Decisions and loops
Programs get interesting when they can choose and repeat.
Choosing uses if, else if and else. The condition goes in parentheses:
if (score >= 90) {
printf("A\n");
} else if (score >= 50) {
printf("pass\n");
} else {
printf("try again\n");
}Here is a C fact worth knowing: there was no built-in boolean type in early C. A condition is just a number. 0 means false, anything else means true. Comparisons like x > 3 produce the int 1 or 0. (<stdbool.h> gives you bool, true and false as friendlier names.)
That is why this classic bug compiles happily:
if (x = 5) // assigns 5, which is nonzero, so always true!You wanted ==. Many compilers will warn you. Listen to them.
Repeating uses loops. A for loop packs start, condition and step into one line:
for (int i = 1; i <= 5; i++) {
printf("%d\n", i);
}A while loop just keeps going while its condition is true, which suits "I do not know how many times yet" situations. break leaves a loop early, continue skips to the next round.
The % operator gives the remainder of a division: 10 % 3 is 1. If n % 2 == 0, n is even. You will use this constantly.
Try it yourself
Edit it. Break it. Run it again.#include <stdio.h>
int main(void) {
int total = 0;
int n = 1;
while (total < 20) {
total = total + n;
printf("added %d, total is %d\n", n, total);
n++;
}
if (total % 2 == 0) {
printf("%d is even\n", total);
} else {
printf("%d is odd\n", total);
}
return 0;
}
Your turn
Type it yourself. That is the whole trick.1FizzBuzz
For each number from 1 to 15, print Fizz if it divides by 3, Buzz if it divides by 5, FizzBuzz if it divides by both, and otherwise the number itself. One per line.
#include <stdio.h>
int main(void) {
for (int i = 1; i <= 15; i++) {
printf("%d\n", i);
}
return 0;
}
2Count down from input
The program reads a number n with scanf. Print a countdown from n to 1, one number per line, then print Liftoff!. With the input below, the output is:
54321Liftoff!
The checker feeds this input to your program.
#include <stdio.h>
int main(void) {
int n;
scanf("%d", &n);
// count down here
return 0;
}