book c++ programing 5th edition by DS Malik S hort Essay Questions A. Selection
ID: 3559330 • Letter: B
Question
book c++ programing 5th edition by DS Malik
Short Essay Questions
A. Selection structures can be used interchangeably for many circumstances. There are a few occasions when one is clearly preferable to another based on input data. Some of these situations were touched on in the switch statement discussion. Review these, give an overview of all of the selection structures discussed in the chapter, and then discuss how to determine which structure to use when there is no clear indication.
B. Repetition structures can be used interchangeably for many circumstances. There are a few occasions when one is clearly preferable to another. Review these, give an overview of all of the repetition structures discussed in the chapter, and then discuss how to determine which structure to use when there is no clear indication.
Explanation / Answer
A) Selection statements: if and else The if keyword is used to execute a statement or block, if, and only if, a condition is fulfilled. Its syntax is: if (condition) statement Here, condition is the expression that is being evaluated. If this condition is true, statement is executed. If it is false
he switch statement has a somewhat peculiar syntax inherited from the early times of the first C compilers, because it uses labels instead of blocks. In the most typical use (shown above), this means that break statements are needed after each group of statements for a particular label. If break is not included, all statements following the case (including those under any other labels) are also executed, until the end of the switch block or a jump statement (such as break) is reached.
If the example above lacked the break statement after the first group for case one, the program would not jump automatically to the end of the switch block after printing x is 1, and would instead continue executing the statements in case two (thus printing also x is 2). It would then continue doing so until a break statement is encountered, or the end of the switch block. This makes unnecessary to enclose the statements for each case in braces {}, and can also be useful to execute the same group of statements for different possible values.
Notice that switch is limited to compare its evaluated expression against labels that are constant expressions. It is not possible to use variables as labels or ranges, because they are not valid C++ constant expressions.
To check for ranges or values that are not constant, it is better to use concatenations of if and else if statements.
B)
Loops repeat a statement a certain number of times, or while a condition is fulfilled. They are introduced by the keywords while, do, and for.
The while loop
The simplest kind of loop is the while-loop. Its syntax is:
while (expression) statement
The while-loop simply repeats statement while expression is true. If, after any execution of statement, expression is no longer true, the loop ends, and the program continues right after the loop
he first statement in main sets n to a value of 10. This is the first number in the countdown. Then the while-loop begins: if this value fulfills the condition n>0 (that n is greater than zero), then the block that follows the condition is executed, and repeated for as long as the condition (n>0) remains being true.
The whole process of the previous program can be interpreted according to the following script (beginning in main):
A thing to consider with while-loops is that the loop should end at some point, and thus the statement shall alter values checked in the condition in some way, so as to force it to become false at some point. Otherwise, the loop will continue looping forever. In this case, the loop includes --n, that decreases the value of the variable that is being evaluated in the condition (n) by one - this will eventually make the condition (n>0) false after a certain number of loop iterations. To be more specific, after 10 iterations, n becomes 0, making the condition no longer true, and ending the while-loop.
Note that the complexity of this loop is trivial for a computer, and so the whole countdown is performed instantly, without any practical delay between elements of the count (if interested, see sleep_for for a countdown example with delays)
The do-while loop
A very similar loop is the do-while loop, whose syntax is:
do statement while (condition);
It behaves like a while-loop, except that condition is evaluated after the execution of statement instead of before, guaranteeing at least one execution of statement
The do-while loop is usually preferred over a while-loop when the statement needs to be executed at least once, such as when the condition that is checked to end of the loop is determined within the loop statement itself. In the previous example, the user input within the block is what will determine if the loop ends. And thus, even if the user wants to end the loop as soon as possible by entering goodbye, the block in the loop needs to be executed at least once to prompt for input, and the condition can, in fact, only be determined after it is executed.
The for loop
The for loop is designed to iterate a number of times. Its syntax is:
for (initialization; condition; increase) statement;
Like the while-loop, this loop repeats statement while condition is true. But, in addition, the for loop provides specific locations to contain an initialization and an increase expression, executed before the loop begins the first time, and after each iteration, respectively. Therefore, it is especially useful to use counter variables as condition.
It works in the following way:
ecause each of the fields is executed in a particular time in the life cycle of a loop, it may be useful to execute more than a single expression as any of initialization, condition, or statement. Unfortunately, these are not statements, but rather, simple expressions, and thus cannot be replaced by a block. As expressions, they can, however, make use of the comma operator (,): This operator is an expression separator, and can separate multiple expressions where only one is generally expected. For example, using it, it would be possible for a for loop to handle two counter variables, initializing and increasing both:
Range-based for loop
The for-loop has another syntax, which is used exclusively with ranges:
for ( declaration : range ) statement;
This kind of for loop iterates over all the elements in range, where declaration declares some variable able to take the value of an element in this range. Ranges are sequences of elements, including arrays, containers, and any other type supporting the functions begin and end; Most of these types have not yet been introduced in this tutorial, but we are already acquainted with at least one kind of range: strings, which are sequences of characters.
Jump statements
Jump statements allow altering the flow of a program by performing jumps to specific locations.
The break statement
break leaves a loop, even if the condition for its end is not fulfilled. It can be used to end an infinite loop, or to force it to end before its natural end. For example, let's stop the countdown before its natural end
The continue statement
The continue statement causes the program to skip the rest of the loop in the current iteration, as if the end of the statement block had been reached, causing it to jump to the start of the following iteration
goto allows to make an absolute jump to another point in the program. This unconditional jump ignores nesting levels, and does not cause any automatic stack unwinding. Therefore, it is a feature to use with care, and preferably within the same block of statements, especially in the presence of local variables.
The destination point is identified by a label, which is then used as an argument for the goto statement. A label is made of a valid identifier followed by a colon (:).
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