'toupper' and 'tolower' have no effect on char array in C - c

Question

I am creating a function that creates a username. The user is asked for first name then last name. The last name is concatenated to the first letter of the first name. Also, any upper case letter should be transformed into lowercase letters. So input 'John DoE' should equal 'jdoe'.
Another function counts the number of lowercase letters in the string "THiS SentENCE HAS SOMe LoWEr CASE ChARAcTERs". Lower case letters are then converted to upper case and the string is printed.
The first function will not convert the uppercase letters to lowercase. The second function does not even recognize the ASCII value of any character.
I've tried using pointers for the two functions but there was no change in output.
void createUsername()
{
int j = 1;
char firstName[15], lastName[15], userName[20];
printf("Enter your first name : ");
fgets(firstName, 15, stdin);
firstName[strlen(firstName) - 1] = '\0';
printf("Enter your last name : ");
fgets(lastName, 15, stdin);
lastName[strlen(lastName) - 1] = '\0';
userName[0] = tolower(firstName[0]);
for (int i = 0, j; lastName[i] != '\0'; i++, j++)
userName[j] = tolower(lastName[i]);
userName[j+1] = '\0';
printf("User name : %s", userName);
}
void lowerToUpperCase()
{
char sentence[] = "THiS SentENCE HAS SOMe LoWEr CASE ChARAcTERs";
int lowerCases = 0;
for (int i = 0; sentence[i] != '\0'; i++)
{
if ((sentence[i] >= 97) && (sentence[i] <= 122))
{
lowerCases++;
sentence[i] = toupper(sentence[i]);
}
}
printf("Number of lower case characters= %d \n", lowerCases);
printf("Upper case sentence : %s \n", sentence);
}
expected output of Joe SMITH should be jsmith. The number of lower case letters should be 10 and the sentence should print all uppercase.
The actual output gives JSMITH, the number of lowercase letters is 0 and the sentence prints the original.

Answer 1

Lets start with simple things and we will go from there:
You can easily compare a with sentence[i]. Do this:
if ((sentence[i] >= 'a') && (sentence[i] <= 'z'))
{
lowerCases++;
sentence[i] = toupper(sentence[i]);
}
than comes this bit:
i < lastName[i] != '\0'
What the hell on earth is this? If you take a look at this page:
https://en.cppreference.com/w/c/language/operator_precedence
you will see that < has precedence over !=.
EDIT:
for (int i = 0, j; lastName[i] != '\0'; i++, j++)
j is reinitialized here as local variable for for() loop so it might get 0 but in my case it always got some garbage and setfault. So put this:
for (int i = 0; lastName[i] != '\0'; i++, j++)
and you will be set to go.

Answer 2

Apart from the problems mentioned in other answers, you also have problems with memory overwrites. Typing names longer than the buffers cause great pain in production. Better allocate enough memory on the fly to accommodate the names.
#include <ctype.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void createUsername() {
printf("Enter your first name : ");
char *firstName;
errno = 0;
int n = scanf("%ms", &firstName);
if (n != 1) {
perror("scanf");
exit(EXIT_FAILURE);
}
printf("Enter your last name : ");
char *lastName;
errno = 0;
n = scanf("%ms", &lastName);
if (n != 1) {
perror("scanf");
free(firstName);
exit(EXIT_FAILURE);
}
char *userName = (char *) malloc(1u + strlen(lastName) + 1u);
userName[0] = tolower(firstName[0]);
char *userNameIter = userName + 1;
char *lastNameIter = lastName;
while ((*userNameIter = tolower((unsigned char) *lastNameIter))) {
userNameIter++;
lastNameIter++;
}
printf("User name : %s\n", userName);
free(userName);
free(lastName);
free(firstName);
}
void lowerToUpperCase() {
char sentence[] = "THiS SentENCE HAS SOMe LoWEr CASE ChARAcTERs";
int lowerCases = 0;
for (int i = 0;; i++) {
unsigned char const ch = (unsigned char) sentence[i];
if (islower(ch)) {
lowerCases++;
sentence[i] = toupper(ch);
} else if (ch == '\0') {
break;
}
}
printf("Number of lower case characters = %d\n", lowerCases);
printf("Upper case sentence : %s\n", sentence);
}