Name Tag

What a variable is, in C

C will not let you make a variable without saying what kind of value it will hold. char name[] says two things at once: char is the type of value stored — one byte apiece, the type C uses for a run of text — and [] says there will be more than one of them, laid out in a row, with the compiler counting how many from whatever you initialise it with.

char name[] = "Sam";

That line makes an array of four bytes: the letters S, a, m, and one invisible byte with the value zero that marks where the text ends. From then on, writing name anywhere in your code hands that whole piece of text back to you, and %s inside a printf call is the placeholder that knows how to print it.

Think of a labelled shelf built to hold exactly this delivery. You stock it once. Anyone who reaches for it afterwards gets whatever is sitting there right now. Restock it with something else and every later reach finds the new thing — you never have to go and tell each printf call separately.

One detail decides everything here: the quotes.

• printf("name") prints the four letters n, a, m, e. Quotes mean this exact text.
• printf("%s", name) prints whatever name is holding. No quotes around the variable means look this name up.

A worked example

A different card, so the answer to this exercise is not given away:

#include <stdio.h>

int main(void) {
    char city[] = "Pune";

    printf("Weather report\n");
    printf("City: %s\n", city);
    printf("Have a good day in %s\n", city);
    return 0;
}

Running it prints:

Weather report
City: Pune
Have a good day in Pune

The value "Pune" is written once, at the top, and used on two later lines through %s. Change that first line to char city[] = "Nagpur"; and both of those lines say Nagpur instead. The printf calls themselves never need editing — they only ever ask %s to print whatever city is holding right now.

Your turn

The editor already prints a name-tag card: a border, a header, a greeting, an owner line and a signature. Run it first, before changing anything, and you will see the card appear with the placeholder in it.

Now find the short line near the top:

char name[] = "YOUR NAME";

Replace YOUR NAME with your own name, keeping the quotes, and press Run again.

Three lines of the card change at once, because all three printf calls ask the same variable, through %s, for its value. You edited one line and the card followed.

Try it a second time with a nickname. That is the whole idea of a variable, and it is worth feeling rather than just reading.

If something goes wrong

The most common slip is dropping the quotes off your value, as in char name[] = Sam;. The compiler then treats Sam as a name it is supposed to already know about, finds nothing declared under it, and reports an undeclared identifier before a single line runs.

The other one is quoting the variable by accident inside a printf call, as in printf(" Welcome, %s\n", "name");. Your card then greets someone called name on every row, because quotes turn it into ordinary text rather than a lookup.

Neither is damage. Read the line the message points at, put the quotes back where they belong, and run it again.