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1 /*
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2 Very Simple Calculator
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3 */
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4
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5 #include <stdio.h>
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6
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7 static char *ptr,*last_ptr;
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8 static int value,lvalue;
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9 static int last_token;
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10 static int variable[48];
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11
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12 static int expr();
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13 static int aexpr();
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14 static int mexpr();
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15 static int term();
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16 static int token();
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11
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17 static void error(char *);
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0
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18
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19
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20 static int
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21 token()
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22 {
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23 int c,d;
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24
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25 last_ptr = ptr; /* for error position */
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26 c= *ptr;
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27 if(!c) {
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28 last_token = EOF;
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29 return last_token;
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30 }
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31 ptr++;
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32 if (c<=' ') { /* comment */
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33 while(*ptr++);
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34 ptr--;
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35 last_token = EOF;
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36 last_ptr = ptr;
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37 return last_token;
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38 }
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39 if('0'<=c && c<='9') { /* Decimal */
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40 d = c-'0';
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15
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41 while((c= *ptr++)) {
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0
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42 if('0'<=c && c<='9') {
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43 d = d*10 + (c - '0');
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44 } else {
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45 break;
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46 }
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47 }
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48 c && ptr--;
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49 value = d;
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50 last_token = '0';
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51 return last_token;
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52 } else if ('a'<=c && c<='z') { /* variable */
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53 value = c-'a'; /* return variable reference */
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54 last_token = 'v';
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55 return last_token;
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56 } else {
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57 last_token = c;
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58 return last_token;
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59 return c;
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60 }
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61 }
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62
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63 static int
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64 expr()
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65 {
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66 int d,assign;
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67
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68 d = aexpr();
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69 assign = lvalue;
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70 switch(last_token) {
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71 case '>':
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72 d = (d > aexpr());
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73 return d;
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74 case '=':
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75 if(assign>=0) {
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76 d = expr();
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77 variable[assign] = d;
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78 return d;
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79 } else {
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80 error("Bad assignment");
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81 return 0;
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82 }
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83 case ')':
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84 return d;
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85 case EOF:
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86 return d;
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87 default:
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88 error("Bad expression");
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89 return d;
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90 }
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91 }
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92
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93 static int
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94 aexpr()
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95 {
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96 int d;
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97
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98 d = mexpr();
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99 switch(last_token) {
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100 case '-':
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101 d -= aexpr();
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102 return d;
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103 case '+':
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104 d += aexpr();
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105 return d;
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106 default:
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107 return d;
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108 }
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109 }
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110
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111 static int
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112 mexpr()
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113 {
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114 int d;
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115
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116 d = term();
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117 switch(last_token) {
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118 case '*':
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119 d *= mexpr();
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120 return d;
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121 case '/':
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122 d /= mexpr();
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123 return d;
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124 default:
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125 return d;
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126 }
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127 }
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128
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129 static int
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130 term()
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131 {
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132 int d;
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133
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134 lvalue= -1;
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135 token();
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136 if(last_token==EOF) {
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137 error("Term expected");
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138 }
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139 switch(last_token) {
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140 case '0':
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141 d = value;
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142 token();
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143 return d;
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144 case 'v':
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145 d = lvalue = value;
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146 token();
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147 return variable[d];
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148 case '(':
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149 d = expr();
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150 if(last_token != ')') {
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151 error("Unbalanced parenthsis");
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152 }
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153 token();
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154 return d;
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155 default:
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156 token();
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157 error("Unknown term");
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158 return 0;
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159 }
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160 }
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161
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11
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162 static int lineno = 0;
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163
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164 void
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165 error(char *msg)
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166 {
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167 fprintf(stderr,"%s on line %d\n",msg, lineno);
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168 }
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169
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0
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170 int
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171 main()
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172 {
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173 int d;
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174 char buf[BUFSIZ];
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175
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176 while (fgets(buf,BUFSIZ,stdin)) {
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177 ptr = buf;
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178 d = expr();
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179 printf("%s = 0x%08x = %d\n",buf,d,d);
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180 fflush(stdout);
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11
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181 lineno++;
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0
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182 }
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183 return 0;
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184 }
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185
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186 /* end */
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