Assume that x is a char variable that has been declared and already given a value. Write an expression whose value is true if and only if x is an hexadecimal (Base 16) digit (0-9 plus A-F or a-f).

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This is the answer:

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(( x>='0' && x<='9') || ( x>='a' && x<='f' ) || ( x>='A' && x<='F'))

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411: You are given a file named phonedir that consists of many lines containing three strings: lastname firstname emailaddress. Write a utility program that reads commands (from stdin). Each command has one of two possible forms: lookup string add lastname firstname emailaddress In the case of the first command (lookup), the program looks for a line in the phonedir file where either the firstname or lastname matches the string given. If it finds such a line it prints out all three parts of the line, separated by spaces: lastname firstname emailaddress If it does not find a match in the file it prints out the string it was looking for, followed by a colon followed by a space followed by the message "not found": string: not found In the case of the second command (add), the program appends the three strings given to the file phonedir. Hints and suggestions: (1) Define and use two functions named lookup and add. When your program reads the string lookup, your lookup function is called; when the program reads the string add, your add function is called. When either of these two functions are called, they then read whatever else is necessary for their command (one string for lookup and three strings -- lastname, firstname, and emailaddress-- for add). And then these functions do... whatever it takes to carry out the command. (2) When doing a lookup or an add, open up the file phonedir ... carry out the operation .. and then close up the file. EXAMPLE: suppose the phonedir file looked like this: arnow david arnow@panix.com bush george president@whitehouse.gov gates bill bill@microsoft.com here then is a sample session with the program (program output is in bold): lookup david arnow david arnow@panix.com lookup joe joe: not found add theplumber joe joetheplumber@nowhere.com lookup arnow arnow david arnow@panix.com lookup joe theplumber joe joetheplumber@nowhere.com and the phonedir file would now be: arnow david arnow@panix.com bush george president@whitehouse.gov gates bill bill@microsoft.com theplumber joe joetheplumber@nowhere.com

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. . Click on the title for the solution . . This is the answer: : #include <iostream> #include <fstream> #include <string> using namespace std; void lookup(); void add(); int main() { string type; while (cin >> type) { if (type == "lookup") lookup(); else  add(); } return 0; } void lookup() { ifstream fin; fin.open("phonedir"); if(!fin.is_open()) cout << "Not Open" << endl; bool check = false; string name,firstName,lastName,email; cin >> name; while (fin >> firstName >> lastName >> email) { if (firstName== name || lastName==name) { cout<<firstName<<" "<<lastName<<" "<<email<<endl; check=true; } } if(!check) cout << name << ": not found" << endl; fin.close(); } void add() { ofstream fout; string name; string fir...
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