Find the number of occurence of a character in a string and store it in another array.

#include <stdio.h>

/* Function returns number of times c occurs in a string */
int numTimes(const char *str, const char c){
    int num=0;
    while(*str){
        if(*str == c) num++;
        str++;
    }
    return num;
}

/* Checks for the presence of the character in res array to avoid duplication */
int isPresent(const char *arr, const char c){
    while(*arr){
        if(c == *arr) return 1;
        arr++;
    }
    return 0;
}

int main(){
    char arr[20] = "AAABDDDFF";
    char res[20];
    char check;
    int num, i;
    int j=0;
    for(i=0;*(arr+i);i++){
        check = arr[i];
        if(isPresent(res, check)==0){
            printf("check: %c\n", check);
            res[j++] = check;
            num = numTimes(arr, check);
            check = (char)((int)'0'+num);
            res[j++] = check;
            res[j] = '\0';
        }
    }
    printf("%s\n",res);
    return 0;
}

// Output: A3B1D3F2

STRCASECMP : Implementation in C

#include <stdio.h>

int mytoupper(char c){
    // 'A' = 65
    // 'a' = 97
    if(('a'<=c) && (c <= 'z'))
        c = 'A' + (c - 'a');
    return c;
}

int mytolower(char c){
    // 'A' = 65
    // 'a' = 97
    if(('A' <= c) && (c <= 'Z'))
        c = 'a' + (c - 'A');
    return c;
}

int mystrcasecmp(const char *s1, const char *s2){
    while((mytoupper(*s1)==mytoupper(*s2)) && *s1 && *s2) s1++, s2++;
    return *s1-*s2;
}

int main(){
    char *a = "Umesh";
    char b[] = "uMeSh";
    int diff = mystrcasecmp(a,b);
    if(diff==0)
        printf("Equal\n");
    else if(diff < 0)
        printf("Less than\n");
    else
        printf("Greater\n");
    return 0;
}

// Output: Equal

MEMSET Implementation in C

#include <stdio.h>

void *mymemset(void *ptr, int value, size_t num){
    size_t i;
    // Typecast to char so that pointer increment can be done as
    // void pointer can't be increment because compiler 
    // does not know the size of "ptr"
    char *cPtr = ptr;
    for(i = 0; i<num; i++,cPtr++){
        *cPtr = value;
    }
    return ptr;
}

int main(){
    char a[]="Umesh is typing this line for testing mymemset";
    mymemset(a,'0',10);
    printf("%s\n", a);
    return 0;
}

STRSTR Implementation in C

const char * strstr ( const char * str1, const char * str2 );
      char * strstr (       char * str1, const char * str2 );

Returns a pointer to the first occurrence of str2 in str1, or a 
null pointer if str2 is not part of str1.

The matching process does not include the terminating 
null-characters, but it stops there.

#include 
char *mystrstr(const char *str1, const char *str2){
    char *a, *b;
    while(*str1){
        a = str1;
        b = str2;
        while(*a && *b && (*a == *b)) a++, b++;
        if(*b == '\0') return a;
        str1++;
    }
    return NULL;
}

int main(){
    char *a = "Andaman and Nicobar is a beautiful island";
    char *b = "and ";
    char *res = mystrstr(a,b);
    if(res)
        printf("%s\n", res);
    else
        printf("Substring not found\n");
    return 0;
}

//Output: Nicobar is a beautiful island