You use a storage class specifier to tell the compiler the variable exists but it's not in this file:
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/*report.cpp*/
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extern p_r_s player_choice;
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extern p_r_s machine_choice;
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Then just define the variables in another file:
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/*select.cpp*/
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p_r_s player_choice;
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p_r_s machine_choice;
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The linker will hook the variable in one file to the code in the other file.