CS156: Enter Matters

The Enter Key Matters

The Enter key (also called newline, or return) matters. When you type something using the keyboard, you have to press Enter at the end.

We tend to ignore Enter, because it’s not what we’re interested in. However, it’s still there. We have to deal with it.

The Straightforward Method

When reading characters, make sure that you’ve accounted for Enter:

char cmd;           // single-character command
char enter;         // dump the enter key here
double value;

printf("Enter a command: ");
scanf("%c%c", &cmd, &enter);
printf("Now a number: ");
scanf("%lf%c", &value, &enter);

This reads the command into cmd, and reads Enter into the variable called enter, where it is ignored.

Sometimes You Can Ignore Enter

If all that your program reads are numbers, then you don’t have to worry about Enter. That’s because scanf is smart when reading numbers, and skips over leading whitespace (spaces, tabs, and the Enter key).

However, if your program is reading both characters and numbers, then you have to deal with Enter.

This Makes it a Bit Easier

Here’s how to avoid having an enter variable:

char cmd;           // single-character command

printf("Enter a command: ");
scanf("%c%*c", &cmd);

The * in %*c means “Don’t save that character that we read—throw it away.”
This way, we don’t need an enter variable, because we’re not saving the Enter key anywhere.

Spaces matter, too

We tend to think that spaces don’t matter.
Well, sometimes they do, sometimes they don’t.
Spaces matter in a scanf format string.
A space in a scanf format string means “skip whitespace”.
- space
- tab
- enter (alias newline)
A space before %d, %f, or %s, is redundant, but ok.
- %d, %f, and %s automatically skip leading whitespace.
- %c does not.

Yet Another Way to Do It

Here’s another technique:

char cmd;
printf("Enter a command: ");
scanf(" %c", &cmd);

Did you notice the space before the %c?
That skipped any pending whitespace (including enter keys) before the %c.

Don’t Get Space-Happy

Consider this poor code:

char cmd;
printf("Enter a command: ");
scanf(" %c ", &cmd);                    // BAD CODE

The leading space will consume any whitespace before the command.
The trailing space will consume any whitespace after the command.
- “any whitespace” means any amount of whitespace—not just a single character.
- How is scanf supposed to know how much whitespace you’re going to type?

CS156: Intro to C, Part I

Spring 2018

Enter Matters