Programming in C Control structure I

Transcription

1 Programming in C Control structure I! Relational operators! Logical operators! Equal and not equal operators! Blocks & scope! Branch statements - if statement - if else statement - switch statement! Summary 1

2 Relational operators There are four binary operators which can be used for comparing values of arithmetic expressions: < less than <= less than or equal > greater than >= greater than or equal Since there is no boolean type in C++ the int type is used to represent true or false. A non-zero value (usually 1) denotes true, zero - false. k = 2 > 1; // assigns 1 (true) k = 2 < 1; // assigns 0 (false) k = 2 >= 2; // assigns 1 (true) k = 3.4 <= 2.4; // assigns 0 (false) Relational operators associate left to right! k = a < b < c; // k = (a < b) < c The operands of the relational operators can be expressions. k = (j + 1) >= (m - 72); 2

3 Logical operators The logical operators are:! negation && and or These operators all give a result of type int and value 0 or 1. The logical negation operator is an unary operator and like all unary operators associates right to left. The evaluation of all operands is guaranteed to be left to right. If the left operand of AND is zero right one is never evaluated!!! If the left operand of OR is non-zero then the right is never evaluated. 3

4 Equal and not equal operators The binary operators representing these operations are: == equal!= not equal Both operands are used to test operands of arithmetic type. The equality operator gives a result of one if both operands are identical and zero otherwise. The binary not equal operator takes arithmetic operands. The result is zero if the two operands are identical and one otherwise. int i = 0, j = 10, k; double z = 10.0, y = 2.0, z = 5.0; k = i ==j; // assigns 0 to k (i not equal j) k = z * y == x; // assigns either 0 or 1 to k k = z!= 5.0; // assigns 0 to k 4

5 Blocks and scope A pair of braces {} can be used to group statements, definitions and declarations into a compound statement or block. Any definitions made within a block are only valid within the block and hide definitions made for the same identifiers outside. Note that there is no semicolon after the second (terminating) brace of the block. Block can be empty!!! Blocks can be both nested and parallel. Some compilers issue helpful warnings when a definition hides a previous definition. The parts of a program where a particular identifier is valid (that is where is visible) are known as the scope of that identifier. 5

6 Branch statements: if statement The if statement takes the form if (expression) statement where the expression is any valid arithmetic expression. The expression is evaluated and if it is non-zero the statement is executed. if (i ==0) x = 100.0; // is assigned if i is zero It is of course possible for if statement to involve a block. if (!i) // if i equals zero { x = ; // these two statements y = 100.0; // will be executed } 6

7 Branch statements: if statement There is common error involving the equality operator which is notoriously difficult to detect. if (k = 0) x = 123.4; The code is syntactically correct but is equivalent to: k = 0; x = 123.4; whereas the programmer almost certainly meant to write if (k == 0) x = 123.4; Some programmers avoid such errors by writing any constant as first. if (0 == k) 7

8 Branch statements: if statement Another common error is to omit the braces for a compound statement following an if: if (!i) // if i equals zero x = ; // only first statement y = 100.0; // will be executed!!! Since non-zero and zero are used to represent true and false the readability of code can be improved by defining TRUE and FALSE identifiers: const int TRUE = 1; const int FALSE = 0; Notice the traditional use of upper case letters as to distinguish predefined constants. 8

9 Branch statements: if else statement The if else statement takes the form: if (expression_1) statement_1 else if (expression_2) statement_2 // more else ifs else statement_n If expression_1 is non-zero statement_1 is executed and control passed beyond statement_2. By contrast if expression_1 is zero expression_2 is tested. The sequence continues until one of the statements (which may be the last one) is executed. 9

10 Branch statements: if else statement Dangling else trap is quite common problem in C++. if (m == 0) if (n == 0) cout << Both m and n are zero\n ; else cout << m is non-zero\n ; The else is dangling. There is an ambiguity as to whether it is associated with the first or second if. In fact the dangling else is always really associated with the previous if. if (m == 0) { if (n == 0) cout << Both m and n are zero\n ; } else 10

11 Branch statements: switch statement The switch statement takes the general form: switch (expression) { case constant_1: statement_1; // more case-statement pairs default: last_statement; } Each statement_n should be finished by break which causes control to pass whatever follows the switch statement. It is the break statement which alters the flow of control rather than the case or default statements. If there is no break corresponding to a particular case statement then the flow of control is unchanged. 11

12 Summary! The relational operators are: <, <=, >, >=. They are binary operators, returning zero if the relation is false and one if it is true.! The negation operator! returns one if the operand is zero, otherwise the operator returns zero.! The logical AND operator && returns one if both operands are non-zero, otherwise the operator returns zero.! The logical OR operator returns zero if both operands are zero, otherwise the operator returns one.! The equality operator == returns one if both operands are equal and zero otherwise.! The not equal operator!= returns zero if both operands are equal and one otherwise.! A compound statement or block is denoted by pair of braces {}. 12