Z

0

Simple string reverse function without string.h library functions :

#include <stdio.h>
void reverse(char *str, int n);
int main()
{
    char str[100];
    int n;
    printf("Enter a string\n");
    scanf("%s",&str);
    for( n = 0; str[n] != '\0'; n++)
    {

    }
    reverse(str,n);
    puts(str);
    return 0;
}
void reverse(char *str,int n)
{
    printf("Length = %d\n",n);
    printf("Reversed :\n");
    int i;
    char ch;
    for(i = 0; i<n/2; i++)
    {
        ch = str[i];
        str[i] = str[n-i-1];
        str[n-i-1] = ch;
    }
}

answered Sep 3, 2016 at 13:57

Tavij

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