Reversing Array in C?

Question

Hi i trying to implement a reverse array code but it doesnt seem to work and im really not sure why. The For loop just doesnt seem to work. I dont know why because the logic seems pretty right to me.

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

void reverse(char, int);

int main()
{
    char a[100];
    gets(a);

    reverse(a, strlen(a)-1);

    printf("%s\n",a);
    getchar();
    getchar();
    getchar();
    return 0;
}

void reverse(char ar[], int n)
{
    char c;
    int i = 0;
    printf("n = %d" , n);
    for ( i = 0; i >= n ; i++){
        c = ar[i];
        ar[i] = ar[n];
        ar[n] = c;
        printf("Processed");
        n--;}

}


/*
if (begin >= n)
return;

c          = *(x+begin);
*(x+begin) = *(x+n);
*(x+n)   = c;
offs = x++;
printf("Begin = %d   ,  n = %d, offs = %p  \n", begin, n, offs);
reverse(x, ++begin, --n); */

Answer 1

void reverse(char, int);  //declaration wrong

void reverse(char[], int);
                 ^^^ 

---

Your loop

for ( i = 0; i >= n ; i++) // this fails i=0, n=some size

should be

for ( i = 0; i <= n ; i++)

Avoid using gets() use fgets() instead.

Answer 2

I think better use macro for this task. In code below it is a macro SWAP.

---

Content a file main.c

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

// swap values with respect a type it
#ifndef SWAP
    #define SWAP(type, a, b) \
    { \
        type temp = a; \
        a = b; \
        b = temp; \
    }
#endif


/*
    Print an array integer items
 */
void
printIntArray(int array[], size_t length) {
    char ending_charapter[] = ", ";
    putchar('[');
    for (size_t i = 0; i < length; ++i) {
        printf("%d", array[i]);
        if (i < length - 1) {
            printf("%s", ending_charapter);
        }
    }
    puts("]");
}


/*
    Print an array float items
 */
void
printFloatArray(float array[], size_t length) {
    char ending_charapter[] = ", ";
    putchar('[');
    for (size_t i = 0; i < length; ++i) {
        printf("%f", array[i]);
        if (i < length - 1) {
            printf("%s", ending_charapter);
        }
    }
    puts("]");
}


/*
    Reverse an integer array in place
 */
static int
reverseIntArray(int *array, const size_t length) {
    for (int i = 0; i < length / 2; ++i) {
        SWAP(int, array[i], array[length - i - 1]);
    }
    return 0;
}


/*
    Reverse an float array in place
 */
static int
reverseFloatArray(float *array, const size_t length) {
    for (int i = 0; i < length / 2; ++i) {
        SWAP(float, array[i], array[length - i - 1]);
    }
    return 0;
}


/*
    Reverse an string
 */
static int
reverseString(char string[]) {
    size_t str_len = strlen(string);
    for (int i = 0; i < str_len / 2; ++i) {
        SWAP(char, string[i], string[str_len - i - 1]);
    }
    return 0;
}


int
main (const int argc, const char *argv[])
{
    puts("An example reverse for a int array");
    int arrInt[4] = {1, -2, 3, -4};
    printIntArray(arrInt, 4);
    reverseIntArray(arrInt, 4);
    printIntArray(arrInt, 4);

    puts("An example reverse for a float array");
    float arrFloat[4] = {0.1, -2.12, 1.3, -4.2};
    printFloatArray(arrFloat, 4);
    reverseFloatArray(arrFloat, 4);
    printFloatArray(arrFloat, 4);

    puts("An example reverse for a string");
    char str[] = "Simple text";
    puts(str);
    reverseString(str);
    puts(str);

    return 0;
}

Compilation as:

gcc std=c11 -I /usr/include/ -o main main.c

---

Result:

An example reverse for a int array
[1, -2, 3, -4]
[-4, 3, -2, 1]
An example reverse for a float array
[0.100000, -2.120000, 1.300000, -4.200000]
[-4.200000, 1.300000, -2.120000, 0.100000]
An example reverse for a string
Simple text
txet elpmiS

---

Notes:

1. Just working
2. Workint with any built-in type
3. Poorly tested and used only the GCC compiler

4. Based on

   4.1 Define a preprocessor macro swap(t, x, y)

   4.2 Reversing an array In place

   4.3 The answers on this question

---

Testing environment

$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 8.6 (jessie)
Release:    8.6
Codename:   jessie
$ uname -a
Linux localhost 3.16.0-4-amd64 #1 SMP Debian 3.16.36-1+deb8u2 (2016-10-19) x86_64 GNU/Linux
$ gcc --version
gcc (Debian 4.9.2-10) 4.9.2
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Answer 3

for loop condition should be 'i < n'. and prototype declaration should match.

Answer 4

for loop condition should be 'i < n'. and prototype declaration should match.

and "int n" is the size of array. So "i<=n" would make the same array reversed from end to mid, and again from mid to top. So result is same as array. make "n" as half of the array size.