do

         
do                                                       
  stmt                        body of the loop
while (expression)

• similar to while loop except:
• at least one iteration (cf. while stmt zero or more iterations)
• test at end/bottom of loop body
• confusingly uses the word while but this while is not the while's while

----------------------------

// input a negative number from user
do {
  cout < "Enter negative number: ";
  cin >> num;
} while (num >= 0);

 User given no indication if input is "wrong". see below for improved version of this

// input valid grade from user
do {
  cin >> gr;
} while (gr!='A' && gr!='B' && gr!='C' && gr !='D' && gr!='F');

----------------------------

// input valid grade.  Error message to user
do {
  cin >> gr;
  if (gr!='A' && gr!='B' && gr!='C' && gr !='D' && gr!='F')
    cout << "ERROR.  Must be ABCDF.  Re-enter: ";
} while (gr!='A' && gr!='B' && gr!='C' && gr !='D' && gr!='F');

----------------------------

// input value in range 1 thru 5
do {
  cin >> choice;
  if (choice<1 || choice>5)
    cout << "ERROR.  Invalid choice: " << choice 
         << "  Must be 1 thru 5.  Re-enter: ";
} while (choice<1 || choice>5);

----------------------------

bool error;
// input a negative number from user
do {
  cout << "Enter negative number: ";
  cin >> num;
  if (error = num>=0)    // assign result of >= (true or false)
    cout << "ERROR.  Must be negative number." << endl;
} while (error);   //will be true or false from the assignment

The GPA program revisited using do and switch. gpa2.cpp

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