Thread: Simple string problem

I would like to create a program where a user inputs their first and last name and the program say it to the user, but my program is only showing the first name.

Any help is greatly appreciated.

Code:

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

int main()
{
    char userfirst[] = "";
    char userlast[] = "";
    
    printf("Please enter in your first name and last name:\n");
    scanf("%s,%s", userfirst, userlast);
    
    printf("Is your first and last name i%s?\n", strcat(userfirst, userlast));
    
    getchar();
    
    return 0;   
}

Your arrays are one byte long, which is only long enough to hold an empty string. C does not have dynamically expanding arrays/strings like some languages, nor will it tell you when you've overflowed a buffer, which will happen here.

There's no simple way to do proper text input in C. The quick fix to your problem is to do something like:

Code:

char userfirst[32], userlast[32];

Now you have room for 31 characters (plus the terminating null character which all strings require). You can still overflow the buffers, but at least this is a start.

Next, you'll want to use "%s%s", not "%s,%s". The comma is not a special separator here. Instead, it tries to read a literal comma. The problem is that %s reads all non-whitespace characters, so it will read a comma, too. Thus whenever %s stops reading characters, you can be guaranteed the next character will not be a comma.

You can get fancy and use "%31s%31s"; the “31”s tell scanf() to read no more than 31 characters. This will prevent buffer overflows, but if a person's name is longer than 31 characters, you'll have other issues to deal with. This is what I mean by text input in C being difficult.

Finally, strcat(), like so many other functions in C, will not do any buffer overflow checking. You have to be sure there's enough space in the target before you tack on the other string. You could do this by making sure the first array is at least twice the length of the second, although you'd probably be better off with:

Code:

printf("%s %s\n", userfirst, usersecond);

You first need to allocate some memory for your arrays.

Code:

    char userfirst[] = "";
    char userlast[] = "";

This is not allocating any memory for these variables.

Jim

Thanks a lot cas, jimblumberg and itsme86!

Originally Posted by benrogers

I would like to create a program where a user inputs their first and last name and the program say it to the user, but my program is only showing the first name.

Any help is greatly appreciated.

Code:

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

int main()
{
    char userfirst[] = "";
    char userlast[] = "";
    
    printf("Please enter in your first name and last name:\n");
    scanf("%s,%s", userfirst, userlast);
    
    printf("Is your first and last name i%s?\n", strcat(userfirst, userlast));
    
    getchar();
    
    return 0;   
}

First off that is not the way to declare stings. Since you used "" you have created a string that will hold exactly 0 characters. Try something like ... char userfirst[16];

Second you have a , in the formatting string for scanf... this will require the user to enter Bill,Jones which is rather unnatural. Just have a space between the two string catchers so the person can enter Bill Jones which is very natural.

Third you can't use strcat where you have it. This has to be a separate operation done before you display the concactinated names... And be sure that userfirst has enough space to hold both the first and last names anyone is likely to enter... char userfirst[128]; or so.

Finally you could have printed the first and last names without using strcat... just use the %s string catcher twice in your printf() statement.