STRCMP implementation in C

//Program to implement strcmp

#include
int my_strcmp( const char *sDest, const char *sSrc){
    while( *sDest == *sSrc){
        if( *sDest == '\0')
            return 0;
        sDest++;
        sSrc++;
    }
    return (*sDest - *sSrc);
}
// OR
int mystrcmp(const char *str1, const char *str2){
    while((*str1 == *str2) && *str1 && *str2) str1++, str2++;
    return (*str1 - *str2);
}
//OR using array
int mystrcmp(const char *a, const char *b)
    int i;
    for(i=0; (a[i] == b[i]) && (a[i] != '\0') && (b[i] != '\0'));i++);
    return (a[i] - b[i])
}

int main(){
    printf("\nstrcmp() = [%d]\n", mystrcmp("Umesh","Umesh"));
    printf("\nstrcmp() = [%d]\n", mystrcmp("Umesh","Guddu"));
      return(0);
}

ONE LINE IMPLEMENTATION USING RECURSION

#include <stdio.h>

//My_strcmp return 0 if both the string are same , 1 if they are diff

int my_strcmp(char *a, char *b){
    return (*a==*b && *b=='\0')?0:(*a == *b)?my_strcmp(++a, ++b):1;
} 

int main(){
    char *a = "Umesh";
    char *b = "Umesh";
    if(my_strcmp(a, b) == 0){
        printf("Same");
    else
        printf("Not same");
    return 0;
}  

STRLEN implementation in C

Two methods mentioned below

Method 1:

//Program to implement strlen

#include <stdio.h>

int my_strlen(const char *src){
    int count = 0;
    while(src[count] != '\0')
        count++;
    return count;
}

int main(){
    char src[] = "umesh";
    printf("length of src = %d\n", my_strlen(src));
    return 0;
}

Method 2:

#include <stdio.h>

int my_strlen(const char *src){
    const char *ptr;
    for(ptr = src; *src; src++);
    return (src - ptr); // Address is subtracted.
} 

int main(){
    char src[] = "umesh";
    printf("length of src = %d\n", my_strlen(src));
    return 0;
}

STRCPY implementation in C

I have mentioned two methods below.

Method 1:

// Program to implement strcpy 

#include <stdio.h>

char *my_strcpy(char *dest, const char *src){
    char *ptr;
    ptr = dest;
    while( *dest++ = *src++);
    return ptr;
}

int main(){
    char dest[6];
    char src[6] = "umesh"; // 5 characters + '\0'
    my_strcpy(dest, src);
    printf("dest = %s\n", dest);
    return 0;
}

* const char *src - const is used to restrict any change in the src

Method 2:

#include <stdio.h>

char *my_strcpy(char dest[], const char src[]){
    int i = 0; // loop variable
    while(src[i] != '\0'){
        dest[i] = src[i];
        i++;
    }
    dest[i]='\0';
    return dest;
}

int main(){
    char dest[6];
    char src[6] = "umesh"; // 5 characters + '\0'
    my_strcpy(dest, src);
    printf("dest = %s\n", dest);
    return 0; 
}

ATOI implementation in C

// Program to implement own atoi function

#include <stdio.h>

int my_atoi(char *p) {
    int k = 0;
    while (*p) {
        k = (k<<3)+(k<<1)+(*p)-'0';
        p++; // Don't increment k
    }
    return k;
}

int main(){
    char pStr[] = "987";
    int myNum = my_atoi(pStr);
    printf(" myNum = %d\n", myNum);
    return 0;
}


Explanation:
1. k = (k<<3) +  (k<<1) +  (*p) - '0';  is equivalent to 
   k = k*10 + (*p) - '0';
2. During first loop value of k is 0 and (*p) has value of char '9' 
   ( ascii  - 57) and ascii of char '0' is 48.
3. Subtraction of them gives the integer value 9 ,stored in k.
4. Increment the pointer p, makes it points to char '8'.
5. Integer k is multiplied with 10  ( k<<3 + k<<1) and 8 is added to it 
   making its value 98.
6. Again p is incremented and k finally stores integer 987.