Write a program which accepts three real numbers, computes their sum and outputs the result. Amend the program so that the result is output to 2 dp.

program ex3(input,output);

var a, b, c, sum: real;
begin
  writeln ('Enter a real number');
  readln (a);
  writeln ('Enter another real number');
  readln (b);
  writeln ('Enter one more real number');
  readln (c);
  sum:=a + b + c;
  writeln ('Sum is: ', sum: 8: 2);
  readln;
end.

We declare 3 real numbers, 3 for input and 1 for the sum. We read the numbers from the keyboard and then use the + to add them. In the output we use 8 for the width of the output and 2 for the number of decimal places.

What if the user fails to input real numbers? What if they enter integers or, worse still, characters such as 'a' or 't'? A simple read is no guarantee at all of data integrity, we need to check this at all times in a serious program. The term 'user friendly' is not very helpful, what we need is something robust, which works in all circumstances. But can we foresee all circumstances?