CSCI 125 & 161 / ENGR 144 Lecture 8

CSCI 125 & 161 / ENGR 144 Lecture 8 Martin van Bommel

Example 1 • “x is not equal to either 2 or 3” if (x != 2 || x != 3) • No, it should be if (!(x == 2 || x == 3)) • or if (x != 2 && x != 3)

Example 2 • “x is in the range from 0 to 10 exclusive” if (0 < x < 10) • No, it should be if (0 < x && x < 10)

Example • Leap year every fourth year, except centuries, then just every fourth century • year is divisible by 4 but not by 100, or • year is divisible by 400 • Try ((y % 4 == 0) && (y % 100 != 0)) || (y % 400 == 0)

Bad Example if (x < 5) cout << ”A”; if (x > 5) cout << ”B”; else cout << ”C”;

If Blocking Rule • For any if statement that requires (1) more than a single line, or (2) an else clause, always use braces to enclose in a separate block the statements under the control of the if statement .

Single-line if statement if (condition) statement where: condition is the Boolean value to test statement is a single statement to be executed if condition is true

Multiline if statement if (condition) { statements; } where condition is the Boolean value to test statements is a block of statements to be executed if condition is true

if-else statements if (condition) { statementsT ; } else { statementsF ; }

Cascading if statement if (condition1) { statements1 ; } else if (condition2) { statements2; . . . } else { statementsnone }

Crafting a Program • Use comments to tell readers what they need to know, including difficult code • Use indentation to show bodies of loops • Use meaningful names • Use convention for names - lastNumber • Use standard idioms when appropriate • Avoid unnecessary complexity

Designing for Change • #define construct - symbolic constants #define symbol value • symbol - name for symbolic constant • value - replaces symbol in precompilation • Why? Easier to modify program

Named Constants • A variable whose content cannot be changed while program is running const double PI = 3.14159; • Variable still has data type and memory location, but is read-only

Simple Statements • Expression followed by semicolon • Assignments total = n1 + n2; • Function calls cout << ”Hello.\n”; • Useless statements n1 + n2;

Embedded Assignments • Assignment expression can be used as part of a larger expression in a statement • Its value is the value assigned z = (x = 6) + y; • x is assigned value 6, then z assigned 6 + y • Difficult to read • Used rarely and only when makes sense

Multiple Assignments • Embedded assignments useful to set several variables to the same value n1 = n2 = n3 = 0; • Assignment operator evaluated right to left • Avoid mixed types; e.g. double d and int i d = i = 1.5; • Assigns i value 1, thus 1 assigned to d

Math Library <cmath> Functions for performing math operations abs(x)absolute value of argument sqrt(x)square root of argument pow(y, x)yx sin(x)sine (argument in radians) cos(x) cosine (argument in radians) tan(x) tangent (argument in radians) log(x) natural logarithm exp(x) ex

CSCI 125 & 161 / ENGR 144 Lecture 8