Reversing String Only Using For Loop

Question

I have been trying to reverse(and print it that way) a given string only using for loops and nothing more. I think I have built up the basic logic, but it has some defects. When run, it only reverses the first two characters and then stops. Please help me find the defect in my logic.

#include<stdio.h>
#include<stdlib.h>
int main()
{
    char a[20];
    int i;
    printf("Enter any String\n");
    gets(a);
    for(i=0;a[i]!=NULL;i++)
    {}
    for(i=1;i>=0;i--)
    {
        printf("%c",a[i]);
    }
}

Answer 1

For starters the function gets is not a standard C function any more. it is unsafe. Instead use the standard C function fgets. The function can append the new line character '\n' to the entered string that should be excluded from the string.

It is unclear from your question whether you are allowed to use standard string functions.

Nevertheless here is a demonstrative program that does the task without using standard C string functions and that uses only for loops (neither while loop nor do-while loop).

#include <stdio.h>

int main(void) 
{
    enum { N = 20 };
    char s[N];

    printf( "Enter any String less than %d symbols: ", N ); 

    fgets( s, N, stdin );

    //  remove the new line character and calculate the length of the string
    size_t n = 0;
    for ( ; s[n] != '\0' && s[n] != '\n'; ) ++n;

    s[n] = '\0';

    //  reverse the string
    for ( size_t i = 0; i < n / 2; i++ )
    {
        char c = s[i];
        s[i] = s[n-i-1];
        s[n-i-1] = c;
    }

    puts( s );

    return 0;
}

Its output might look the following way

Enter any String less than 20 symbols: Hello dev.aniruddha
ahddurina.ved olleH

If you want just to output the original string in the reverse order then the program can look like

#include <stdio.h>

int main(void) 
{
    enum { N = 20 };
    char s[N];

    printf( "Enter any String less than %d symbols: ", N ); 

    fgets( s, N, stdin );

    //  remove the new line character and calculate the length of the string
    size_t n = 0;
    for ( ; s[n] != '\0' && s[n] != '\n'; ) ++n;

    s[n] = '\0';

    //  reverse the string

    for ( ; n-- != 0;  )
    {
        putchar( s[n] );
    }

    putchar( '\n' );

    return 0;
}

Its output is the same as shown above

Enter any String less than 20 symbols: Hello dev.aniruddha
ahddurina.ved olleH

Answer 2

gets() is a bad idea as you can easily get overflows and it is no longer part of the c standard. So let's assume that the string entered fits the array and this is just for an excercise with no reallife usage.

Your first loop finds the terminator. That's good.
Your second loop sets the variable that indicates the terminator to 1, destroying the result. If you remove the assignment i=1, your program compiles with gcc and does what you want.

#include<stdio.h>
#include<stdlib.h>
int main()
{
    char a[20];
    int i;
    printf("Enter any String\n");
    gets(a);
    for(i=0;a[i]!=NULL;i++)
    {}
    for(;i>=0;i--) //removed i=1 here
    {
        printf("%c",a[i]);
    }
}

But there are still some issues to be addressed. You will also reverse the terminator, instead you should start from i-1 I would advise to not use a for loop if you do not have a counter criterion The first loop should rather be a while loop, but as it was part of the assignment you had no choice still I will replace it in my recommendation. As they can easily be swapped.

Then you could use another variable for the second loop for clarity. Also NULL is the NULL-pointer not the value 0 (also namend NUL apperantly) . So you should replace this either with 0 or with '\0'

Also stdlib.h is not required here

#include<stdio.h>
int main()
{
    char a[20];
    int i = 0;

    printf("Enter any String\n");
    gets(a);

    while (a[i] != 0)
    {
        i++;
    }

    for(int j = i-1; j>=0; j--) // -1 to get the value in front of the terminator
    {
        printf("%c",a[j]);
    }
    printf("\n"); //to flush the output.
}

Answer 3

Here is the solution code. The first for loop is to be used for determining the length of the string and the second for loop is for traversing the string from the last position to the first.

#include<stdio.h>
int main()
{
    char a[20];
    int i,j,len;
    printf("Enter a String\n");
    gets(a);
    for(i=0;a[i]!=NULL;i++)
    {}
    len=i;
    for(j=len-1;j>=0;j--)
    {
        printf("%c",a[j]);
    }

}

Answer 4

I think why only two chars are been return is because of the condition statement in your second "for loop".

for(i=1;i>=0;i--) 

Note:

1. it repeats from 1~0 (1,0): meaning it will repeat only twice
2. first iteration: when i == 1
3. second iteration: when i == 0 ; then it ends .

Please note that you created two "for loops" with the first one having no content.

Bonus: I tried to fixed your code but realized that my C language skills isnt the best lol . Anyways, i came up with something that you could reference but it only reverse strings of less than 8 elements.

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

int findlength(char a[]);

int main()
{
    char a[20];
    int i;
    printf("Enter any String\n");
    gets(a);
    int len = findlength(a);
    printf("Lenght of the String is: %d \n",len);
    printf("Reversed String is: ");
    for(i=len;i>-1;i--){
        printf("%c",a[i]);
    }

}

int findlength(char a[]){
    int result = 0;
    int i;
    for(i=0;i<sizeof(a) / sizeof(char);i++){ // sizeof(char) is 1 
        if(a[i] == '\0') //end of string
            return result;
        result += 1;
    }
    return result;
}