String character check - c

Question

i should realize two very similar functions but i am having problems.
I have to read the string "username", this string can only contain letters (upper and lower case) and spaces.
I have to read the string "key", this string can only contain letters (upper and lower case) and numbers.
If the guidelines are not followed, the user must be able to retrieve the input.
Unfortunately, I cannot use special libraries (only stdio and stdlib).
I realized this:
void checkString(char *i){
int cont;
do {
scanf("%s", i);
if (checkStrLen(6, 6, i) != 0) { //function that controls the size of the string (min,max,string)
for(cont=0; cont<6;){
if((i[cont]>='0' && i[cont]<='9')||
(i[cont]>='A' && i[cont]<='Z')||
(i[cont]>='a' && i[cont]<='z')){
cont++;
}else{
printf("Not valid character");
printf("Try again");
}
}
}else{
printf("\nToo large string");
printf("\nTry again");
}
}while(1);
}
I was thinking of doing something similar.
For the first problem I would replace (i[cont]>='0' && i[cont]<='9') with (i[cont]==' ').
the problem is that I don't understand how to get out of the for if I find a forbidden character during the loop.
I was thinking of using a break, but that would get me out of the whole function.
any advice?
PS how does the function look like? can it be okay or is it completely wrong?

Answer 1

I think the do while loop is not necessary here. do the scanf and get user input first then call checkString. Inside checkString keep your if else statement.
char checkString(char *i){
int cont;
if (checkStrLen(6, 6, i) != 0) { //function that controls the size of the string (min,max,string)
for(cont=0; cont<6;){
if((i[cont]>='0' && i[cont]<='9')||
(i[cont]>='A' && i[cont]<='Z')||
(i[cont]>='a' && i[cont]<='z')){
cont++;
}else{
printf("Not valid character");
printf("Try again");
return i;
}
}
}
else{
printf("\nToo large string");
printf("\nTry again");
}
}

Answer 2

#include <stdio.h>
#define MAXSIZE 100
#define SIZELIM 6
#define true 1
#define false 0
// Returns length of string
// If possible, use strlen() from <string.h> instead
int strlen(char *str) {
char i;
for (i = 0; str[i] != 0 && str[i] != '\n'; i++);
return i;
}
// Returns 1 if strings are equal
// If possible, use strcmp() from <string.h> instead
int streq(const char *x, const char *y) {
char chrx = 1, chry = 1, i;
for (i = 0;
chrx != 0 && chry != 0 && chrx == chry;
chrx = x[i], chry = y[i], i++);
return chrx == chry;
}
// Returns 1 if chr is number or letter
// If possible, use isalnum() from <ctype.h> instead
int isalnum(const char chr) {
return (chr >= '0' && chr <= '9' ||
chr >= 'A' && chr <= 'Z' ||
chr >= 'a' && chr <= 'z');
}
// Checks if string contains numbers and letters only
int isvalid(const char *str) {
int valid = true;
for (int i = 0; str[i] != 0 && str[i] != '\n'; i++) {
if (!isalnum(str[i])) {
valid = false;
break;
}
}
return valid;
}
// Main
int main(void) {
char str[MAXSIZE];
for (;;) {
printf("> ");
fgets(str, MAXSIZE, stdin);
if (streq(str, "quit\n"))
break;
if (strlen(str) > SIZELIM || !isvalid(str)) {
if (strlen(str) > SIZELIM)
puts("String too large");
else if (!isvalid(str))
puts("Not a valid string");
puts("Try again"); }
}
return 0;
}

Answer 3

You can code those functions that you cannot import:
int letters_and_spaces(char c)
{
return c == ' ' || C >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z';
}
int letters_and_numbers(char c)
{
return c >= '0' && c <= '9' || C >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z';
}
And to use scanf to read spaces you can't use %s. You could change to:
scanf("%100[^\n]*c", i);
BE CAREFUL: I've put 100, supposing i has enough space for that. It will read up to 100 characters (or as many as the number you put there) or until find the \n.