CS150 Introduction to Computer Science 1 Summary We have still not finished talking about functions. Today we will finish functions and start talking about reading and writing to files. Don’t forget! Assignment is due on Wednesday, October 29, 2003. 2/21/2019 CS150 Introduction to Computer Science 1

Testing a Program Using Stubs When you have a large program with many functions, it is usually a good idea to create dummy functions or stubs. These will have the proper prototypes, but they will not actually doing anything. It is a good way of testing that the values are being passed to and from the functions correctly. 2/21/2019 CS150 Introduction to Computer Science 1

CS150 Introduction to Computer Science 1 Example void computeSum(int, int, int&); int main( void ) { int x, y, sum; x = 4; y = 5; sum = 0; computeSum (x, y, sum); cout << x << y << sum; return 0; } void computeSum(int num1, int num2, int& sum) cout << “Function computeSum entered with the values ”; cout << num1 << “, “ << num2 << “ and “ << sum << endl; sum = 50; 2/21/2019 CS150 Introduction to Computer Science 1

CS150 Introduction to Computer Science 1 Last Word on Functions Arguments can be input (receives) and/or output (sends) When would an argument be both? Can be tricky… How can you identify arguments that are used both for input and output? Do we know any examples? How do we identify them in our comments? 2/21/2019 CS150 Introduction to Computer Science 1

CS150 Introduction to Computer Science 1 Example void computeSum(int, int, int&); int main( void ) { int x, y, sum; x = 4; y = 5; sum = 0; computeSum(x, y, sum); cout << x << y << sum; return 0; } // Function: computeSum // Purpose: computes the sum of // two numbers // In: num1, num2 // Out: sum void computeSum(int num1, int num2, int& sum) { sum = num1 + num2; } 2/21/2019 CS150 Introduction to Computer Science 1

CS150 Introduction to Computer Science 1 Example void computeSumAndChange(int, int&, int&); int main( void ) { int x, y, sum; x = 4; y = 5; sum = 0; computeSumAndChange(x, y, sum); cout << x << y << sum; return 0; } // Function: computeSumAndChange // Purpose: computes the sum of two // numbers and changes // one of the values // In: num1 // Out: sum // In/Out: num2 void computeSumAndChange(int num1, int& num2, int& sum) { sum = num1 + num2; num2 = 7; } 2/21/2019 CS150 Introduction to Computer Science 1

CS150 Introduction to Computer Science 1 Constants Revisited We know that we can declare constants in C++ as follows: const type identifier = constant; For example: const int Pi = 3.14159; There is another way of defining constants: #define identifier replacement-text #define Pi 3.14159 2/21/2019 CS150 Introduction to Computer Science 1

CS150 Introduction to Computer Science 1 #define #define is a compiler directive It’s different from a variable or const Form: #define X Y The compiler replaces all instances of X with Y No memory space is created Used in C primarily and in older C++ code Not considered true C++ programming, but it’s useful NOTE: NO SEMICOLON!! 2/21/2019 CS150 Introduction to Computer Science 1

Ifstreams and Ofstreams So far, we have been reading user input from the keyboard. We have also been writing the output of our programs directly to the screen. What if we want to read in a large amount of data that is stored in a file. What if we want to write a large amount of data and store it in a file. 2/21/2019 CS150 Introduction to Computer Science 1

Steps for External files include<fstream> Declare file variables (pointers) that will correspond to the files you are using Input files type: ifstream Output files type: ofstream Open file Use file for input/output Close files 2/21/2019 CS150 Introduction to Computer Science 1

Declaring File pointers ifstream ifil; ofstream ofil; File variables or pointers are the ways that you refer to the files you are using. Can specify which input/output file to use. May input from more than one file May output to more than one file 2/21/2019 CS150 Introduction to Computer Science 1

CS150 Introduction to Computer Science 1 Opening files <fileptr>.open(<filename>) Same syntax for both input and output files Filename may be string literal or char array variable Example: ifstream ifil; ifil.open(“input.dat”); 2/21/2019 CS150 Introduction to Computer Science 1

CS150 Introduction to Computer Science 1 Checking for errors Should check for errors opening file if(ifil.fail()) { cout << “Error opening input file “; exit(1); } Note: you must open files first before using them! 2/21/2019 CS150 Introduction to Computer Science 1

CS150 Introduction to Computer Science 1 Using file pointers Use input file pointer wherever you use cin Examples: ifil >> ch; Output output file ptr wherever you use cout ofil << ch; Make sure to use correct file pointer! 2/21/2019 CS150 Introduction to Computer Science 1

CS150 Introduction to Computer Science 1 Closing files Must do at end when file is finished ifil.close(); ofil.close(); Same syntax for input and output files 2/21/2019 CS150 Introduction to Computer Science 1

CS150 Introduction to Computer Science 1 Example #include<fstream> #include<iostream> #define inFile "in.txt" int main( void ) { ifstream ifil; //input stream string name; ifil.open(inFile); if (ifil.fail()) cout << "*** Error opening file" << endl; exit (1); } while (!ifil.eof()) ifil >> name; cout << name << " "; ifil.close(); return 0; 2/21/2019 CS150 Introduction to Computer Science 1