Chapter 5 Flashcards
Increment Operator
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)
Decrement Operator
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)
prefix vs postfix
prefix mode: increments or decrements then returns the value of the variable
postfix mode: returns the value, then increments or decrements
Loop
- a control structure that causes a statement or statements to repeat
while loop format
while (expression) {
statement;
statement;
}
how does the while loop work
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
things to watch out for in loops
- 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
using the while loop for input validation
- 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
counters
- 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
do while loop
- 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
do while loop format
do {
statement;
} while (expression);
for loop
- 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
how the for loop works
1) perform initialization
2) evaluate test expression
- if true, execute statement
- if false, terminate loop execution
3) execute update, then re-evaluate test expression
for loop format
for (initialization; test; update) {
statement;
}
when to use the for loop
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
running total
accumulated sum of numbers from each repetition of loop
accumulator
variable that holds running total
sentinel
- 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
deciding which loop to use
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
nested loops
- 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
nested for loop format
for (initialization; test; update) { // outer loop
for (initialization; test; update) { // inner loop
statement;
}
statement;
}
files
- 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
steps for using files
- open the file
- use the file (read from, write to, or both)
- close the file
file stream types
- ifstream: for input from a file
- ofstream: for output to a file
- fstream: for input from or output to a file
define file stream objects
ifstream infile;
ofstream outfile;
opening files
- 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
testing for file open errors
- can test a file stream object to detect if an open operation failed
infile.open(“test.txt”);
if (!infile) {
cout «_space;“File open failure!”;
}
- can also use the fail member function
using files
- can use output file object and «_space; to send data to a file
outfile «_space; “Inventory report”; - can use input file object and»_space; to copy data from file to variables
infile»_space; partNum;
infile»_space; qtyInStock»_space; qtyOnOrder;
using loops to process files
- the stream extraction operator»_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»_space; number) . . .
closing files
- 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
letting user specify a filename
- 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
c_str in older versions of C++
- 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());
breaking out of a loop
- 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
the continue statement
- 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
