Operators, I/O and selection, in one page

1 · Integer division chops; equal ranks go left to right

"what does 2*3/4 - 3/4*2 print?"

7 / 2         // 3   (towards zero; -7/2 is -3)
2*3/4 - 3/4*2 // 6/4 - 0*2 = 1 - 0 = 1

Do * / % strictly left to right. 3/4 is already 0 before the *2.

2 · One double operand makes the operation real; a cast converts one value

averages · "the result should be a double" · (int)(…) questions

(double) sum / n     // real division: 7/2 → 3.5
(double) (sum / n)   // too late: 3.0
int x = 2.5;         // stores 2 (clang: "changes value from 2.5 to 2")

3 · % keeps the sign of the left operand

"negative even values" · odd/even tests

-7 % 2    // -1   so  x % 2 == 1  is FALSE for negative odd x
x % 2 != 0   // the safe "odd" test;  x % 2 == 0  is fine for evens

4 · Comparisons give 1 or 0; nonzero means true; ! is rank 1

!x < 5 · a < b < c · "which has higher precedence than %?"

!x < 5      // (!x) < 5: always 1, since !x is 0 or 1
3 < 2 < 1   // (3<2) < 1 = 0 < 1 = 1   (Apple clang 21 refuses to compile it)

Ladder, strongest first: unary ! ++ -- (type) sizeof › * / % › + - › << >> › < <= > >= › == != › & › && › || › ?: › = += › ,

5 · && and || short-circuit: a skipped side is never run

any ++/--/assignment on the right of && or ||

a || --b     // a nonzero → --b skipped, b unchanged
a && ++j     // a zero    → ++j skipped
!i && ++j || ++k    // means (!i && ++j) || ++k

6 · Single & is bitwise: both sides always run, and "true & true" can be 0

if ((y>=0) & (x=x+1)) · bit counting

1 & 4   // 0: 001 and 100 share no bits  (but 1 && 4 is 1)
6 & 3   // 2: 110 and 011 share the 2s bit
if (n & i) count++;  i = i << 1;   // counts 1-bits; 2273 → 5

The 2025 answer key read & as && twice: Q2b is 16 (key 14) and Q2h is 012348 (key 348). See Worked example 3.

7 · Postfix gives the old value, prefix the new; never change one variable twice

while (i++) · a-- && … · c = (a>=7) ? a++ : ++a

do { … } while (i++);   // tests the OLD i: 0 stops the loop, then i is 1
i = i++ + ++i;          // undefined behaviour (clang: "multiple unsequenced modifications to 'i'")

8 · Comma operator: evaluate all, keep the last

b = (a+1, c+1, 100, a+c)

int b = (a+1, c+1, 100, a+c);   // b = a + c

It's the weakest operator of all, so it needs the brackets. Without them the commas mean something else.

9 · = inside a condition assigns, and the value is what was stored

if (x = 0) · (x = x + 1) inside a test

if (x = 0) …   // x becomes 0; condition is 0 → always false

10 · Characters are numbers: '0'…'9' = 48…57, 'a'…'z' = 97…122, Enter = 10

"sum of the digits in a character stream" · "highest letter"

sum += c - '0';                 // '7' - '0' = 7
if (c >= '0' && c <= '9') …     // same as c >= 48 && c <= 57, but readable
while (c != '\n') …             // '\n' is 10

11 · An else belongs to the nearest unmatched if

"for which inputs does it print s2?" · unbraced nested ifs

if (a > b) if (b > c) s1; else s2;   // else pairs with (b > c)

Read the path to the target: s2 needs a > b and "not b > c", i.e. b <= c, equality included. The lesson has the decision-tree figure.

12 · c ? X : Y evaluates only the chosen branch; chains read like else-if

"rewrite using the conditional operator" · printf inside ?:

g = (s>=90)?'A' : (s>=80)?'B' : (s>=70)?'C' : (s>=60)?'D' : 'F';
(a==1) ? printf("One") : (a==2) ? printf("Two") : printf("Other");

13 · switch: jump to the matching label (else default, wherever it is), then fall through to break

missing breaks · default written first · "why does it print PositiveZero?"

switch (i) { case 2: i*=i; case 4: i*=i; default: i*=i; break; case 16: … }
// i = 2 → 4 → 16 → 256, stop at break.   i = 1 → default only → 1

14 · scanf matches its format like a pattern and stops at the first mismatch

field widths · literal characters · %c · type letters

%2d, %5f   // read AT MOST 2 / 5 characters (the '.' counts)
"%d-%d"    // the '-' must be typed exactly; "10 20" gives n=10, m untouched
" %c"      // %c takes the very next char, even a space; the leading space skips whitespace
%d int · %f float · %lf double · %Lf long double · %c char · %u unsigned

15 · printf returns the number of characters printed

"make if/else print HelloWorld" · printf inside conditions

if (!printf("Hello")) …   // prints Hello, value 5, !5 = 0 → the else runs
printf("%5.2f", 3.14159); // " 3.14"  (width 5, 2 decimals)   %.1f of 2.75 → 2.8