Reading elements from a string in C

Question

I am trying to print the first letter of a word at the position entered by the user. If there is a whitespace, then it will just stop searching. There is a warming message concerning funtion gets(). I have problems with the pointers and funtion, it just does not return the letter. I think the porblem might be on the while loop. Function firstLetter prints the first letter of a word. For instance, if the user enters index 5 of a previously entered string, and in str[5] corresponds to the 'e' of the word 'ice-cream' then the firstLetter will search for the first letter of that word, in this case it will return 'i'.

#include <stdio.h>
#include <string.h>


int firstLetter(int , char *);

int main()
{
    char str[200];
    printf("Insert line: ");
    gets(str);
    
    int pstn;
    scanf("%i,&pstn");
    
    firstLetter(pstn, str);
    
    
    return 0;
}

int firstLetter(int i, char *str)
{
    if(str[i]=' ')
    {
        printf("No letters at this position");
    }
    
    else
    {
        while(str[i]!=' ')
        {
            i--;
        }
        printf("First letter: %c ",str[i+1]);
    }
    return 0;
}

Answer 1

For starters in this line

scanf("%i,&pstn");

you have a typo. At least you need to rewrite it like

scanf("%i",&pstn);

Also within the function firstLetter there is also a typo in the if statement

if(str[i]=' ')

where you are using the assignment operator = instead of the comparison operator ==.

The function gets is unsafe and is not supported by the C Standard.

Instead use the function fgets

fgets( str, sizeof( str ), stdin );

The variable pstn should have the unsigned integer type size_t. Otherwise the user can enter a negative number.

Also you need to check that the entered position is not greater than the length of the string.

For example

size_t pstn = 0;
scanf("%zu", &pstn);

if ( pstn < strlen( str ) ) firstLetter(pstn, str);

The function parameter should have the qualifier const because the passed string is not changed within the function

Also the function has a bug because in this while loop

    while(str[i]!=' ')
    {
        i--;
    }

there is no check whether i is less than 0.

Also the return type int of the function is useless.

The function can be declared and defined the following way

void firstLetter( const char *str, size_t i )
{
    if ( str[i] == ' ' )
    {
        printf("No letters at this position");
    }
    else
    {
        while( i != 0 && str[i-1] != ' ' )
        {
            i--;
        }

        printf("First letter: %c\n", str[i] );
    }
}

So in main the function is called like

size_t pstn = 0;
scanf("%zu", &pstn);

if ( pstn < strlen( str ) ) firstLetter( str, pstn );

Or you can make the check that the specified position in the string is less than the length of the string within the function as for example

void firstLetter( const char *str, size_t i )
{
    if ( !( i < strlen( str ) ) || str[i] == ' ' )
    {
        printf("No letters at this position");
    }
    else
    {
        while( i != 0 && str[i-1] != ' ' )
        {
            i--;
        }

        printf("First letter: %c\n", str[i] );
    }
}

In this case the function is called like

size_t pstn = 0;
scanf("%zu", &pstn);

firstLetter( str, pstn );

Instead of the comparison with the space character ' ' you could compare with the space character ' ' and the tab character '\t' using the standard C function isblank declared in the header <ctype.h>. For example

#include <string.h>
#include <ctype.h>

void firstLetter( const char *str, size_t i )
{
    if ( !( i < strlen( str ) ) || isblank( ( unsigned char )str[i] ) )
    {
        printf("No letters at this position");
    }
    else
    {
        while( i != 0 && !isblank( ( unsigned char )str[i-1] ) )
        {
            i--;
        }

        printf("First letter: %c\n", str[i] );
    }
}