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1 seqlua: Extension for handling sequential data in Lua
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2 =====================================================
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3
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4 This is an experimental package to extend Lua in the following manner:
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5
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6 * allow ``ipairs(seq)`` to accept either tables or functions (i.e function
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7 iterators) as an argument,
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8 * add a new function ``string.concat(separator, seq)`` that concats either
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9 table entries or function return values,
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10 * provide auxiliary C functions and macros to simplify iterating over both
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11 tables and iterator functions with a generic statement.
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12
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13 Existing ``__ipairs`` or ``__index`` (but not ``__len``) metamethods are
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14 respected by both the Lua functions and the C functions and macros. The
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15 ``__ipairs`` metamethod takes precedence over ``__index``, while the
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16 ``__len`` metamethod is never used.
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17
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18 Metamethod handling in detail is explained in the last section
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19 ("Respected metamethods") at the bottom of this README.
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20
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21
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22
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23 Motivation
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24 ----------
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25
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26 Sequential data (such as arrays or streams) is often represented in two
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27 different ways:
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28
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29 * as an ordered set of values (usually implemented as an array in other
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30 programming languages, or as a sequence in Lua: a table with numeric keys
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31 {1..n} associated with a value each),
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32 * as some sort of data stream (sometimes implemented as a class of objects
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33 providing certain methods, or as an iterator function in Lua: a function that
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34 returns the next value with every call, where nil indicates the end of the
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35 stream).
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36
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37 Quite often, when functions work on sequential data, it shouldn't matter in
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38 which form the sequential data is being provided to the function. As an
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39 example, consider a function that is writing a sequence of strings to a file.
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40 Such function could either be fed with an array of strings (a table with
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41 numeric keys in Lua) or with a (possibly infinite) stream of data (an iterator
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42 function in Lua).
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43
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44 A function in Lua that accepts a table, might look like as follows:
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45
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46 function write_lines(lines)
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47 for i, line in ipairs(lines) do
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48 io.stdout:write(line)
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49 io.stdout:write("\n")
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50 end
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51 end
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52
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53 In contrast, a function in Lua that accepts an iterator function would have to
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54 be implemented differently:
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55
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56 function write_lines(get_next_line)
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57 for line in get_next_line do
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58 io.stdout:write(line)
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59 io.stdout:write("\n")
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60 end
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61 end
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62
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63 If one wanted to create a function that accepts either a sequence in form of a
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64 table or an iterator function, then one might need to write:
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65
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66 do
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67 local function write_line(line)
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68 io.stdout:write(line)
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69 io.stdout:write("\n")
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70 end
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71 function write_lines(lines)
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72 if type(lines) == "function" then
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73 for line in lines do
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74 write_line(line)
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75 end
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76 else
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77 for i, line in ipairs(lines) do
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78 write_line(line)
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79 end
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80 end
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81 end
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82 end
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83
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84 Obviously, this isn't something we want to do in every function that accepts
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85 sequential data. Therefore, we usually decide for one of the two first forms
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86 and thus disallow the other possible representation of sequential data to be
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87 passed to the function.
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88
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89 This extension, however, modifies Lua's ``ipairs`` statement in such way that
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90 it automatically accepts either a table or an iterator function as argument.
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91 Thus, the first of the three ``write_lines`` functions above will accept both
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92 (table) sequences and (function) iterators.
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93
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94 In addition to the modification of ``ipairs``, it also provides C functions and
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95 macros to iterate over values in the same manner as a generic loop statement
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96 with ``ipairs`` would do.
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97
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98 Note that this extension doesn't aim to supersede Lua's concept of iterator
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99 functions. While metamethods (see section "Respected metamethods" below) may be
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100 used to customize iteration behavior on values, this extension isn't thought to
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101 replace the common practice to use function closures as iterators. Consider the
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102 following example:
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103
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104 local result = sql_query("SELECT * FROM actor ORDER BY birthdate")
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105 write_lines(result:get_column_entries("name"))
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106
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107 The ``get_column_entries`` method can return a simple function closure that
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108 returns the next entry in the "name" column (returning ``nil`` to indicate the
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109 end). Such a closure can then be passed to another function that iterates
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110 through a sequence of values by invoking ``ipairs`` with the general for-loop
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111 (as previously shown).
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112
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113 Where desired, it is also possible to use metamethods to customize iteration
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114 behavior:
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115
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116 function print_rows(rows)
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117 for i, row in ipairs(rows) do
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118 print_row(row)
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119 end
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120 end
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121 local result = sql_query("SELECT * FROM actor ORDER BY birthday")
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122 assert(type(result) == "userdata")
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123
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124 -- we may rely on the ``__index`` or ``__ipairs`` metamethod to
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125 -- iterate through all result rows here:
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126 print_rows(result) -- no need to use ":rows()" or a similar syntax
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127
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128 -- but we can also still pass an individual set of result rows to the
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129 -- print_rows function:
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130 print_rows{result[1], result[#result]}
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131
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132 This extension, however, doesn't respect the ``__len`` metamethod due to the
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133 following considerations:
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134
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135 * An efficient implementation where ``for i, v in ipairs(tbl) do ... end`` does
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136 neither create a closure nor repeatedly evaluate ``#tbl`` seems to be
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137 impossible.
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138 * Respecting ``__len`` could be used to implement sparse arrays, but this would
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139 require iterating functions to expect ``nil`` as a potential value. This may
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140 lead to problems because ``nil`` is usually also used to indicate the absence
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141 of a value.
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142
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143 Though, if such behavior is desired, it can still be implemented through the
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144 ``__ipairs`` metamethod.
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145
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146 Unless manually done by the user in the ``__ipairs`` metamethod, the ``ipairs``
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147 function as well as the corresponding C functions and macros provided by this
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148 extension never create any closures or other values that need to be garbage
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149 collected.
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150
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151
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152
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153 Lua part of the library
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154 -----------------------
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155
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156 The modified ``ipairs(seq)`` and the new ``string.concat(sep, seq)`` functions
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157 accept either a table or a function as ``seq``. This is demonstrated in the
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158 following examples:
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159
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160 require "seqlua"
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161
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162 t = {"a", "b", "c"}
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163
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164 for i, v in ipairs(t) do
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165 print(i, v)
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166 end
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167 -- prints:
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168 -- 1 a
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169 -- 2 b
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170 -- 3 c
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171
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172 print(string.concat(",", t))
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173 -- prints: a,b,c
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174
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175 function alphabet()
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176 local letter = nil
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177 return function()
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178 if letter == nil then
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179 letter = "a"
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180 elseif letter == "z" then
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181 return nil
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182 else
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183 letter = string.char(string.byte(letter) + 1)
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184 end
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185 return letter
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186 end
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187 end
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188
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189 for i, v in ipairs(alphabet()) do
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190 print(i, v)
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191 end
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192 -- prints:
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193 -- 1 a
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194 -- 2 b
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195 -- 3 c
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196 -- ...
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197 -- 25 y
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198 -- 26 z
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199
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200 print(string.concat(",", alphabet()))
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201 -- prints: a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z
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202
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203 function filter(f)
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204 return function(seq)
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205 return coroutine.wrap(function()
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206 for i, v in ipairs(seq) do f(v) end
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207 end)
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208 end
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209 end
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210
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211 alpha_beta_x = filter(function(v)
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212 if v == "a" then
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213 coroutine.yield("alpha")
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214 elseif v == "b" then
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215 coroutine.yield("beta")
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216 elseif type(v) == "number" then
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217 for i = 1, v do
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218 coroutine.yield("X")
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219 end
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220 end
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221 end)
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222
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223 print((","):concat(alpha_beta_x{"a", 3, "b", "c", "d"}))
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224 -- prints: alpha,X,X,X,beta
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225
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226 print((","):concat(alpha_beta_x(alphabet())))
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227 -- prints: alpha,beta
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228
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229
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230
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231 C part of the library
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232 ---------------------
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233
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234 In ``seqlualib.h``, the following macro is defined:
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235
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236 #define seqlua_iterloop(L, iter, idx) \
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237 for ( \
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238 seqlua_iterinit((L), (iter), (idx)); \
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239 seqlua_iternext(iter); \
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240 )
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241
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242 and
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243
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244 #define seqlua_iterloopauto(L, iter, idx) \
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245 for ( \
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246 seqlua_iterinit((L), (iter), (idx)); \
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247 seqlua_iternext(iter); \
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248 lua_pop((L), 1) \
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249 )
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250
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251 This macro allows iteration over either tables or iterator functions as the
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252 following example function demonstrates:
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253
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254 int printcsv(lua_State *L) {
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255 seqlua_Iterator iter;
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256 seqlua_iterloop(L, &iter, 1) {
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257 if (seqlua_itercount(&iter) > 1) fputs(",", stdout);
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258 fputs(luaL_tolstring(L, -1, NULL), stdout);
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259 // two values need to be popped (the value pushed by
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260 // seqlua_iternext and the value pushed by luaL_tolstring)
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261 lua_pop(L, 2);
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262 }
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263 fputs("\n", stdout);
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264 return 0;
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265 }
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266
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267 printcsv{"a", "b", "c"}
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268 -- prints: a,b,c
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269
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270 printcsv(assert(io.open("testfile")):lines())
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271 -- prints: line1,line2,... of "testfile"
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272
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273 NOTE: During iteration using ``seqlua_iterloop``, ``seqlua_iterloopauto``, or
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274 ``seqlua_iterinit``, three extra elements are stored on the stack (additionally
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275 to the value). These extra elements are removed automatically when the loop ends
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276 (i.e. when ``seqlua_iternext`` returns zero). The value pushed onto the stack
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277 for every iteration step has to be removed manually from the stack, unless
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278 ``seqlua_iterloopauto`` is used.
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279
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280
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281
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282 Respected metamethods
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283 ---------------------
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284
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285 Regarding the behavior of the Lua functions and the C functions and macros
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286 provided by this extension, an existing ``__index`` metamethod will be
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287 respected automatically. An existing ``__ipairs`` metamethod, however, takes
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288 precedence.
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289
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290 If the ``__ipairs`` field of a value's metatable is set, then it must always
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291 refer to a function. When starting iteration over a value with such a
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292 metamethod being set, then this function is called with ``self`` (i.e. the
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293 value itself) passed as first argument. The return values of the ``__ipairs``
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294 metamethod may take one of the following 4 forms:
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295
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296 * ``return function_or_callable, static_argument, startindex`` causes the three
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297 arguments to be returned by ``ipairs`` without further modification. Using
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298 the C macros and functions for iteration, the behavior is according to the
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299 generic loop statement in Lua:
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300 ``for i, v in function_or_callable, static_argument, startindex do ... end``
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301 * ``return "raw", table`` will result in iteration over the table ``table``
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302 using ``lua_rawgeti``
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303 * ``return "index", table_or_userdata`` will result in iteration over the table
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304 or userdata while respecting any ``__index`` metamethod of the table or
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305 userdata value
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306 * ``return "call", function_or_callable`` will use the callable value as
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307 (function) iterator where the function is expected to return a single value
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308 without any index (the index is inserted automatically when using the
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309 ``ipairs`` function for iteration)
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310
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311 These possiblities are demonstrated by the following example code:
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312
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313 require "seqlua"
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314
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315 do
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316 local function ipairsaux(t, i)
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317 i = i + 1
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318 if i <= 3 then
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319 return i, t[i]
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320 end
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321 end
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322 custom = setmetatable(
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323 {"one", "two", "three", "four", "five"},
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324 {
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325 __ipairs = function(self)
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326 return ipairsaux, self, 0
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327 end
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328 }
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329 )
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330 end
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331 print(string.concat(",", custom))
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332 -- prints: one,two,three
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333 -- (note that "four" and "five" are not printed)
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334
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335 tbl = {"alpha", "beta"}
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336
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337 proxy1 = setmetatable({}, {__index = tbl})
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338 for i, v in ipairs(proxy1) do print(i, v) end
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339 -- prints:
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340 -- 1 alpha
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341 -- 2 beta
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342
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343 proxy2 = setmetatable({}, {
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344 __ipairs = function(self)
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345 return "index", proxy1
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346 end
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347 })
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348 for i, v in ipairs(proxy2) do print(i, v) end
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349 -- prints:
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350 -- 1 alpha
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351 -- 2 beta
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352 print(proxy2[1])
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353 -- prints: nil
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354
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355 cursor = setmetatable({
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356 "alice", "bob", "charlie", pos=1
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357 }, {
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358 __call = function(self)
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359 local value = self[self.pos]
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360 if value == nil then
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361 self.pos = 1
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362 else
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363 self.pos = self.pos + 1
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364 end
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365 return value
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366 end,
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367 __ipairs = function(self)
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368 return "call", self
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369 end
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370 })
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371 for i, v in ipairs(cursor) do print(i, v) end
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372 -- prints:
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373 -- 1 alice
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374 -- 2 bob
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375 -- 3 charlie
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376 print(cursor())
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377 -- prints: alice
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378 for i, v in ipairs(cursor) do print(i, v) end
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379 -- prints:
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380 -- 1 bob
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381 -- 2 charlie
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382 -- (note that "alice" has been returned earlier)
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383
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384 coefficients = setmetatable({1.25, 3.14, 17.5}, {
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385 __index = function(self) return 1 end,
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386 __ipairs = function(self) return "raw", self end
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387 })
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388 for i, v in ipairs(coefficients) do print(i, v) end
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389 -- prints:
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390 -- 1 1.25
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391 -- 2 3.14
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392 -- 3 17.5
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393 -- (note that iteration terminates even if coefficients[4] == 1)
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394 print(coefficients[4])
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395 -- prints: 1
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396
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397
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