Umbrella or Not

Asking a question before acting

Everything you have written so far ran straight down the page, every line, every time. That is fine for a banner. It is useless for advice, because advice depends on the situation.

if is how a program asks a question before it acts. Give it something to check inside parentheses, and a brace-enclosed block underneath runs only when the answer is true. Follow that with else and a second block, and you have covered the false case too. Exactly one of the two blocks runs. Never both, never neither.

C has no separate true/false type sitting underneath this. A condition is just an integer expression, and the rule is: zero is false, anything else is true. rain_chance >= umbrella_mark works out to 1 when it holds and 0 when it does not, and if treats those exactly the way you would hope.

One equals sign or two

The question itself goes between the parentheses, and it is usually a comparison — two values held up against each other:

• == asks whether two values are the same
• >= asks whether the left one is at least as big as the right one

That first one catches almost everybody once, because one equals sign and two equals signs are completely different instructions in C:

• = stores. rain_chance = 70; puts 70 into rain_chance. It is a command, and it changes something.
• == compares. rain_chance == 70 asks whether what is in there happens to be 70. It is a question, and on its own it changes nothing.

Unlike some languages, C will not stop you writing = inside an if — an assignment is a perfectly legal expression, and its value is whatever was just stored. if (rain_chance = 0) compiles, assigns 0 and then tests that 0, which is false, so the block quietly never runs; if (rain_chance = 5) compiles too, and because 5 is nonzero the block runs on every single pass. Most compilers will warn about this. Read the warning.

A worked example

A cinema till, not a weather report, so the answer to this exercise stays yours to write:

#include <stdio.h>

int main(void) {
    int age = 14;
    int adult_from = 18;

    printf("Age ...... %d\n", age);
    if (age >= adult_from) {
        printf("Ticket type: adult\n");
    } else {
        printf("Ticket type: child\n");
    }
    printf("Enjoy the film.\n");
    return 0;
}

Running that prints three lines:

Age ...... 14
Ticket type: child
Enjoy the film.

14 is not at least 18, so the first block is skipped over entirely and the second one runs. Only one ticket line ever appears. Notice the last line too: it sits outside both braces, so it prints either way. Change age to 30 and the middle line changes while the other two stay put.

Your turn

The editor holds a rain report. Two numbers sit at the top: rain_chance is today's forecast, and umbrella_mark is the point at which carrying an umbrella is worth the bother.

Run it before changing anything. The card prints, and then tells you to leave the umbrella at home on a day with a seventy percent chance of rain. That is because the test currently reads:

if (rain_chance == 0) {

which asks whether the chance is exactly zero — a question with nothing to do with umbrellas, and one whose answer today is no.

Replace that test with one asking whether rain_chance has reached umbrella_mark: at the mark or above it counts as take it. Everything else stays exactly as it is.

Then change rain_chance to 20 and run it again. The same program should now tell you to leave the umbrella at home. That second run is the real proof — it shows the program is reading the numbers rather than printing a fixed answer.

If something goes wrong

The likeliest slip is the equals sign. if (rain_chance = umbrella_mark) compiles — it is legal C — and the block then runs on every single pass, whatever the real forecast is, because it is testing the value it just stored rather than comparing anything. If the advice never changes no matter what you set rain_chance to, count the equals signs first.

Next likeliest is a stray semicolon straight after the closing parenthesis, if (...) ;. C reads that as an if whose body is the empty statement that does nothing, and the printed advice below runs unconditionally, looking exactly as if it belonged to the if.

Nothing here can break anything you cannot immediately fix. Read the diagnostic, find the line it names, and run it again.