Largest Palindrome of 3 Numbers

If you have any doubts, let me know
#include <stdio.h>
#include <string.h>
#include<limits.h>

main()
{
    int i=999,j=999,sum=0,r=0,y,x,sum1=0,a[3000],k=0,temp;
    for(i=999;i>100;i--)
    {
        for(j=999;j>100;j--)
        {
            sum=i*j;
            x=sum;
            while(sum>0)
            {
             sum1=sum1*10;
             sum1=sum1+sum%10;
             sum=sum/10;
            }
            if(sum1==x)
         {
             a[k]=x;
             k++;
         }
            sum1=0;
        }
    }
   
    for(i=0;i<k;i++)
    for(j=0;j<k;j++)
    {
        if(a[i]<a[j])
        {
            temp=a[i];
            a[i]=a[j];
            a[j]=temp;
        }
    }
printf("%d",a[k-1]);

}

Decrypting a message program

Feel free to use it, modify it or anything.

Try using the input

y!sr@xo#dt$pa%or^ko*vb(da&ql

You'll get the output laboratory

I've wrote in the comments what each function does. According to that, modify it to make a suitable input

#include <stdio.h>

#include <string.h>

char string[50],t;

int l,k;

main()

{

int i,j;

  

  gets(string);

   printf(" string is %s\n",string);

    l=strlen(string);

    k=l;

    letsreverse();

}

letsreverse() //function reverses a string

{

    

int i=0,j,q;

char *ptr;

ptr=string; 

q=l;

q--;

for(i=0;i<=(l/2)-1;i++)

  {

 t=*(ptr+i);

 *(ptr+i)= *(ptr+q);

 *(ptr+q)=t;

 q--;

 }

 printf("Reverse string is %s\n",string);

 wedontlikejunk();

}

wedontlikejunk()     // kills off junk

 {

    int i=0,z,c=0,x;

    char clean[20];

    for(x=0;x<k;x++)

    {

         if(string[c] >= 'a' && string[c] <= 'z')

         {

         clean[i]=string[c];

         i++;

         }

         c++;

         

    }

    clean[i]='\0';

    printf("semi string is %s\n",clean);

    for(i=0;clean[i]!='\0';i++)

    string[i]=clean[i];

    string[++i]='\0';

    letskillsomeletters();

}

letskillsomeletters()     //kills alternate letters

{

    int i=0,z;

    char final[20];

    for(z=(10%10);z<k;z++)

    {

        final[i]=string[z];

        i++;

        z++;

        

    

    }

    printf("Final string is %s\n",final);

}

Program to copy a file

C program to copy file. If you have any doubts, let me know

#include <stdlib.h>

#include <stdio.h>
main()
{
   
FILE *source, *target;

char x, sourcef[100], targetf[100];
   printf("File to copy?");
     gets(sourcef);
   source=fopen(sourcef, "r"); 
   printf("Target File?");
      gets(targetf);
   target=fopen(targetf, "w");
       while((x=fgetc(source))!=EOF)
           fputc(x, target);
  printf("The file has been copied.");
}