What's the better way to check if input is string? - c

Question

I'm making a program in which i ask for the username name, and i'd like to only accept strings with valid characters only (alphabetic).
I found that i can either use
do{
//since scanf returns the number of currectly input
if(scanf("%s", &name) == 1)
break;
else printf("Please enter a valid name.\n);
}while(1);
or
do{
check = 0;
scanf("%s", &name);
for(i=0; i<strlen(name; i++){
//since isalpha() returns != 0 if it's a letter
if(isalpha(name[i]) == 0){
printf("Invalid character. Please enter a valid name.\n");
check = 1;
break;
}
}
}while(check == 1);
But i'm not sure if any of those work, and what would be better to check if there isn't anything except alphabetic letters.
Also though about making all input letters (after this verification) on lower case and make the first letter upper case with
//all to lower except the first letter
for(i=1; i<strlen(name); i++){
name[i] = tolower(name[i]);
}
//first letter to upper
name[0] = toupper(name[i]);
x=1;
while(name[x] != '\0'){
//if the letter before is a white space, even the first letter, it should place the first letter of a name upper
if(name[x-1] == ' ')
name[x] = toupper(name[x]);
x++;
}
Would this work?

Answer 1

if(scanf("%s", &name) reads in all non-white-space, not just letters, into name and does not return if input is only "\n".
if(isalpha(name[i]) == 0){ loop is not bad, but scanf("%s", &name) still does not return if input is only "\n" or just white-space.
for(i=1; i<strlen(name); i++) name[i] = tolower(name[i]) works to make all following letters lower case, but if inefficient as code repeatedly calculates the string length.
Separate reading data and parsing data. Use fgets() to read the data and various code to test the data for correctness.
char buf[200];
fgets(buf, sizeof buf, stdin);
int n = 0;
// Skip leading white-space
// Look for A-Z, a-z or space (like a space between first & last)
// Skip white-space like \n
// Save into 'n' the current scan position
sscanf(buf, " %*[A-Za-z ] %n", &n);
if (n > 0 && buf[n] == '\0') Success(); // #user3121023
Should code need to rid buf of a potential trailing "\n", suggest:
buf[strcspn(buf, "\n")] = 0;

Answer 2

Let's look at each option.
First option:
do {
//since scanf returns the number of currectly input
if(scanf("%s", &name) == 1)
break;
else printf("Please enter a valid name.\n");
} while(1);
This won't quite work the way you expect. First off, what exactly is name? I'm almost sure that you want scanf("%s", name) instead (name instead of &name), unless you declared it as char name;, which would be catastrophic anyway.
Anyway, the problem I see with this approach is that you don't really validate the string. Read the man page section about %s:
s - Matches a sequence of non-white-space characters; the next pointer
must be a pointer to character array that is long enough to hold
the input sequence and the terminating null byte ('\0'), which is
added automatically. The input string stops at white space or at the
maximum field width, whichever occurs first.
Nothing says that the string is composed of alphabetic characters only.
Second option:
do{
check = 0;
scanf("%s", &name);
for(i=0; i<strlen(name); i++){
//since isalpha() returns != 0 if it's a letter
if(isalpha(name[i]) == 0){
printf("Invalid character. Please enter a valid name.\n");
check = 1;
break;
}
}
}while(check == 1);
Again, you probably want name rather than &name. You also shouldn't be calling strlen() in the for loop condition, because it's inefficient (strlen() is O(n)). A smart compiler may optimize it away, but it's hard for the compiler to know when it is safe to do so. Just call strlen() before the loop and store the result in a variable.
isalpha() expects an integer as an argument, which is expected to be either EOF or an unsigned char converted to int. Again, you don't show the declaration for name, but assuming that it is a character array, you should cast name[i] to unsigned char before calling isalpha(), so that you don't get any sign extension surprises:
if (isalpha((unsigned char) name[i]) == 0) { /* ... */ }
In fact, gcc nowadays will most likely give you a warning if you call any of the ctype family macros / functions with a plain char. The macros are deliberately written in such a way that a warning is shown, precisely because this is a common mistake. It is implementation-defined whether a plain char is signed or unsigned. You would get problems in a platform with signed chars because of sign extension (this is because typically, things like isalpha() are implemented using lookup tables, and extending the sign yields a negative number that would index the lookup table with a negative index - Oops!)
Other than this, this approach seems ok to me.
A third, maybe better option:
Since you mentioned fgets(), I think you could do this easily by combining fgets() with sscanf(). First, you read a line with fgets(). Then, you use sscanf() to match a string consisting of only characters in the range [a-zA-Z]. This can be done with the format specifier %[a-zA-Z]s. Then, you just have to check if this matched the entire line. Here's a working program:
#include <stdio.h>
#include <string.h>
int main(void) {
static char buf[512];
static char name[512];
int is_valid = 0;
while (!is_valid) {
fgets(buf, sizeof(buf), stdin);
size_t line_len = strlen(buf);
if (line_len > 0 && buf[line_len-1] == '\n') {
buf[line_len-1] = '\0';
line_len--;
}
int n = 0;
if (sscanf(buf, " %[a-zA-Z] %n", name, &n) == 1 && buf[n] == '\0') {
is_valid = 1;
} else {
printf("Please enter a valid name.\n");
}
}
printf("Name: %s\n", buf);
return 0;
}
Make sure your buffers are large enough; this code is vulnerable to buffer overflow for arbitrarily long names / lines.
Now let's see the code to make the first letter upper case:
//all to lower except the first letter
for(i=1; i<strlen(name); i++){
name[i] = tolower(name[i]);
}
//first letter to upper
name[0] = toupper(name[i]);
x=1;
while(name[x] != '\0'){
//if the letter before is a white space, even the first letter, it should place the first letter of a name upper
if(name[x-1] == ' ')
name[x] = toupper(name[x]);
x++;
}
Again, remove strlen() from the loop condition. toupper() and tolower() also expect an int as an argument representing either EOF or an unsigned char converted to int. You should cast it to unsigned char to avoid problems with possible sign extension, as I said earlier with the other example.
This is wrong:
//first letter to upper
name[0] = toupper(name[i]);
It should be:
//first letter to upper
name[0] = toupper(name[0]);
(The argument to toupper() is name[0], not name[i]).
Finally, this is useless:
x=1;
while(name[x] != '\0'){
//if the letter before is a white space, even the first letter, it should place the first letter of a name upper
if(name[x-1] == ' ')
name[x] = toupper(name[x]);
x++;
}
%s will never give you a string with whitespaces (refer to the manpage quote I pasted above).

Answer 3

Assuming that you want your name to have only characters a through z or A through Z then you could use this function
//Returns 1 if non alphabetic character is found, 0 otherwise
int NonAlphaCharsFound(char *name)
{
int FoundNonChar = 0;
int i, nameLength;
nameLength = strlen(name);
for(i = 0; i < nameLength; i++)
{
if((name[i] >= 'a' && name[i] <= 'z') || (name[i] >= 'A' && name[i] <= 'Z') || name[i] == ' ')
{
//do nothing if it's an alphabect character
//name[i] == ' ' is to allow for spaces if you want spaces in the name
}
else
{
FoundNonChar = 1;
break;
}
}
return FoundNonChar;
}