Statements So far, we ve used return statements and expression - PDF document

1/30/14 ¡ Statements • So far, we ’ ve used return statements and expression statements. Control Flow Statements • Most of C ’ s remaining statements fall into three categories: o Selection statements: if and switch o Iteration statements: while , do , and for Based on slides from K. N. King o Jump statements: break , continue , and goto . ( return also belongs in this category.) • Other C statements: Bryn Mawr College CS246 Programming Paradigm o Compound statement o Null statement 2 1 Logical Expressions Relational Operators • In many programming languages, an expression • C ’ s relational operators: such as i < j would have a special “ Boolean ” or < less than “ logical ” type. greater than > <= less than or equal to • In C, a comparison such as i < j yields an integer: >= greater than or equal to either 0 (false) or 1 (true). • These operators produce 0 (false) or 1 (true) when used in expressions. • The relational operators can be used to compare integers and floating-point numbers, with operands of mixed types allowed. 3 4 Relational Operators Relational Operators • The precedence of the relational operators is lower • Consider the expression than that of the arithmetic operators. i < j < k o For example, i + j < k - 1 means (i + j) < (k - o Is it legal? YES 1) . o What does it test? • The relational operators are left associative. Since the < operator is left associative, this expression is equivalent to (i < j) < k The 1 or 0 produced by i < j is then compared to k. • How to test whether j lies between i and k ? The correct expression is i < j && j < k . 5 6 1 ¡

1/30/14 ¡ Equality Operators Logical Operators • C provides two equality operators: • More complicated logical expressions can be built == equal to from simpler ones by using the logical operators: != not equal to ! logical negation • The equality operators are left associative and produce && logical and either 0 (false) or 1 (true) as their result. || logical or • The equality operators have lower precedence than the • The ! operator is unary, while && and || are relational operators, so the expression binary. i < j == j < k • The logical operators produce 0 or 1 as their result. is equivalent to • The logical operators treat any nonzero operand as (i < j) == (j < k) a true value and any zero operand as a false value. 7 8 Logical Operators Logical Operators • Behavior of the logical operators: • Both && and || perform “ short-circuit ” evaluation: they first evaluate the left operand, then the right one. ! expr has the value 1 if expr has the value 0. • If the value of the expression can be deduced from the expr1 && expr2 has the value 1 if the values of expr1 left operand alone, the right operand isn ’ t evaluated. and expr2 are both nonzero. • Example: expr1 || expr2 has the value 1 if either expr1 or expr2 (or (i != 0) && (j / i > 0) both) has a nonzero value. (i != 0) is evaluated first. If i isn ’ t equal to 0, then • In all other cases, these operators produce the value (j / i > 0) is evaluated. 0. • If i is 0, the entire expression must be false, so there ’ s no need to evaluate (j / i > 0) . Without short-circuit evaluation, division by zero would have occurred. 9 10 Logical Operators Logical Operators • Thanks to the short-circuit nature of the && and || • The ! operator has the same precedence as the operators, side effects in logical expressions may unary plus and minus operators. not always occur. • The precedence of && and || is lower than that • Example: of the relational and equality operators. i > 0 && ++j > 0 o For example, i < j && k == m means (i < j) If i > 0 is false, then ++j > 0 is not evaluated, so j && (k == m) . isn ’ t incremented. • The ! operator is right associative; && and || • The problem can be fixed by changing the are left associative. condition to ++j > 0 && i > 0 or, even better, by incrementing j separately. 11 12 2 ¡

1/30/14 ¡ The if Statement The if Statement • The if statement allows a program to choose • Confusing == (equality) with = (assignment) is between two alternatives by testing an expression. perhaps the most common C programming error. • In its simplest form, the if statement has the form • The statement if ( expression ) statement if (i == 0) … • When an if statement is executed, expression is tests whether i is equal to 0. evaluated; if its value is nonzero, statement is • The statement executed. if (i = 0) … • Example: assigns 0 to i , then tests whether the result is if (line_num == MAX_LINES) nonzero. line_num = 0; 13 14 The if Statement Compound Statements • Often the expression in an if statement will test • In the if statement template, notice that statement whether a variable falls within a range of values. is singular, not plural: • To test whether 0 ≤ i < n : if ( expression ) statement • To make an if statement control two or more if (0 <= i && i < n) … statements, use a compound statement. • To test the opposite condition ( i is outside the • A compound statement has the form range): { statements } if (i < 0 || i >= n) … • Putting braces around a group of statements forces the compiler to treat it as a single statement. 15 16 The else Clause The else Clause • It ’ s not unusual for if statements to be nested inside • An if statement may have an else clause: other if statements: if ( expression ) statement else statement if (i > j) • The statement that follows the word else is if (i > k) max = i; executed if the expression has the value 0. else max = k; • Example: else if (j > k) if (i > j) max = j; max = i; else max = k; else • Aligning each else with the matching if makes the max = j; nesting easier to see. 17 18 3 ¡

1/30/14 ¡ The else Clause Cascaded if Statements • To avoid confusion, don ’ t hesitate to add braces: • A “ cascaded ” if statement is often the best way to test a series of conditions, stopping as soon as one if (i > j) { of them is true. if (i > k) max = i; • Example: else if (n < 0) max = k; printf("n is less than 0\n"); } else { else if (j > k) if (n == 0) max = j; printf("n is equal to 0\n"); else else max = k; printf("n is greater than 0\n"); } 19 20 Cascaded if Statements Cascaded if Statements • Although the second if statement is nested inside • This layout avoids the problem of excessive the first, C programmers don ’ t usually indent it. indentation when the number of tests is large: • Instead, they align each else with the original if : if ( expression ) statement if (n < 0) else if ( expression ) printf("n is less than 0\n"); statement else if (n == 0) … printf("n is equal to 0\n"); else if ( expression ) else statement printf("n is greater than 0\n"); else statement 21 22 Program: Calculating a Broker ’ s Commission Program: Calculating a Broker ’ s Commission • When stocks are sold or purchased through a broker, the • The broker.c program asks the user to enter the broker ’ s commission often depends upon the value of the amount of the trade, then displays the amount of stocks traded. the commission: • Suppose that a broker charges the amounts shown in the following table: Enter value of trade: 30000 Transaction size Commission rate Commission: $166.00 Under $2,500 $30 + 1.7% • The heart of the program is a cascaded if $2,500–$6,250 $56 + 0.66% statement that determines which range the trade $6,250–$20,000 $76 + 0.34% $20,000–$50,000 $100 + 0.22% falls into. $50,000–$500,000 $155 + 0.11% Over $500,000 $255 + 0.09% • The minimum charge is $39. 23 24 4 ¡