2007 Copyright Pearson Education, Inc. Publishing as Pearson Addison-Wesley
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Chapter
5
Looping
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5.1
The Increment and Decrement Operators
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The Increment and Decrement Operators
++ is the increment operator. It adds one to a variable. val++; is the same as val = val + 1;
++ can be used before (prefix) or after (postfix) a variable: ++val; val++;
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The Increment and Decrement Operators
-- is the decrement operator. It subtracts one from a variable. val--; is the same as val = val - 1;
-- can be also used before (prefix) or after (postfix) a variable: --val; val--;
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(Program Continues)
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Prefix vs. Postfix
++ and -- operators can be used in complex statements and expressions In prefix mode (++val, --val) the operator increments or decrements, then returns the value of the variable In postfix mode (val++, val--) the operator returns the value of the variable, then increments or decrements
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Prefix vs. Postfix - Examples
int num, val = 12; cout << val++; // displays 12, // val is now 13; cout << ++val; // sets val to 14, // then displays it num = --val; // sets val to 13, // stores 13 in num num = val--; // stores 13 in num, // sets val to 12
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Notes on Increment, Decrement
Can be used in expressions: result = num1++ + --num2; Must be applied to something that has a location in memory. Cannot have: result = (num1 + num2)++; Can be used in relational expressions: if (++num > limit) pre- and post-operations will cause different comparisons
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5.2
Introduction to Loops: The while Loop
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Introduction to Loops: The while Loop
Loop: a control structure that causes a statement or statements to repeat General format of the while loop: while (expression) statement; statement; can also be a block of statements enclosed in { }
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while Loop How It Works
while (expression) statement; expression is evaluated if true, then statement is executed, and expression is evaluated again if false, then the the loop is finished and program statements following statement execute
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The Logic of a while Loop
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How the Loop in Lines 9 through 13 Works
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Flowchart of the Loop
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while is a Pretest Loop
expression is evaluated before the loop executes. The following loop will never execute: int number = 6; while (number <= 5) { cout << "Hello\n"; number++; }
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Watch Out for Infinite Loops
The loop must contain code to make expression become false Otherwise, the loop will have no way of stopping Such a loop is called an infinite loop, because it will repeat an infinite number of times
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An Infinite Loop
int number = 1; while (number <= 5) { cout << "Hello\n"; }
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5.3
Using the while Loop for Input Validation
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Using the while Loop for Input Validation
Input validation is the process of inspecting data that is given to the program as input and determining whether it is valid. The while loop can be used to create input routines that reject invalid data, and repeat until valid data is entered.
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Using the while Loop for Input Validation
Here's the general approach, in pseudocode:
Read an item of input. While the input is invalid Display an error message. Read the input again. End While
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Input Validation Example
cout << "Enter a number less than 10: "; cin >> number; while (number >= 10) { cout << "Invalid Entry!" << "Enter a number less than 10: "; cin >> number; }
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Flowchart
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Input Validation Example from Program 5-4
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5.4
Counters
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Counters
Counter: a variable that is incremented or decremented each time a loop repeats Can be used to control execution of the loop (also known as the loop control variable) Must be initialized before entering loop
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5.5
The do-while Loop
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The do-while Loop
do-while: a posttest loop execute the loop, then test the expression General Format: do statement; // or block in { } while (expression); Note that a semicolon is required after (expression)
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The Logic of a do-while Loop
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do-while Example
int x = 1; do { cout << x << endl; } while(x < 0);
Although the test expression is false, this loop will execute one time because do-while is a posttest loop.
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do-while Loop Notes
Loop always executes at least once Execution continues as long as expression is true, stops repetition when expression becomes false Useful in menu-driven programs to bring user back to menu to make another choice (see Program 5-7 in the book)
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The for Loop
Useful for counter-controlled General loop Format: for(initialization; test; update) statement; // or block in { }
No semicolon after 3rd expression or after the )
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for Loop - Mechanics
for(initialization; test; update)
statement; // or block in { } Perform initialization Evaluate test expression If true, execute statement If false, terminate loop execution Execute update, then re-evaluate test expression
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for Loop - Example
int count; for (count = 1; count <= 5; count++)
cout << "Hello" << endl;
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A Closer Look at the Previous Example
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Flowchart for the Previous Example
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A Closer Look at Lines 13 through 14 in Program 5-8
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Flowchart for Lines 13 through 14 in Program 5-8
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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
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The for Loop is a Pretest Loop
The for loop tests its test expression before each iteration, so it is a pretest loop. The following loop will never iterate:
for (count = 11; count <= 10; count++) cout << "Hello" << endl;
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for Loop - Modifications
You can have multiple statements in the initialization expression. Separate the statements with a comma: Initialization Expression
int x, y; for (x=1, { cout << << << } y=1; x <= 5; x++) x << " plus " << y " equals " << (x+y) endl;
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for Loop - Modifications
You can also have multiple statements in the test expression. Separate the statements with a comma: Test Expression
int x, y; for (x=1, { cout << << << } y=1; x <= 5; x++, y++) x << " plus " << y " equals " << (x+y) endl;
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for Loop - Modifications
You can omit the initialization expression if it has already been done:
int sum = 0, num = 1; for (; num <= 10; num++) sum += num;
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for Loop - Modifications
You can declare variables in the initialization expression: int sum = 0; for (int num = 0; num <= 10; num++) sum += num; The scope of the variable num is the for loop.
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5.7
Keeping a Running Total
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Keeping a Running Total
running total: accumulated sum of numbers from each repetition of loop accumulator: variable that holds running total
int sum=0, num=1; // sum is the while (num <= 10) // accumulator { sum += num; num++; } cout << "Sum of numbers 1 10 is" << sum << endl;
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(Program Continues)
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5.8
Sentinels
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Sentinels
sentinel: value in a list of values that indicates end of data Special value that cannot be confused with a valid value, e.g., -999 for a test score Used to terminate input when user may not know how many values will be entered
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(Program Continues)
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5.9
Using a Loop to Read Data from a File
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Using a Loop to Read Data from a File
The stream extraction operator >> 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 >> number) ...
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5.10
Deciding Which Loop to Use
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Deciding Which Loop to Use
while: pretest loop; loop body may not be executed at all do-while: posttest loop; loop body will always be executed at least once for: pretest loop with initialization and update expression; useful with counters, or if precise number of repetitions is needed
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5.11
Nested Loops
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Nested Loops
A nested loop is a loop inside the body of another loop Inner (inside), outer (outside) loops: for (row=1; row<=3; row++) //outer for (col=1; col<=3; col++)//inner cout << row * col << endl;
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Lines from Program 5-14
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Nested Loops - Notes
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.
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5.12
Breaking Out of a Loop
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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
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5.13
The continue Statement
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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
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