Putting uppercase letters taken from user and putting into an array

Question

In this code im trying to take a string from user and put the uppercase letters into a other array then print it. But im doing something horribly wrong i can tell, can someone help me with this code?

#include<string.h>

int main() {
   char arr[100],uppercaseletters[50];
   printf("Enter array:");
   scanf("%s",arr);
   strlen(arr);
   printf("The lenght of string is: %d\n",strlen(arr));
   for(int i=0;i<=strlen(arr);i++){
    if(arr[i]>='A' && arr[i]<='Z'){
        arr[i]+=uppercaseletters[50];
        printf("%s",uppercaseletters);
       }





   }

}

Answer 1

For starters according to the C Standard the function main without parameters shall be declared like

int main( void )

It is unclear why the array uppercaseletters has less elements than the array arr.

char arr[100],uppercaseletters[50];

The user can enter a string consisting only from upper case characters.

This statement

strlen(arr);

does not have an effect.

This statement

arr[i]+=uppercaseletters[50];

does not make sense. You have to fill the array uppercaseletters. Moreover the element uppercaseletters[50] does not exist because the valid range of indices is [0, 50).

In this statement

printf("%s",uppercaseletters);

you are trying to output a non-initialized array.

The program can look the following way.

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

int main(void) 
{
    enum { N = 100 };
    char s[N];

    printf( "Enter a string: " );

    fgets( s, sizeof( s ), stdin );

    size_t n = 0;

    for ( size_t i = 0; s[i] != '\0'; i++ )
    {
        if ( isupper( ( unsigned char )s[i] ) ) ++n;
    }

    char *upper_case_letters = malloc( n + 1 );

    n = 0;

    for ( size_t i = 0; s[i] != '\0'; i++ )
    {
        if ( isupper( ( unsigned char )s[i] ) )
        {
            upper_case_letters[n++] = s[i];
        }
    }

    upper_case_letters[n] = '\0';

    puts( upper_case_letters );

    free( upper_case_letters );

    return 0;
}

The program output might look for example like

Enter a string: Hello World!
HW

Without dynamically allocating an array for upper case letters the program can look the following way.

#include <stdio.h>
#include <ctype.h>

int main(void) 
{
    enum { N = 100 };
    char s[N];
    char upper_case_letters[N];

    printf( "Enter a string: " );

    fgets( s, sizeof( s ), stdin );

    size_t n = 0;

    for ( size_t i = 0; s[i] != '\0'; i++ )
    {
        if ( isupper( ( unsigned char )s[i] ) )
        {
            upper_case_letters[n++] = s[i];
        }
    }

    upper_case_letters[n] = '\0';

    puts( upper_case_letters );

    return 0;
}