How String Functions (string.h) Work?

In the previous article String
Manipulation Functions (string.h), we had a look at some of the commonly
used string manipulation functions. There is no denying the fact that those
functions are useful but have you ever wondered how those functions actually
work or what is the algorithm behind their working?

If yes then read on…

In this article I am going to present you with our own version of the string
manipulation functions that we had discussed, namely strlen(), strcpy(),
strcat() and strcmp(). Our versions will do the same thing as done
by the original functions but surely they would teach us a lot!

Let's have a look at them one-by-one:

mystrlen

  // mystrlen- function 
  #include<iostream.h>

  int mystrlen(const char *);

  void main(void)
  {
   char ch[]="This is great!";
   cout<<"Length:"<<mystrlen(ch);
  }

  int mystrlen(const char *str)
  {
   int len=0;

   while(str[len]!='\0')
     len++;
   return len;
  }

mystrcpy

  // mystrcpy- function 
  #include<iostream.h>

  void mystrcpy(char *,const char *);

  void main(void)
  {
   char ch[]="This is great!";
   char ch2[20];

   mystrcpy(ch2,ch);

   cout<<ch;
   cout<<endl;
   cout<<ch2;
  }

  void mystrcpy(char *str1,const char *str2)
  {
   int i=0;

   // copy each character
   while(str2[i]!='\0')
   {
    str1[i]=str2[i];
    i++;
   }
   // put the end of 
   // string identifier
   str1[i]='\0';
  }

mystrcat

  // mystrcat- function 
  #include<iostream.h>

  void mystrcat(char *,const char *);

  void main(void)
  {
   char ch[]="This is great!";
   char ch2[25]="Yes ";

   mystrcat(ch2,ch);

   cout<<ch;
   cout<<endl;
   cout<<ch2;
  }

  void mystrcat(char *str1,const char *str2)
  {
   int i=0;
   int len=0;

   // skip to the end of the first
   // string(target)
   while(str1[len]!='\0')
     len++;

   // start copying characters
   // from the start of the 
   // second string (source)
   // to the end of the
   // first (target)
   while(str2[i]!='\0')
   {
    str1[len]=str2[i];
    i++;len++;
   }
   str1[len]='\0';
  }

mystrcmp

  // mystrcmp- function
  #include<iostream.h>

  int mystrcmp(char *,const char *);

  void main(void)
  {
   char ch[]="C++";
   char ch2[]="C++";

   cout<<mystrcmp(ch2,ch);
  }

  int mystrcmp(char *str1,const char *str2)
  {
   int i=0,cmp=-1;

   while(str1[i]!='\0')
   {
    if(str1[i]==str2[i])
    {
     // check one character
     // following it, so that
     // end of string is also
     // compared
     if(str1[i+1]==str2[i+1])
       cmp=0;
     // if not same then check
     // to see which string has 
     // higher ASCII value for the
     // non-matching charcter
     else if(str1[i+1]<str2[i+1])
     {
      cmp=-1;
      break;
     }
     else
     {
      cmp=1;
      break;
     }
    }
    else if(str1[i]<str2[i])
    {
     cmp=-1;
     break;
    }
    else
    {
     cmp=1;
     break;
    }
    i++;
   }
   return cmp;
  }

Good-Bye!

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  String
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