CS101 Fundamentals of Computer Programmingstejada/csci101/slides/wk4b-arrays2.pdfCS101: Fundamentals of Computer Programming! Dr. Tejada [email protected] stejada Week 4: Arrays,Strings,

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

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

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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;

}

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4

void printGrade(double score)

{

if (score >= 90)

cout << “A” << endl;

else if (score >= 80)

cout << “B” << endl;


cout << “C” << endl;


cout << “D” << endl;

else

cout << “F” << endl;

}

int main()

{

const int NUM_OF_STUDENTS = 10;

double averages[NUM_OF_STUDENTS]= {0};

//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]);

} }

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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:

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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

7

Accessing 2D Array Components


8

Example: sales[5][3] = 25.75;

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: int board[4][3]= {{2, 3, 1}

{15, 25, 13}

{20, 4, 7} {11, 18, 14}};

9

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


10

Ini,aliza,on � Examples:

�  To ini,alize row number 4 (fiPh row) to 0:

�  To ini,alize the en,re matrix to 0:


11

Input � Examples:

�  To input into row number 4 (fiPh row):

�  To input data into each component of matrix:


12

Sum by Row � Example:

�  To find the sum of row number 4:


13

Sum by Column � Example:

�  To find the sum of each individual column:

14

Find the Largest Element

15

•  Example: – To find the largest element in each row:

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


16

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


17

Arrays of Strings and Character Arrays

18

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‘

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� 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

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� 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>

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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"

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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

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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

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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

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� 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

26

Forma`ng Output

�  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

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Addi,onal Output Forma`ng Tools

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


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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; ifile.close(); ofile.close();

return 0; }

5 -3.5

input.txt

Int from file is 5 Double from file is -3.5

output.txt

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

I t w a s t h e b e s t o f

...

fp

char c; ifile >> c;

a t f e r . T h e E n d ! EOF

fp


fp

...

...

...

char c; ifile >> c; // get first char while( ifile.good() ) { // process c ifile >> c; // get next char

}

File text

>> 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 c; ifile >> c; // skips space and returns ‘w’


...

char ch; ifile.get(ch); // get next char while( ifile.good() ) { // process ch ifile.get(ch); // get next char

}

File text

fp

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)

#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; }

The fox jumped over the log.

The bear ate some honey.

The CS student solved a hard problem.

1: The fox jumped over the log.

2: The bear ate some honey.

3: The CS student solved a hard problem.

input.txt

output

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; }

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)

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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;

}

35

36

void printGrade(double score)

{

if (score >= 90)

cout << “A” << endl;


cout << “B” << endl;


cout << “C” << endl;


cout << “D” << endl;

else

cout << “F” << endl;

}

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)

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Documents

CS101 Fundamentals of Computer Programmingstejada/csci101/slides/wk4b-arrays2.pdfCS101: Fundamentals of Computer Programming! Dr. Tejada [email protected] stejada Week 4: Arrays,Strings,