Thread: i need a little help here

Code:

    scanf("%s",operation);
    scanf("%f",x);
    scanf("%f",y);

When reading anything but strings, you need to use an & on the variable to make it a pointer.

Code:

    scanf("%s",operation);
    scanf("%f",&x);
    scanf("%f",&y);

You don't need anything for strings, because a string is already a pointer -- an array of chars, usually. Nor do you need the &'s when displaying the values, because printf() doesn't need to set the value to anything else.

But there's more than that going on here. You're using %s to read a float, which you then compare with an int. Oops. Here are the rules:

• %f for floats.
• %i or %d for ints.
• %s for strings. (Remember, don't use an &.)
• %c for chars.

So make up your mind which one you want to use, and use it correctly.

Also, your indentation really needs some work.
http://cpwiki.sourceforge.net/Indentation
http://cpwiki.sourceforge.net/User:Elysia/Indentation

Reading strings with scanf is dangerous. Read this.

Eh, but it's not a string, it's actually a float! But wait, it's being compared with an int . . . .

Don't forget to check for divide-by-zero.

As a suggestion: read in the operator as a char. That way the user can enter '+' or '/' instead of 1 or 3 or something.

Yes, the OP is reading into a float. Of course, it could be done either way. Read a float or read a string. I'd avoid the read a string, as you'll have to read that link otherwise.

Originally Posted by Elysia

Yes, the OP is reading into a float. Of course, it could be done either way. Read a float or read a string. I'd avoid the read a string, as you'll have to read that link otherwise.

Or read a char, as I suggested. A float makes no sense as an operator, and while a string would work, most operators can be represented with a single character . . . +, -, *, /, and so on.

Another thing: using one getchar() at the end of your program wouldn't work most of the time, as there would usually be at least a newline in the input buffer, left over from the scanf(). (Unless you typed CTRL-Z or something strange.) I suggest you use something like this:

Code:

while(getchar() != '\n');
while(getchar() != '\n');

Also see:

• http://cpwiki.sourceforge.net/Pause_console
• http://cpwiki.sourceforge.net/Fflush

What else . . . oh, yes. Printing a space before every line probably looks rather weird when you run the program.

(@Elysia: you should make that page into a cpwiki article!)

Originally Posted by dwks

Another thing: using one getchar() at the end of your program wouldn't work most of the time, as there would usually be at least a newline in the input buffer, left over from the scanf(). (Unless you typed CTRL-Z or something strange.) I suggest you use something like this:

Or just skip scanf completely and use fgets and not have to worry about the input buffer at all.

(@Elysia: you should make that page into a cpwiki article!)

Yeah, I know. Lazyness and such
Still, I have a draft saved at the computer so I'll see if I can make it into a wiki article.

Originally Posted by Elysia

Or just skip scanf completely and use fgets and not have to worry about the input buffer at all.

Indeed -- but I think it's important to master scanf() first. Or at least understand when to use & and when not to, and what %f means. Because if you use fgets(), you may use sscanf() to get a number out of the string anyway -- or strtol(), which would be smart, or atoi(), which would not.

So many functions, just to get a number. I think scanf() is a good place to start.

Yeah, I know. Lazyness and such
Still, I have a draft saved at the computer so I'll see if I can make it into a wiki article.

You make it sound like you're not at "the computer". Aha! You have a direct connection to your brain . . . .

Originally Posted by dwks

Indeed -- but I think it's important to master scanf() first.

I say bah to scanf. Throw that stupid function into the fire.
It's typically unsafe since you never specify buffer size and can easily leave crap in the input buffer which there's no single, compact, easy way to clear.
The idea of input buffers is just stupid IMO.

Or at least understand when to use & and when not to, and what %f means.

You're bound to learn what & means since pointers are a must in C. You don't need scanf for that

So many functions, just to get a number. I think scanf() is a good place to start.

I believe it's better to use a function such sprintf or perhaps sscanf. They're better than scanf.

You make it sound like you're not at "the computer". Aha! You have a direct connection to your brain . . . .

Hmmm, indeed, do I?

I never said that scanf() was better than line-oriented input like fgets()+strtol(). I said that it was easier. And when someone's just learning how to program, do you think this

Code:

scanf("%f", &f);

or this inspires them more?

Code:

if(get_number(&f)) {
    perror("Error reading number");
    exit(1);
}

int get_number(double *num) {
    char buffer[BUFSIZ], *p, c;

    do {
        if(!fgets(buffer, sizeof buffer, stdin)) return 1;
        *num = strtod(buffer, &p);
    } while(p == buffer);
    return 0;
}

Well, it's not a question about inspiring or easier, is it? It's about doing the right thing.
If scanf would just take another argument that specifies buffer size, it would all be swell, but nope, it doesn't, for some reason.

It can, you know.

Code:

char buffer[10];
scanf("%9[^\n]", buffer);

That's direct from your article. And you're the one who wrote it! . . . .

Or a more general solution:

Code:

scanf("%*[^\n]", sizeof buffer, buffer);

Yes, I mean it can, but what sort of bizzare and newbie-unfriendly syntax is that?
It's better to just do (if it worked),

Code:

scanf("%s", sizeof(buffer), buffer);

But it doesn't work! So you have to apply petty syntax tricks to make it work.
And it doesn't solve the input buffer problem either which are the two reasons that makes it so bad in my eyes.