Decisions in Java IF Statements

Transcription

1 Boolean Values & Variables In order to make decisions, Java uses the concept of true and false, which are boolean values. Just as is the case with other primitive data types, we can create boolean variables to hold these values. boolean readytoprogram = true; Boolean Expressions A boolean expression is similar to a mathematical expression, except that the result is true or false, rather than a numeric value. To create a boolean expression, we use the relational operators to compare the values of various data types, such as integers, floats, characters, or strings, using a relational expression. Relational Operator Meaning Example Result == is equal to 5 == 5 TRUE!= is not equal to 5!= 5 FALSE < is less than 3 < 7 TRUE <= is less than or equal to 4 <= 4 TRUE > is greater than 3 > 7 FALSE >= is greater than or equal to 7 >= 3 TRUE The following rules apply to the use of relational operators with different data types: 1. Values of any of the primitive numeric data types (e.g., int, float, and all their variations) can be used with any of the relational operators. 2. Boolean values can only be tested as equal to or not equal to. 3. Values of type char are ordered according to the Unicode encoding system. A character the occurs earlier in the system is less than a character that occurs later in the system. a) For alphabetic characters, this means that 'a' is less than 'z', and 'A' is less than 'Z', as expected. b) In the Unicode system, all uppercase letters occur earlier than all lowercase letters. Thus we get the relational ordering of: 'A' < 'B' < 'C' <... < 'Z' < 'a' < 'b' < 'c' <... <'z' c) Representing numbers as characters, such as when you type on a keyboard, keeps the same ordering, so that '0' < '1' < '2' <... < '9'. Page 1 of 6

2 Comparing Strings The relational operators used with the primitive data types should not be used to compare strings. Recall that a String variable does not actually contain the string, but only the location (address) of the string in memory. Thus any comparison between strings using relational operators would actually be comparing two addresses, rather than the String values. Java provides a number of methods for comparing strings, but for now we will introduce only the equals method. This is a String method, which means it can be called from any String variable. For example, given the declaration String s = "Same"; then the equals method would yield the following results: a) s.equals("same") will return true b) s.equals("same") will return false because 'S' is not equal to 's' c) s.equals("same ") will return false because of the spaces at the end of the word Boolean Operators It is possible to combine two or more boolean values, variables, or expressions into a more complicated boolean expression using the boolean operators. Unlike the relational operators, the boolean operators can only work on boolean values. You can use a relational operator to compare any data type, which forms a boolean expression. Multiple boolean expressions can be combined using boolean operators. The boolean operators are summarized in the following table. boolean value not p p AND q p OR q p q!p p && q p q TRUE TRUE FALSE TRUE TRUE TRUE FALSE FALSE FALSE TRUE FALSE TRUE TRUE FALSE TRUE FALSE FALSE TRUE FALSE FALSE Page 2 of 6

3 1.!p (not p) has the true/false value opposite to p. 2. p && q (p AND q) is true if and only if both p and q are both true. 3. p q (p OR q) is true if p is true, q is true, or both p and q are true. One Choice IF-THEN Statements if (<boolean expression>) <statements> The simplest decision is to choose between doing something, or doing nothing. In this general code outline, the <boolean expression> may be quite simple or very sophisticated. Similarly, the <statements> could be a single line of Java code, or a very complicated block of code. Nonetheless, there is a single decision here if the boolean expression evaluates to true, the statements will be executed. If the expression is false, no code from this block is executed. Two Choices IF-THEN-ELSE Statements Rather than doing nothing, it is far more common to use our decision to choose between two possible options one for true, one for false. The additional code, for the false condition, is placed within the block of the statement. This is the most commonly occurring decision statement. class IfDemo public static void main (String[] args) System.out.print("Please give one integer: "); int first = In.getInt(); System.out.print("and a second: "); int second = In.getInt(); if (first == second) System.out.println("The values are equal"); System.out.println("The values are not equal"); Page 3 of 6

4 Many Choices Nested IF Statements Decisions in Java IF Statements It is possible to make more than two choices with multiple if statements: if (x < 0) System.out.println("value is negative"); if (x > 0) System.out.println("value is positive"); if (x == 0) System.out.println("value is zero"); This code involves three tests, which may not seem significant now, but inefficient testing of conditions is one of the major sources of slow and inefficient programs. The following code improves the efficiency by reducing the code to a maximum of two comparisons using nesting. if (x < 0) System.out.println("value is negative"); if (x > 0) System.out.println("value is positive"); System.out.println("value is zero"); An alternate format is also available which has a more linear appearance than the nested code. if (x < 0) System.out.println("value is negative"); if (x > 0) System.out.println("value is positive"); // (x == 0) is the only remaining option System.out.println("value is zero"); Page 4 of 6

5 Exercises 1. Study this program and then answer the questions that follow it. Try it on paper first, and then code the program to check your answers. class ChipChoice public static void main (String [] args) System.out.println("Preferred flavour? (first letter only) "); char flavour = In.getChar(); System.out.println("Preferred style? (first letter only)"); char style = In.getChar(); System.out.print("The client preferred "); if (flavour == 'B') System.out.print("bar-b-que"); if (flavour == 'V') System.out.print("vinegar"); System.out.print("other"); if (style == 'C') System.out.print(", crinkled"); if (style == 'R') System.out.print(", regular"); System.out.println(" chips."); What would the program print given each set of data as input? (a) V C (b) B R (c) V T (d) B C (e) C V (f) v r 2. How would you modify the ChipChoice program so that it would accept both upper case and lower case letters in the responses from the user? Page 5 of 6

6 3. One way of giving a direction is to simply use one of the letters N, E, S, or W, indicating the nearest compass point. Another way is to give a bearing as a number from 0 to 359 (since 360 would be back to 0 again). A bearing of 0 corresponds to N, a bearing of 90 corresponds to E, and so on. Write a program that prompts the user for a bearing from 0 to 359 and prints the corresponding letter of he compass point nearest to the bearing. For example, input of 73 is between N and E, but it is closer to E, so the program should output E. Bearings exactly halfway between two compass points should produce either N or S (never E or W). 4. In the country of Rahmania, the cost of mailing a letter is 40 sinas for letters up to 30 grams. Between 30 g and 50 g, it is 55 sinas. Between 50 g and 100 g, it is 70 sinas. Over 100 g, it costs 70 sinas plus an additional 25 sinas for each additional 50 g (or part thereof). For example, 101 grams would cost = 95 sinas. 149 g would also cost 95 sinas, because both of these packages only use the first 50 g over 100 g. Write a program that prompts the user for a mass and then gives the cost of mailing a letter having that mass. Page 6 of 6