Thread: Help in java code

I'm learning data structure from "Introduction to Java Programming Liang" and here are the codes from Liang's book but I don't understand. Please help me out.
From line 38 to line 57 there are two conditions:
(this program asked to calculate a string when your entered on google searchlike (3+2)-3+5*3/3 well, something like that)

else if (token.charAt(0) == '+' || token.charAt(0) == '-')

and the other one is :

else if (token.charAt(0) == '*' || token.charAt(0) == '/')

What is the purpose of that split? Why didn't he put them all like this:

if (token.charAt(0) == '+' || token.charAt(0) || token.charAt(0) == '*' || token.charAt(0) == '/' == '-')

and why in the while loop, the first one we need 4 conditions to end the loop
(operatorStack.peek() == '+' ||
operatorStack.peek() == '-' ||
operatorStack.peek() == '*' ||
operatorStack.peek() == '/')) {

but on the second "if else" there were only two
operatorStack.peek() == '*' ||
operatorStack.peek() == '/'))

Here is the complete codes from Liang's book



1 import java.util.Stack;
2
3 public class EvaluateExpression {
4 public static void main(String[] args) {
5 // Check number of arguments passed
6 if (args.length != 1) {
7 System.out.println(
8 "Usage: java EvaluateExpression \"expression\"");
9 System.exit(1);
10 }
11
12 try {
13 System.out.println(evaluateExpression(args[0]));
14 }
15 catch (Exception ex) {
16 System.out.println("Wrong expression: " + args[0]);
17 }
18 }
19
20 /** Evaluate an expression */
21 public static int evaluateExpression(String expression) {
22 // Create operandStack to store operands
23 Stack<Integer> operandStack = new Stack<Integer>();
24
25 // Create operatorStack to store operators
26 Stack<Character> operatorStack = new Stack<Character>();
27
28 // Insert blanks around (, ), +, -, /, and *
29 expression = insertBlanks(expression);
30
31 // Extract operands and operators
32 String[] tokens = expression.split(" ");
33
34 // Phase 1: Scan tokens
35 for (String token: tokens) {
36 if (token.length() == 0) // Blank space
37 continue; // Back to the while loop to extract the next token
38 else if (token.charAt(0) == '+' || token.charAt(0) == '-') {
39 // Process all +, -, *, / in the top of the operator stack
40 while (!operatorStack.isEmpty() &&
41 (operatorStack.peek() == '+' ||
42 operatorStack.peek() == '-' ||
43 operatorStack.peek() == '*' ||
44 operatorStack.peek() == '/')) {
45 processAnOperator(operandStack, operatorStack);
46 }
47
48 // Push the + or - operator into the operator stack
49 operatorStack.push(token.charAt(0));
50 }
51 else if (token.charAt(0) == '*' || token.charAt(0) == '/') {
52 // Process all *, / in the top of the operator stack
53 while (!operatorStack.isEmpty() &&
54 (operatorStack.peek() == '*' ||
55 operatorStack.peek() == '/')) {
56 processAnOperator(operandStack, operatorStack);
57 }
58
59 // Push the * or / operator into the operator stack
60 operatorStack.push(token.charAt(0));
61 }
62 else if (token.trim().charAt(0) == '(') {
63 operatorStack.push('('); // Push '(' to stack
64 }
65 else if (token.trim().charAt(0) == ')') {
66 // Process all the operators in the stack until seeing '('
67 while (operatorStack.peek() != '(') {
68 processAnOperator(operandStack, operatorStack);
69 }
70
71 operatorStack.pop(); // Pop the '(' symbol from the stack
72 }
73 else { // An operand scanned
74 // Push an operand to the stack
75 operandStack.push(new Integer(token));
76 }
77 }
78
79 // Phase 2: process all the remaining operators in the stack
80 while (!operatorStack.isEmpty()) {
81 processAnOperator(operandStack, operatorStack);
82 }
83
84 // Return the result
85 return operandStack.pop();
86 }
87
88 /** Process one operator: Take an operator from operatorStack and
89 * apply it on the operands in the operandStack */
90 public static void processAnOperator(
91 Stack<Integer> operandStack, Stack<Character> operatorStack) {
92 char op = operatorStack.pop();
93 int op1 = operandStack.pop();
94 int op2 = operandStack.pop();
95 if (op == '+')
96 operandStack.push(op2 + op1);
97 else if (op == '-')
98 operandStack.push(op2 - op1);
99 else if (op == '*')
100 operandStack.push(op2 * op1);
101 else if (op == '/')
102 operandStack.push(op2 / op1);
103 }
104
105 public static String insertBlanks(String s) {
106 String result = "";
107
108 for (int i = 0; i < s.length(); i++) {
109 if (s.charAt(i) == '(' || s.charAt(i) == ')' ||
110 s.charAt(i) == '+' || s.charAt(i) == '-' ||
111 s.charAt(i) == '*' || s.charAt(i) == '/')
112 result += " " + s.charAt(i) + " ";
113 else
114 result += s.charAt(i);
115 }
116
117 return result;
118 }
119 }

Thank you very much!

Wow! Your first post of 2 total was nearly 2 years ago. You said at that time you'd been studying Java for a year and a half. Welcome back to the forum, and I wish you luck at continuing your study.

Please read this topic to learn how to post code in code or highlight tags and other useful info for new members.

Please post your code correctly using code or highlight tags per the above link.

@GregBrannon: Yes, and I quitted it, took web design class, have front-end web design job. Now, want to work on back-end(pay more) therefore, I'm going back to java.

I've been studying web development too. I pick up the back end stuff pretty quickly but front end always gives me trouble. HTML and CSS aren't really tough but I feel like good front end stuff requires an artistic bent that I don't have.

Like the others have said, embed your code stuff in the code tags. It's tough to read as is.