1 | #include "../../std_lib_facilities.h"
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2 |
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3 | // Klassen----------------------------------------------------------------------
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4 |
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5 | class Token {
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6 | public:
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7 | char kind; // what kind of token
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8 | double value; // for numbers: a value
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9 | Token(char ch) // make a Token from a char
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10 | :kind(ch), value(0) { }
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11 | Token(char ch, double val) // make a Token from a char and a double
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12 | :kind(ch), value(val) { }
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13 | };
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14 |
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15 | class Token_stream {
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16 | public:
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17 | Token_stream(); // make a Token_stream that reads from cin
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18 | Token get(); // get a Token (get() is defined elsewhere)
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19 | void putback(Token t); // put a Token back
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20 | private:
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21 | bool full; // is there a Token in the buffer?
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22 | Token buffer; // here is where we keep a Token put back using putback()
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23 | };
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24 |
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25 |
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26 |
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27 | // Memberfunktionen-------------------------------------------------------------
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28 |
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29 | // The constructor just sets full to indicate that the buffer is empty:
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30 | Token_stream::Token_stream()
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31 | :full(false), buffer(0) // no Token in buffer
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32 | {
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33 | }
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34 |
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35 | // The putback() member function puts its argument back into the Token_stream's buffer:
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36 | void Token_stream::putback(Token t)
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37 | {
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38 | if (full) error("putback() into a full buffer");
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39 | buffer = t; // copy t to buffer
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40 | full = true; // buffer is now full
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41 | }
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42 |
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43 | Token Token_stream::get()
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44 | {
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45 | if (full) { // do we already have a Token ready?
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46 | // remove token from buffer
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47 | full=false;
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48 | return buffer;
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49 | }
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50 |
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51 | char ch;
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52 | cin >> ch; // note that >> skips whitespace (space, newline, tab, etc.)
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53 |
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54 | switch (ch) {
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55 | case '=': // for "print"
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56 | case 'x': // for "quit"
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57 | case '(': case ')': case '+': case '-': case '*': case '/':
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58 | return Token(ch); // let each character represent itself
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59 | case '.':
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60 | case '0': case '1': case '2': case '3': case '4':
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61 | case '5': case '6': case '7': case '9':
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62 | {
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63 | cin.putback(ch); // put digit back into the input stream
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64 | double val;
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65 | cin >> val; // read a floating-point number
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66 | return Token('8',val); // let '8' represent "a number"
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67 | }
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68 | default:
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69 | error("Bad token");
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70 | }
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71 | }
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72 |
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73 |
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74 |
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75 | // Objekterstellung und Vorwärtsdeklaration-------------------------------------
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76 |
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77 | Token_stream ts; // provides get() and putback()
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78 |
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79 | double expression(); // declaration so that primary() can call expression() d.h. Vorwärtdeklaration
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80 |
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81 |
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82 |
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83 | // Definitionen für primary(), term(), expression()-----------------------------
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84 |
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85 | // deal with numbers and parentheses
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86 | double primary()
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87 | {
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88 | Token t = ts.get();
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89 | switch (t.kind) {
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90 | case '(': case '{': // handle '(' expression ')'
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91 | {
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92 | double d = expression();
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93 | t = ts.get();
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94 | if (t.kind != ')' || t.kind != '}') error("')' expected");
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95 | return d;
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96 | }
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97 | case '8': // we use '8' to represent a number
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98 | return t.value; // return the number's value
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99 | default:
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100 | error("primary expected");
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101 | }
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102 | }
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103 |
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104 | // deal with *, /, and %
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105 | double term()
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106 | {
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107 | double left = primary();
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108 | Token t = ts.get(); // get the next token from token stream
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109 |
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110 | while(true) {
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111 | switch(t.kind) {
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112 | case '*':
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113 | left *= primary();
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114 | t = ts.get();
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115 | break;
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116 | case '/':
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117 | {
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118 | double d = primary();
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119 | if (d == 0) error("divide by zero");
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120 | left /= d;
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121 | t = ts.get();
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122 | break;
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123 | }
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124 | default:
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125 | ts.putback(t); // put t back into the token stream
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126 | return left;
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127 | }
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128 | }
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129 | }
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130 |
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131 | // deal with + and -
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132 | double expression()
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133 | {
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134 | double left = term(); // read and evaluate a Term
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135 | Token t = ts.get(); // get the next token from token stream
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136 |
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137 | while(true) {
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138 | switch(t.kind) {
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139 | case '+':
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140 | left += term(); // evaluate Term and add
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141 | t = ts.get();
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142 | break;
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143 | case '-':
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144 | left -= term(); // evaluate Term and subtract
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145 | t = ts.get();
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146 | break;
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147 | default:
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148 | ts.putback(t); // put t back into the token stream
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149 | return left; // finally: no more + or -: return the answer
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150 | }
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151 | }
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152 | }
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153 |
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154 |
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155 |
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156 | // main-Funktion---------------------------------------------------------------
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157 |
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158 | int main()
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159 | try
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160 | {
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161 | cout << "Willkommen zu unserem einfachen Taschenrechnerprogramm\n"
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162 | << "Bitte geben Sie einen Ausdruck mit reelen Zahlen ein.\n\n"
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163 | << "Erlaubte Operatoren sind + - * /\n"
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164 | << "'==' startet berechnung, 'x' beendet das Programm\n\n";
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165 | double val = 0;
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166 | while (cin) {
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167 | Token t = ts.get();
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168 | if(t.kind == 'x') break; // break verlässt die momentane Schleife
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169 | switch(t.kind) {
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170 | case '=': // '==' for "print now"
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171 | t = ts.get();
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172 | if(t.kind == '=')
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173 | cout << "=" << val << '\n';
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174 | break;
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175 | default:
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176 | ts.putback(t);
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177 | val = expression();
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178 | break;
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179 | }
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180 | }
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181 | keep_window_open();
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182 | }
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183 | catch (exception& e) {
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184 | cerr << "error: " << e.what() << '\n';
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185 | keep_window_open();
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186 | return 1;
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187 | }
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188 | catch (...) {
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189 | cerr << "Oops: unknown exception!\n";
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190 | keep_window_open();
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191 | return 2;
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192 | }
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