Useful tools for building programs or program segments
x == y // x is equal to y x != y // x is not equal to y x < y // x is less than y x <= y // x is less than or equal to y x > y // x is greater than y x >= y // x is greater than or equal to y
We also have Boolean operators for combining expressions. Again, these operators return 0 for false and 1 for true
x && y // the AND operator -- true if both x and y are true x || y // the OR operator -- true if either x or y (or both) are true !x // the NOT operator (negation) -- true if x is false
These operators will be commonly used as test expressions in selection
statements or repetition statements (loops).
In the Boolean AND expression (x && y), if x is false, there is no need to evaluate y (so the evaluation stops).
Similarly, for the Boolean OR operation (x || y), if the first
part is true, the whole thing is true, so there is no need to continue the
evaluation. The computer only evaluates as much of the expression as it
needs. This can allow the programmer to write faster executing code.
Example:
bool answer; answer = true;
if (expression)
statement
else
statement
The else clause is optional.
Note: In this format, the expression part can be any expression, but it must be enclosed in parintheses ( ). Any expression that evaluates to 0 is interpreted as false. Anything else is interpreted as true. The statement part can be a single line statement, or it can be a compound statement. If it is a compound statement, remember that it must be enclosed in set braces { }.
Examples:
if (grade >= 68)
cout << "Passing";
// Notice that there is no else clause. If the grade is below 68, we move on.
if (x == 0)
cout << "Nothing here";
else
cout << "There is a value";
// This example contains an else clause. The bodies are single
statements.
if (y != 4)
{
cout << "Wrong number";
y = y * 2;
counter++;
}
else
{
cout << "That's it!";
success = 1;
}
Multiple statements are to be executed as a result of the condition
being true or false. In this case, notice the compound statement
to delineate the bodies of the if and else clauses.
// What output will it produce if val = 2? Does the "too bad" statement really go with the "else" here?
if (val < 5)
cout << "True\n";
else
cout << "False\n";
cout << "Too bad!\n";
* Indentation is only for people! It improves readability, but means nothing to the compiler.
Miscellaneous if/else examples
The switch statement: Another type of selection statement is the switch statement, which is good for occasions in which you are selecting among many different choices of values. The syntax format is:
switch (expression)
{
case constant:
statements
case constant:
statements
... (as many case labels as needed)
default: // optional label
statements
}
The switch statement evaluates the expression, and then compares it to the values in the case labels. If it finds a match, execution of code jumps to that case label. The values in case labels must be constants, which means that you may use an integer literal, a character literal, or a constant variable. You may not have case labels with regular variables, strings, or floating point literals. The compiler must know the exact value of each label, without ambiguity. If you want to execute code only in the case that you jump to, remember to end the case with a break statement!
Switch Example 1 -- syntactically correct, but logically incorrect
Switch Example 2 -- corrected version of Example 1
Example 3: This examples uses character literals for the case labels. It also allows for both upper and lower case menu choices.
cout << "Enter menu choice: ";
cin >> option;
switch(option)
{
case 'A':
case 'a':
result = a + b;
break;
case 'S':
case 's':
result = a - b;
break;
case 'M':
case 'm':
result = a * b;
break;
default:
result = 0;
}
Formats:
while (expression)
statement
do
statement
while (expression);
for (initial condition; test expression; iterative statement)
statement
Examples: Each of the following loops adds up all of the numbers between 1 and 50. However, here are three separate ways of doing it.
// while loop example
// loop body runs 50 times, condition checked 51 times
int i = 1, sum = 0;
while (i <= 50)
{
sum += i;
i++;
}
cout << "Sum of numbers from 1 through 50 is " << sum;
// do/while loop example
// loop body runs 50 times, condition checked 50 times
int i = 1, sum = 0;
do
{
sum += i;
i++;
} while (i <= 50);
cout << "Sum of numbers from 1 through 50 is " << sum;
// for loop example
// loop body runs 50 times, condition checked 51 times
int sum = 0;
for (int i = 1; i <= 50; i++)
{
sum += i;
}
cout << "Sum of numbers from 1 through 50 is " << sum;
for (int counter = 0; counter < 10; counter++)
{ (loop body) }
// The variable "counter" is now out of scope
This can be avoided by declaring the control variable before the loop itself.
int counter; // declaration of control variable
for (counter = 0; counter < 10; counter++)
{ (loop body) }
// The variable "counter" is still in scope
2) For loops also do not have to count one-by-one, or even upward. Examples:
for (i = 100; i > 0; i--) for (c = 3; c <= 30; c+=4)
The first example gives a loop header that starts counting at 100 and
decrements its control variable, counting down to 1 (and quitting when
i reaches 0). The second example shows a loop that begins counting
at 3 and counts by 4's (the second value of c will be 7, etc).
break: causes immediate exit from switch
structures and all three types of loops.
continue: only used in loops. A continue
statement in a loop causes that loop execution to end, and the next loop
iteration begins (i.e. continue causes the remainder of the loop body to
be skipped for that loop iteration).