Question

Create C function for String.c and Strings.h that

#include <string.h>

/* substring - return a pointer to the substring beginning at the iPos-th position.
* If iPos is out-of-range, return (char*)NULL
*/
char* substring(char* str, int iPos);

/* charPosition - return the position of the first occurance of c in str.
* If c not present, return -1
*/
int charPosition(char* str, char c);

Answer 1

required functions

char* substring(char* str, int iPos)
{
    int i=0;
    while(str[i]!='\0')  /* string is terminated with '\0' in C. traverse string */
    {
        if(i == iPos)   /* if index is same as position passed to function then return address of that charcter*/
            return &str[i];
        
        i++;
    }
    return (char*)NULL;  /* if control comes out of the loop that means position exceeds length of string so 
    return NULL */
}

int charPosition(char* str, char c)
{
    int i=0;
    while(str[i]!='\0')  /* traverse string until '\0' is found */
    {
        if(str[i] == c)   /*if character at position i is equal to character to be found then return position*/
            return i;
            
        i++;
    }
    return -1;  /* if control comes out of loop that means character not found then return -1*/
}

complete program with driver code

#include <stdio.h>

char* substring(char* str, int iPos)
{
    int i=0;
    while(str[i]!='\0')  /* string is terminated with '\0' in C. traverse string */
    {
        if(i == iPos)   /* if index is same as position passed to function then return address of that charcter*/
            return &str[i];
        
        i++;
    }
    return (char*)NULL;  /* if control comes out of the loop that means position exceeds length of string so 
    return NULL */
}

int charPosition(char* str, char c)
{
    int i=0;
    while(str[i]!='\0')  /* traverse string until '\0' is found */
    {
        if(str[i] == c)   /*if character at position i is equal to character to be found then return position*/
            return i;
            
        i++;
    }
    return -1;  /* if control comes out of loop that means character not found then return -1*/
}

int main()
{
    
    char str[] ="Hello World";
    char *str1=substring(str,6);
    if(str1 != NULL)
        printf("\n%s",str1);
    else
        printf("\nposition is out of string length");
        
    str1 = substring(str,45);
    if(str1 != NULL)
        printf("\n%s",str1);
    else
        printf("\nposition is out of string length");
    
    int pos = charPosition(str,'l');
    if(pos==-1)
    {
        printf("\ncharacter not found");
    }
    else
    {
        printf("\n Character found at position %d ",pos);
    }
    
    pos = charPosition(str,'z');
    if(pos==-1)
    {
        printf("\ncharacter not found");
    }
    else
    {
        printf("\n Character found at position %d ",pos);
    }
    return 0;
}

output screenshot