Part 5 - String class, Files and Streams, C++ vs Java

Question 1

WHat will

string s4(5, ‘x’);

do?

Answer 1

Make a string with 5 ‘x’ s i.e.

“xxxxx”

Question 2

What does the .at method do in strings?

What is the implication of this?

Answer 2

Returns a reference to the character at that position

Can do

s.at(2) = ‘e’;

Question 3

What does the append function do to strings?

Answer 3

s1.append(s2) will append s2 to the end of s1

same as +=

Question 4

What will s2.append(3, ‘!’);

do?

Answer 4

append 3 !s to the end of s2

Question 5

Demonstrate substrings, replacing strings and erasing parts of strings

Answer 5

s1. substr(m, n) will get a copy of the string n characters long starting at m
s1. replace(m, n, x) will replace substring or length n starting at m with a copy x
s1. erase(m, n) will erase n characters starting with m

Question 6

How do the < <= > and >= operators compare strings?

Answer 6

Lexicographically using ASCII ordering

Question 7

what does the compare function do with strings?

Answer 7

s1.compare(s2) returns 0 if they are equal, negative number if s1 is before s2 and positive number of s2 is before s1

Question 8

What will the find and rfind functions do with strings?

Answer 8

s. find(‘l’) finds the index of the first instance of l in the string s
s. rfind finds the index of the last instance of l in the string s

if l is not found string::npos is returned

Question 9

What is the second argument that can optionally be used with the find or rfind functions on the string class?

Answer 9

start index

Question 10

Write a function to count the number of instances of a given character in a string

Answer 10

int count(string s, char c) {

int tot = 0;

int pos = s.find(c);

while (pos != string::npos) {

tot++;

pos = s.find(c);

}

return tot;

}

Question 11

What do the find_first_of and find_first_not_of functions do?

Answer 11

s. find_first_of(“ \t\n”) finds the first white space character
s. find_first_not_of(“ \t\n”) finds the first non whitespace character

Question 12

What is the getline function used for? How is it used?

Answer 12

To get a line of text that may contain whitespace

Takes reference to a stream and reference to string where result will be stored, e.g.

getline(cin, s2)

Question 13

What are the benefits of a string stream?

Answer 13

Input and output to be performed to and from string objects

Formatting like setw and setfill can be used

Question 14

What class and header can be used for an output string stream?

Give an example of a method to output something using one

Answer 14

<sstream></sstream>

ostringstream

void fun(string s) {

ostringstream os;

os << setfill(‘0’) << setw(10) << s;

return os.str();

}

Question 15

Give an example using istringstream to convert a string to an int

Answer 15

int stoi2(string s) {

istringstream str(s);

int i;

str >> i;

return i;

}

Question 16

How can files be

a) input
b) output

using file streams?

Answer 16

a) ifstream
b) ofstream

Question 17

What header is needed for filestreams?

Answer 17

<fstream></fstream>

Question 18

What are the 4 different access modes for input/output fstreams?

Answer 18

ios: :in (input)
ios: :out (output)
ios: :app (append)
ios: :binary (binary files)

Question 19

Write the body of a method that will append “hi” to a file “output.txt” using an output file stream

Answer 19

ofstream os;

os.open(“output.txt”, ios::app);

if (os) { os << “hi” << endl }

os.close();

Question 20

What does the get function do with input streams?

What if the end of the file has been reached?

Answer 20

Gets the next character in the file (possibly whitespace)

If end of file reached, EOF will be returned

Question 21

what does the function eof() do to an input stream?

Answer 21

Returns true if the end of the file has been reached

Question 22

How does the getline function work with input streams?

Answer 22

is.getline(addrofbuffertostorelinein, maxlinelength);

Question 23

Describe what the following code will do:

ifstream str(“data.txt”);

const int SIZE = 80;

char buffer[SIZE];

while (!str.eof()) {

str.getline(buffer, SIZE);

// process buffer

}

Answer 23

Read data.txt line by line

if any line contains more than 79 characters, it will be read as multiple 79 character lines

Question 24

How can you switch between fixed and scientific outputs in output streams?

Answer 24

cout << fixed << pi << endl;

cout << scientific << pi << endl;

Question 25

How can you change the precision of floats in output streams?

Answer 25

cout << precision(5) << pi << endl;

Question 26

What two alignment types are permitted in fixed witdh fields of output streams?

Answer 26

cout << left << setw(5) << n << endl;

cout << right << setw(5) << n << endl;