CS101: Fundamentals of Computer Programming. Dr. Tejada www-bcf.usc.edu/~stejada Week 4: Arrays,Strings, File I/O

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1 CS101: Fundamentals of Computer Programming Dr. Tejada www-bcf.usc.edu/~stejada Week 4: Arrays,Strings, File I/O

2 2 Problem: Write a program to calculate the average test score for 10 students on 3 tests and printouts their letter grade based on the average. Use an array to hold the average test score of the 10 students. 1. Create a function to calculate the average test score 1. The function should ask the user enter 3 test scores to average 2. Then calculate the average test score 3. Return the average test score 2. Create a function that printouts the student s grade based on this average. 1. The function should based on the average score determine the student s grade: score >=90 is A, score>=80 is B, score >=70 is C, score >=60 is D, else F

3 3 double calculateavg(int studentnum) { double sum = 0; //sum of all test scores double testscore; //individual student test score cout << Student: << studentnum << endl; } for (int i=0; i< 3; i++) { } cout << Enter test score << i+1 << : ; cin >> testscore; sum = sum + testscore; return sum/3;

4 void printgrade(double score) { if (score >= 90) cout << A << endl; else if (score >= 80) cout << B << endl; else if (score >= 70) cout << C << endl; else if (score >= 60) cout << D << endl; else cout << F << endl; } 4

5 int main() { const int NUM_OF_STUDENTS = 10; double averages[num_of_students]= {0}; 5 //calculates and stores 10 students averages for (int i=0; i<num_of_students; i++) averages[i] = calculateavg(i); } //prints out the grade for 10 students for (int i=0; i<num_of_students; i++) { } cout << Student: << i << endl; printgrade(averages[i]);

6 6 Two- and Mul,dimensional Arrays Two-dimensional array: collection of a fixed number of components arranged in two dimensions Sometimes called matrices or tables Declaration syntax: intexp1 and intexp2 are expressions with positive integer values specifying the number of rows and columns in the array To access a component:

7 7 Mul,dimensional Arrays n- dimensional array: collec,on of a fixed number of elements arranged in n dimensions (n >= 1) Declara,on syntax: To access a component: C++ Programming: Program Design Including Data Structures, Sixth Edition

8 8 Accessing 2D Array Components Example: sales[5][3] = 25.75; C++ Programming: Program Design Including Data Structures, Sixth Edition

9 Two- Dimensional Array Ini,aliza,on During Declara,on Two- dimensional arrays can be ini,alized when they are declared: Elements of each row are enclosed within braces and separated by commas All rows are enclosed within braces For number arrays, unspecified elements are set to 0 Example: 9 int board[4][3]= {{2, 3, 1} {15, 25, 13} {20, 4, 7} {11, 18, 14}};

10 10 Processing Two- Dimensional Arrays Ways to process a two- dimensional array: Process en,re array Row processing: process a single row at a,me Column processing: process a single column at a,me Each row and each column of a two- dimensional array is a one- dimensional array To process, use algorithms similar to processing one- dimensional arrays C++ Programming: Program Design Including Data Structures, Sixth Edition

11 11 Ini,aliza,on Examples: To ini,alize row number 4 (fiph row) to 0: To ini,alize the en,re matrix to 0: C++ Programming: Program Design Including Data Structures, Sixth Edition

12 12 Input Examples: To input into row number 4 (fiph row): To input data into each component of matrix: C++ Programming: Program Design Including Data Structures, Sixth Edition

13 13 Sum by Row Example: To find the sum of row number 4: C++ Programming: Program Design Including Data Structures, Sixth Edition

14 14 Sum by Column Example: To find the sum of each individual column:

15 15 Find the Largest Element Example: To find the largest element in each row:

16 16 Passing Two- Dimensional Arrays as Parameters to Func,ons Two- dimensional arrays are passed by reference as parameters to a func,on Base address is passed to formal parameter Two- dimensional arrays are stored in row order When declaring a two- dimensional array as a formal parameter, can omit size of first dimension, but not the second C++ Programming: Program Design Including Data Structures, Sixth Edition

17 17 Arrays of Strings and the string Type To declare an array of 100 components of type string: string list[100]; Basic opera,ons, such as assignment, comparison, and input/output, can be performed on values of the string type The data in list can be processed just like any one- dimensional array C++ Programming: Program Design Including Data Structures, Sixth Edition

18 Arrays of Strings and Character Arrays 18

19 19 Character Arrays and C- Strings Character array: an array whose components are of type char C- strings are null- terminated ('\0 ) character arrays Example: 'A' is the character A "A" is the C- string A "A" represents two characters, 'A' and '\0 C++ Programming: Program Design Including Data Structures, Sixth Edition

20 20 Character Arrays and C- Strings Example: char name[16]; Since C-strings are null terminated and name has 16 components, the largest string it can store has 15 characters If you store a string whose length is less than the array size, the last components are unused C++ Programming: Program Design Including Data Structures, Sixth Edition

21 21 Character Arrays and C- Strings Size of an array can be omitted if the array is initialized during declaration Example: char name[] = "John"; Declares an array of length 5 and stores the C-string "John" in it Useful string manipulation functions strcpy, strcmp, and strlen #include <cstring> C++ Programming: Program Design Including Data Structures, Sixth Edition

22 22 String Comparison C- strings are compared character by character using the colla,ng sequence of the system Use the func,on strcmp If using the ASCII character set: "Air" < "Boat" "Air" < "An" "Bill" < "Billy" "Hello" < "hello" C++ Programming: Program Design Including Data Structures, Sixth Edition

23 23 Reading and Wri,ng Strings Most rules for arrays also apply to C- strings (which are character arrays) Aggregate opera,ons, such as assignment and comparison, are not allowed on arrays C++ does allow aggregate opera,ons for the input and output of C- strings C++ Programming: Program Design Including Data Structures, Sixth Edition

24 24 String Input Example: cin >> name; Stores the next input C-string into name To read strings with blanks, use get function: cin.get(name, m+1); Stores the next m characters into name but the newline character is not stored in name If input string has fewer than m characters, reading stops at the newline character C++ Programming: Program Design Including Data Structures, Sixth Edition

25 25 String Output Example: cout << name; Outputs the content of name on the screen << continues to write the contents of name until it finds the null character If name does not contain the null character, then strange output may occur << continues to output data from memory adjacent to name until a '\0' is found C++ Programming: Program Design Including Data Structures, Sixth Edition

26 26 Forma`ng Output manipulator is used to format the output Two types of manipulators: With parameters Without parameters Parameterized: require iomanip header setprecision, setw, and setfill Nonparameterized: require iostream header endl, showpoint, left, and right C++ Programming: Program Design Including Data Structures, Sixth Edition

27 27 Addi,onal Output Forma`ng Tools setw manipulator Outputs the value of an expression in a specified number of columns cout << setw(5) << name << endl; If number of columns exceeds the number of columns required by the expression Output of the expression is right- jus,fied Unused columns to the lep are filled with spaces setfill manipulator Output stream variables can use setfill to fill unused columns with a character cout << setfill('#'); left and right manipulators lep- jus,fies or right- jus,fies the output unsetf manipulator to disable other manipulator

28 28 File Input/Output File: area in secondary storage to hold info File I/O is a five- step process 1. Include fstream header 2. Declare file stream variables 3. Associate the file stream variables with the input/ output sources 4. Use the file stream variables with >>, <<, or other input/output func,ons 5. Close the files C++ Programming: Program Design Including Data Structures, Sixth Edition

29 Text File I/O #include <iostream> #include <fstream> using namespace std; int main () { int x; double y; ifstream ifile ( input.txt ); ofstream ofile; ofile.open( output.txt"); if( ifile.good() ){ // make sure not at end ifile >> x >> y; } else { return 1; } ofile << Int from file is << x << endl; ofile << Double from file is << y << endl; input.txt output.txt Int from file is 5 Double from file is -3.5 ifile.close(); ofile.close(); return 0; }

30 File Access Though the data is on disk we can imagine a file pointer (abbreviated fp ) to the current position in the file Similar analogy to a play head in a tape recorder / VHS player Your ifstream object (ifile) implicitly keeps track of where you are in the file via this pointer Any access to the file implicitly moves/updates the location of the pointer EOF (end-of-file) or other error means no more data can be read. Use the good() function to ensure file pointer is okay or read/write operation succeeded fp I t w a s t h e b e s t o f fp char c; ifile >> c; I t w a s t h e b e s t o f a t f e r. T h e E n d! EOF char c; ifile >> c; // get first char while( ifile.good() ) { // process c ifile >> c; // get next char } File text fp

31 >> Operator Recall that with cin the >> operator stops getting a value when it encounters whitespace and also skips whitespace to get to the next value In the example on this slide, the spaces will NOT be read in They will be skipped by the >> operator To get raw data from the file (including whitespaces) use the get() function fp char ch; ifile.get(ch); // get next char while( ifile.good() ) { // process ch ifile.get(ch); // get next char } I t w a s t h e b e s t o f char c; ifile >> c; // skips space and returns w fp File text I t w a s t h e b e s t o f......

32 Getting Lines of Text Using the >> operator to get an input string of text implicitly stops at the first whitespace How can we get a whole line of text (including spaces) cin.getline(char buf[], int bufsize); ifstream.getline(char buf[], int bufsize); Reads max of bufsize-1 characters (including newline) input.txt The fox jumped over the log. The bear ate some honey. The CS student solved a hard problem. #include <iostream> #include <fstream> using namespace std; int main () { } char myline[100]; int i = 1; ifstream ifile ( input.txt ); if( ifile.fail() ){ // exit if bad return 1; } ifile.getline(myline, 100); while (ifile.good()) { cout << i++ << : << myline << endl; ifile.getline(myline, 100); } ifile.close(); return 0; output 1: The fox jumped over the log. 2: The bear ate some honey. 3: The CS student solved a hard problem.

33 stdin, stdout, stderr Most OS es map console I/O (keyboard and monitor I/O) to 3 predefined FILE pointers: stdin (input from keyboard) = cin stdout (output to monitor) = cout Normal output stderr (output to monitor) = cerr Exception / error information Unix/Linux can allow you to redirect stdout vs. stderr separately./prog > log_of_stdout.txt./prog >& log_of_stderr.txt int main() { char first_char; char first_line[80]; } // read char from keyboard cin << first_char; // read entire line of text from // keyboard cin.getline(first_line, 80); // echo line back to stdout cout << first_line; // output to stderr cerr << I had an error. << endl; return 0;

34 34 Problem: Write a program to calculate the average test score for each student in 3 classes of 10 students on 3 tests and printouts all the students letter grades based on their average. Use a 2D array to hold the average test scores of the 10 students for the 3 classes. Create a function to calculate the average test scores for 1 class of 10 students Reuse these functions: 1. double calculateavg(int studentnum) 2. void printgrade(double score)

35 35 double calculateavg(int studentnum) { double sum = 0; //sum of all test scores double testscore; //individual student test score cout << Student: << studentnum << endl; } for (int i=0; i< 3; i++) { } cout << Enter test score << i+1 << : ; cin >> testscore; sum = sum + testscore; return sum/3;

36 void printgrade(double score) { if (score >= 90) cout << A << endl; else if (score >= 80) cout << B << endl; else if (score >= 70) cout << C << endl; else if (score >= 60) cout << D << endl; else cout << F << endl; } 36

37 37 Take Home Problem: Write a program to calculate the average test score for each student in 3 classes of 10 students on 3 tests and printouts all the students letter grades based on their average. Use a 2D array to hold the average test scores of the 10 students for the 3 classes. Create a function to calculate the average test scores for 1 class of 10 students. Use this function header: 1. void calculateclassavgs(double studentavgs[], int classnum, int numofstudents) Reuse these functions: 1. double calculateavg(int studentnum) 2. void printgrade(double score)