The problem in Software

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21 The problem
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The program in 12/12 wrote a greeting with a frame around it For example, if our user gave us the name Estragon, our program would write
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******************** * * * Hello, Estragon! * * * ********************
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The program built up the output a line at a time It defined variables named first and second to contain the first and second lines of the output, and wrote the greeting itself, surrounded by some characters, as the third line We didn't need separate variables for the fourth or fifth output lines, because those were the same as the second and first lines respectively This approach has a major shortcoming: Each line of the output has a part of the program-and a variable-that corresponds to it Therefore, even a simple change to the output format, such as removing the spaces between the greeting and the frame, would require rewriting the program We would like to produce a more flexible form of output without having to store each line in a local variable We will approach this problem by generating each character of the output separately, except for the greeting itself, which we already have available as a string What we shall discover is that there is no need to store the output characters in variables, because once we have written a character, we don't need it any more
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22 Overall structure
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We'll begin by reviewing the part of the program that we don't have to rewrite:
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#include <iostream> #include <string> int main() { // ask for the person's name std::cout << "Please enter your first name: "; // read the name std::string name; std::cin >> name;
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// build the message that we intend to write const std::string greeting = "Hello, " + name + "!"; // we have to rewrite this part return 0; }
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As we rewrite the part of the program that the we have to rewrite this part comment represents, we shall already be in a context that defines name, greeting, and the relevant names from the standard library We will build up the new version of the program a piece at a time, and then, in 254/29, we'll put all the pieces together
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23 Writing an unknown number of rows
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We can think of our output as a rectangular array of characters, which we must write one row at a time Although we don't know how many rows it has, we do know how to compute the number of rows The greeting takes up one row, as do the top and bottom rows of the frame We've accounted for three rows so far If we know how many blank rows we intend to leave between the greeting and the frame, we can double that number and add three to obtain the total number of rows in the output:
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// the number of blanks surrounding the greeting const int pad = 1; // total number of rows to write const int rows = pad * 2 + 3;
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We want to make it easy to find the part of our program that defines the number of blanks, so we give that number a name The variable called pad represents the amount of padding around the frame Having defined pad, we use it in computing rows, which will control how many rows we write The built-in type int is the most natural type to use for integers, so we've chosen that type for pad and rows We also said that both variables are const, which we know from 12/13 is a promise that we will not change the value of either pad or rows Looking ahead, we intend to use the same number of blanks on the left and right sides as on the top and bottom, so one variable will serve for all four sides If we are careful to use this variable every time we want to refer to the number of blanks, changing the size of the frame will require only changing the program to give the variable a different value We have computed how many rows we need to write; our next problem is to do so:
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// separate the output from the input std::cout << std::endl; // write rows rows of output int r = 0; // invariant: we have written r rows so far while (r != rows) { // write a row of output (as we will describe in 24/22) std::cout << std::endl; ++r; }
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We start, as we did in 12/12, by writing a blank line, so that there will be some space between the input and the output The rest of this fragment contains so many new ideas that we need to look at it closely Once we've understood how it works, we'll think about how to write each individual row
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