Chapter 5

Question 1

Increment Operator

Answer 1

prefix: val++;
postfix: ++val;
- adds one to a variable
- can be used in expressions: (result = num1++ + –num2)
- can be used in relational expressions (pre/postfix operations will cause different comparisons)

Question 2

Decrement Operator

Answer 2

prefix: –val;
postfix: val–;
- subtracts one from a variable
- can be used in expressions: (result = num1++ + –num2)
- can be used in relational expressions (pre/postfix operations will cause different comparisons)

Question 3

prefix vs postfix

Answer 3

prefix mode: increments or decrements then returns the value of the variable
postfix mode: returns the value, then increments or decrements

Question 4

Loop

Answer 4

• a control structure that causes a statement or statements to repeat

Question 5

while loop format

Answer 5

while (expression) {
statement;
statement;
}

Question 6

how does the while loop work

Answer 6

expression is evaluated before the loop executes
- if true, then statement is executed, and expression is evaluated again
- if false, then the loop is finished and program statements following statement execute

Question 7

things to watch out for in loops

Answer 7

• loop must contain code to make expression become false; otherwise, the loop will have no way of stopping - called an infinite loop because it will repeat an infinite number of times

Question 8

using the while loop for input validation

Answer 8

• the while loop can be used to create input routines that reject invalid data, and repeat until valid data is entered.
• general approach in pseudocode
  read an item of input
  while the input is invalid
  display an error message
  read the input again
  end while

Question 9

counters

Answer 9

• a variable that is incremented or decremented each time a loop repeats
• can be used to control execution of the loop (AKA: loop control variable)
• must be initialized before entering loop

Question 10

do while loop

Answer 10

• a posttest loop, execute the loop, then test the expression
• always executes at least once
• execution continues as long as expression is true, stops repeating when expression becomes false
• useful in menu-driven programs to bring user back to menu selection

Question 11

do while loop format

Answer 11

do {
statement;
} while (expression);

Question 12

for loop

Answer 12

• useful for counter-controlled loop
• pretest loop: tests its test expression before each iteration
• can have multiple statements in the initialization expressions
• can have multiple statements in the test expression
• can omit the initialization expression if var has already been initialized
• can declare variables in the initialization expression: then. the scope of the variable is the for loop

Question 13

how the for loop works

Answer 13

1) perform initialization
2) evaluate test expression
- if true, execute statement
- if false, terminate loop execution
3) execute update, then re-evaluate test expression

Question 14

for loop format

Answer 14

for (initialization; test; update) {
statement;
}

Question 15

when to use the for loop

Answer 15

in any situation that clearly requires
- an initialization
- a false condition to stop the loop
- an update to occur at the end of each iteration

Question 16

running total

Answer 16

accumulated sum of numbers from each repetition of loop

Question 17

accumulator

Answer 17

variable that holds running total

Question 18

sentinel

Answer 18

• value in a list of values that indicates end of data
• special value cannot be confused with a valid value
• used to terminate input when user may not know many values will be entered

Question 19

deciding which loop to use

Answer 19

1) the while loop is a conditional pretest loop
- iterates as long as a certain condition exits
- validating input
- reading lists of data terminated by a sentinel

2) the do-while loop is a conditional posttest loop
- always iterates at least once
- repeating a menu

3) the for loop is a pretest loop
- built-in expressions for initializing, testing, and updating
- situations where the exact number of iterations is known

Question 20

nested loops

Answer 20

• a loop inside the body of another loop
• inner loop goes through all repetitions for each repetition of outer loop
• inner loop repetitions complete sooner than outer loop
• total number of repetitions for inner loop is product of number of repetitions of the two loops

Question 21

nested for loop format

Answer 21

for (initialization; test; update) { // outer loop
for (initialization; test; update) { // inner loop
statement;
}
statement;
}

Question 22

files

Answer 22

• can use files instead of keyboard, monitor screen for program input, output
• allows data to be retained between program runs
• fstream header file for file access

Question 23

steps for using files

Answer 23

1. open the file
2. use the file (read from, write to, or both)
3. close the file

Question 24

file stream types

Answer 24

• ifstream: for input from a file
• ofstream: for output to a file
• fstream: for input from or output to a file

Question 25

define file stream objects

Answer 25

ifstream infile;
ofstream outfile;

Question 26

opening files

Answer 26

• create a link between file name (outside the program) and file stream object (inside the program) by using the open member function
• open member function:
  infile.open(“inventory.dat”);
  outfile.open(“report.txt”);
• filename may include drive, path info
• output file will be created if necessary; existing file will be erased first
• input file must exist for ‘open’ to work

Question 27

testing for file open errors

Answer 27

• can test a file stream object to detect if an open operation failed

infile.open(“test.txt”);
if (!infile) {
cout &laquo_space;“File open failure!”;
}

• can also use the fail member function

Question 28

using files

Answer 28

• can use output file object and &laquo_space; to send data to a file
  outfile &laquo_space; “Inventory report”;
• can use input file object and&raquo_space; to copy data from file to variables
  infile&raquo_space; partNum;
  infile&raquo_space; qtyInStock&raquo_space; qtyOnOrder;

Question 29

using loops to process files

Answer 29

• the stream extraction operator&raquo_space; returns true when a value was successfully read, false otherwise
• can be tested in a while loop to continue execution as long as values are read from the file:
  while (inputFile&raquo_space; number) . . .

Question 30

closing files

Answer 30

• use the close member function:
  infile.close();
  outfile.close();
• dont wait for operating system to close files at program end
  
  • may be limit on number of open files
  • may be buffered output data waiting to send to file

Question 31

letting user specify a filename

Answer 31

• in many cases, you will want the user to specify the name of a file for the program to open
• in C++, you can pass a string object as an argument to a file stream object’s open member function

Question 32

c_str in older versions of C++

Answer 32

• prior to c++ 11, the open member function requires that you pass the name of the file as a null-terminated string, AKA C-string
• string literals are stored in memory as null-terminated C-strings, but string objects are not
• string objects have a member function named c_str
  
  • it returns the contents of the object formatted as a null-terminated C-string
  • general format:
    stringObject.c_str()
  • example :
    inputFile.open(filename.c_str());

Question 33

breaking out of a loop

Answer 33

• can use break to terminate execution of a loop
• use sparingly if at all - makes code harder to understand and debug
• when used in an inner loop, terminates that loop only and goes back to outer loop

Question 34

the continue statement

Answer 34

• can use continue to go to end of loop and prepare for next repetition
  
  • while, do-while loops: go to test, repeat loop if test passes
  • for loop: perform update step, then test, then repeat loop if test passes
• use sparingly - like break, can make program logic hard to follow