C for Dummies 2nd edition phần 5 pptx

The < symbol between them means “less than.” The test reads “If the value of the variable number is less than 5.” If this is true, the cluster of statements following the if keyword is e

The if keyword allows you to put these types of decisions into your pro­grams The decisions are based on a comparison For example:

 If the contents of the variable calories are very high, it must taste very

good

 If it ain’t broke, don’t fix it

 If Doug doesn’t ask me out to the prom, I’ll have to go with Charley All these examples show important decisions, similar to those you can make

in your C programs by using the if keyword However, in the C programming language, the if keyword’s comparisons are kind of, sort of — dare I say it? — mathematical in nature Here are more accurate examples:

 If the value of variable A is equal to the value of variable

 If the contents of variable ch

 If the value of variable zed

These examples are really simple, scales-of-justice evaluations of variables and values The if keyword makes the comparison, and if the comparison is true, your program does a particular set of tasks

 if is a keyword in the C programming language It allows your programs

to make decisions

 if decides what to do based on a comparison of (usually) two items

 The comparison that if makes is mathematical in nature: Are two items equal to, greater than, less than — and so on — to each other? If they are, a certain part of your program runs If not, that part of the program doesn’t run

 The if keyword creates what is known as a selection statement in the C

language I wrote this topic down in my notes, probably because it’s in

some other C reference I have read at some time or another Selection

statement Impress your friends with that term if you can remember it Just

throw your nose in the air if they ask what it means (That’s what I do.)

The following program is GENIE1.C, one of many silly computer number programs you write when you find out how to program Computer scientists used to play these games for hours in the early days of the com­puter They would probably drop dead if we could beam a Sony PlayStation back through time

guess-the-What GENIE1.C does is to ask for a number, from 0 through 9 You type that number at the keyboard Then, using the magic of the if statement, the com­

puter tells you whether the number you entered is less than 5 This program was a major thigh-slapper when it was first written in the early 1950s

Enter the following source code into your text editor The only new stuff comes with the if statement cluster, near the end of the program Better double-double-check your typing

#include <stdio.h>

#include <stdlib.h>

int main() {

char num[2];

int number;

printf(“I am your computer genie!\n”);

printf(“Enter a number from 0 to 9:”);

gets(num);

number=atoi(num);

if(number<5) {

printf(“That number is less than 5!\n”);

} printf(“The genie knows all, sees all!\n”);

return(0);

}

I am your computer genie!

Enter a number from 0 to 9:

Type a number, somewhere in the range of 0 through 9 For example, you can type 3 Press Enter and you see:

That number is less than 5!

The genie knows all, sees all!

 The #include <stdlib.h> part is necessary because the program uses the atoi() function

 The if command is followed by parentheses, which contain the compar­ison that the if keyword tests

 The comparison that if makes tests the value of the variable number

with 5 The < symbol between them means “less than.” The test reads “If the value of the variable number is less than 5.” If this is true, the cluster

of statements following the if keyword is executed If the test proves false, the cluster of statements is skipped

 Remember the < — less than — from school? Good!

 Notice that the if test isn’t followed by a semicolon! Instead, it’s fol­lowed by a statement enclosed in curly braces The statements (there can be more than one) “belong” to the if command and are executed

only if the condition is true

 If you see only the line The genie knows all, sees all!, you proba­bly typed a number greater than 4 (which includes 5 and higher) The reason is that the if statement tests only for values less than 5 If the

value is less than 5, That number is less than 5! is displayed The next section elaborates on how it all works

 No, the computer genie doesn’t know all and see all if you type a number

Figure 12-1:

How

printf("whatever");

if(chins>3) {

printf("Where is your neck?");

} printf("something else");

Program execution

?

False True

if is followed by a set of parentheses in which a comparison is made The

comparison is mathematical in nature, using the symbols shown in Table 12-1

What’s being compared is usually the value of a variable against a constant value, or two variables against each other (See Table 12-1 for examples.)

If the result of the comparison is true, the statement (or group of statements)

between the curly braces is executed If the result is false, the stuff in the curly braces is conveniently skipped over — ignored like a geeky young lad

at his first high school dance and with a zit the size of Houston on his chin

Yes, the curly braces that follow if can contain more than one statement

And, each of the statements ends with a semicolon All are enclosed in the

curly braces It’s technically referred to as a code block It shows you which

statements “belong” to if The whole darn thing is part of the if statement

Table 12-1 Operators Used in if Comparisons

Comparison Meaning or Pronunciation “True” Examples

Table 12-1 (continued) Comparison Meaning or Pronunciation “True” Examples

printf(“That number is less than 5!\n”);

}

The first line is the if keyword and its comparison in parentheses What’s being compared is the value of the numeric variable number and the constant value 5 The comparison is “less than.” Is number less than 5? If so, the state­ment in curly braces is executed If not, the whole deal is skipped over Consider these modifications:

if(number==5) {

printf(“That number is more than 4!\n”);

}

This time, the test is greater than or equal to: Is the number that is entered 5

or more than 5? If the number is greater than or equal to 5, it must be more than 4, and the printf() statement goes on to display that important info on the screen

The following modification to the GENIE1.C program doesn’t change the if

comparison, as in the previous examples Instead, it shows you that more than one statement can belong to if:

if(number<5) {

printf(“That number is less than 5!\n”);

printf(“By goodness, aren’t I smart?\n”);

}

Everything between the curly braces is executed when the comparison is true

Advanced C programs may have lots of stuff in there; as long as it’s between the curly braces, it’s executed only if the comparison is true (That’s why it’s indented — so that you know that it all belongs to the if statement.)

 The comparison that if makes is usually between a variable and a value

It can be a numeric or single-character variable

 if cannot compare strings For information on comparing strings, refer

to my book C All-in-One Desk Reference For Dummies (Wiley)

 Less than and greater than and their ilk should be familiar to you from basic math If not, you should know that you read the symbols from left

to right: The > symbol is greater than because the big side comes first;

the < is less than because the lesser side comes first

 The symbols for less than or equal to and greater than or equal to always appear that way: <= and >= Switching them the other way generates an error

 The symbol for “not” in C is the exclamation point So, != means “not equal.” What is !TRUE (not-true) is FALSE “If you think that it’s butter, but it’s !.” No, I do ! want to eat those soggy zucchini chips

 When you’re making a comparison to see whether two things are equal,

you use two equal signs I think of it this way: When you build an if

statement to see whether two things are equal, you think in your head

“is equal” rather than “equals.” For example:

if(x==5)

Read this statement as “If the value of the x variable is equal to 5, then .” If you think “equals,” you have a tendency to use only oneequal sign — which is very wrong

 If you use one equal sign rather than two, you don’t get an error; how­ever, the program is wrong The nearby Technical Stuff sidebar attempts

to explain why

 If you have programmed in other computer languages, keep in mind that the C language has no 2ewd or fi word The final curly brace signals to the compiler that the if statement has ended

 Also, no then word is used with if, as in the if-then thing they have in the BASIC or Pascal programming language

A question of formatting the if

The if statement is your first “complex” C language statement The C lan­guage has many more, but if is the first and possibly the most popular, though I doubt that a popularity contest for programming language words has ever been held (and, then again, if would be great as Miss Congeniality but definitely come up a little thin in the swimsuit competition)

Though you probably have seen the if statement used only with curly braces, it can also be displayed as a traditional C language statement For example, consider the following — one of the modifications from the GENIE1 program:

if(number==5) {

printf(“That number is 5!\n”);

}

In C, it’s perfectly legitimate to write this as a more traditional type of state­ment To wit:

if(number==5) printf(“That number is 5!\n”);

This line looks more like a C language statement It ends in a semicolon Everything still works the same; if the value of the number variable is equal

to 5, the printf() statement is executed If number doesn’t equal 5, the rest

of the statement is skipped

Although all this is legal and you aren’t shunned in the C programming com­munity for using it, I recommend using curly braces with your if statements until you feel comfortable reading the C language

Clutter not thy head with this comparison nonsense

The comparison in the if

have to use any symbols at all! Strange but true

What the C compiler does is to figure out what you have put between the parentheses Then it For a comparison using <, >, ==, or any of the whether the comparison is true or false

any valid C statement — between the paren­

theses and the compiler determines whether it works out to true or false For example:

if(input=1)

This if

the value of the input variable is equal to 1

No, you need two equal signs for that Instead,

what happens between these parentheses is that the numeric variable input is given the value 1

input=1;

The C compiler obeys this instruction, stuffing 1 into the input variable Then, it sits back and strokes its beard and thinks, “Does that work out to be true or false?” Not knowing any true It tells the if keyword, and the cluster of statements that belong to the ifstatement are then executed

statement doesn’t

weighs whether it’s true or false

horde in Table 12-1, the compiler figures out However, you can stick just about anything —

statement doesn’t figure out whether

It’s the same as

better, it figures that the statement must be

The final solution to the income-tax problem

I have devised what I think is the fairest and most obviously well-intentioned way to decide who must pay the most in income taxes You should pay more taxes if you’re taller and more taxes if it’s warmer outside Yessir, it would be hard to dodge this one

This problem is ideal for the if keyword to solve You pay taxes based on either your height or the temperature outside, multiplied by your favorite number and then 10 Whichever number is higher is the amount of tax you pay To figure out which number is higher, the program TAXES.C uses the if

keyword with the greater-than symbol It’s done twice — once for the height value and again for the temperature outside:

#include <stdio.h>

#include <stdlib.h>

int main() {

tax1 = atoi(height) * atoi(favnum);

tax2 = atoi(temp) * atoi(favnum);

if(tax1>tax2) {

printf(“You owe $%d in taxes.\n”,tax1*10);

} if(tax2>=tax1) {

printf(“You owe $%d in taxes.\n”,tax2*10);

} return(0);

}

This program is one of the longer ones in this book Be extra careful when you’re typing it It has nothing new in it, but it covers almost all the informa­tion I present in the first several chapters Double-check each line as you type

it into your editor

Save the file to disk as TAXES.C

Compile TAXES.C Fix any errors you see

Run the program:

Enter your height in inches:

Type your height in inches Five feet is 60 inches; six feet is 72 inches The average person is 5'7" tall or so — 67 inches Press Enter

What temperature is it outside?

Right now, in the bosom of winter in the Pacific Northwest, it’s 18 degrees That’s Fahrenheit, by the way Don’t you dare enter the smaller Celsius number

If you do, the IRS will hunt you down like a delinquent country music star and make you pay, pay, pay

Enter your favorite number:

Type your favorite number Mine is 11 Press Enter

If I type 72 (my height), 18, and 11, for example, I see the following result, due April 15:

You owe $7920 in taxes

Sheesh! And I thought the old system was bad I guess I need a smaller favorite number

 The second if comparison is “greater than or equal to.” This catches the case when your height is equal to the temperature If both values are equal, the values of both the tax1 and tax2 variables are equal The first if comparison, “tax1 is greater than tax2,” fails because both are equal The second comparison, “tax1 is greater than or equal to tax2,”

passes when tax1 is greater than tax2 or when both values are equal

 If you enter zero as your favorite number, the program doesn’t say that you owe any tax Unfortunately, the IRS does not allow you to have zero —

or any negative numbers — as your favorite number Sad, but true

Hold on to that tax problem!

No, not the one the government created Instead, hold on to the TAXES.C source code introduced in the preceding section If it’s already in your text editor, great Otherwise, open it in your editor for editing

The last part of the TAXES.C program consists of two if statements The second if statement, which should be near Line 23 in your editor, really isn’t necessary Rather than use if in that manner, you can take advantage of another word in the C language, else

Change Line 23 in the TAXES.C program It looks like this now:

Save the file back to disk

Compile TAXES.C Run the final result The output is the same because the program hasn’t changed (and assuming that it hasn’t gotten any warmer and you haven’t grown any taller in the past few moments) What you have done

is to create an if-else structure, which is another way to handle the

decision-making process in your C programs

 The else keyword is a second, optional part of an if cluster of state­ments It groups together statements that are to be executed when the condition that if tests for isn’t true

 Or else what?

 Alas, if you enter the same values as in the old program, you still owe the same bundle to Uncle Sam

Covering all the possibilities with

The if-else keyword combination allows you to write a program that can make either-or decisions By itself, the if keyword can handle minor deci­sions and execute special instructions if the conditions are just so But when

if is coupled with else, your program takes one of two directions, depend­ing on the comparison if makes Figure 12-2 illustrates how this can happen

printf("whatever");

if(chins>3) {

printf("Where is your neck?");

} else { printf("My, but what a slender neck."); }

printf("something else");

Program execution

If the comparison is true, the statements belonging to the if statement are executed But, if the comparison is false, the statements belonging to the

else are executed The program goes one way or the other, as illustrated

in Figure 12-2 Then, after going its own way, the statement following the

else’s final curly brace is executed, like this: “You guys go around the left side of the barn, we’ll go around the right, and we’ll meet you on the other side.”

The if format with

The else keyword is used in an if statement The keyword holds its own group of statements to be executed (okay, “obeyed”) when the if compari­

son isn’t true Here’s the format:

if(comparison)

{

statement(s);

} else {

statement(s);

}

The if keyword tests the comparison in parentheses If it’s a true comparison — no foolin’ — the statements that appear in curly braces right after the if statement are executed But, if the comparison is false, those statements following the else keyword and enclosed in curly braces are executed One way or another, one group of statements is executed and the other isn’t

The else keyword, like all words in the C language, is in lowercase It isn’t followed by a semicolon Instead, a set of curly braces follows the else The curly braces enclose one or more statements to be run when the comparison that if makes isn’t true Notice that those statements each must end in a semicolon, obeying the laws of C first etched in stone by the ancient Palo Altoites

The statements belonging to the else keyword are executed when the condi­

tion that the if keyword evaluates is false Table 12-2 illustrates how it works, showing you the opposite conditions for the comparisons that an if keyword would make

Table 12-2 if Comparisons and Their Opposites

if Comparison else Statement Executed By This Condition

< >= (Greater than or equal to)

> <= (Less than or equal to)

<= > (Greater than)

 I don’t know about you, but I think that all those symbols in Table 12-2 would certainly make an interesting rug pattern

 The else keyword is used only with if

 Both if and else can have more than one statement enclosed in their curly braces if’s statements are executed when the comparison is true;

else’s statements are executed when the comparison is false

 To execute means to run C programs execute, or run, statements from

the top of the source code (the first line) to the bottom Each line is executed one after the other unless statements like if and else are encountered In that case, the program executes different statements, depending on the comparison that if makes

 When your program doesn’t require an either-or decision, you don’t have to use else For example, the TAXES program has an either-or deci­sion But, suppose that you’re writing a program that displays an error message when something doesn’t work In that case, you don’t need

else; if an error doesn’t occur, the program should continue as normal

 If you’re the speaker of another programming tongue, notice that the C language has no end-else word in it This isn’t smelly old Pascal, for goodness’ sake The final curly brace signals the end of the else state­ment, just as it does with if

The strange case of else-if

and even more decisions

The C language is rich with decision making The if keyword helps if you need to test for only one condition True or false, if handles it And, if it’s true, a group of statements is executed Otherwise, it’s skipped over (After the if’s group of statements is executed, the program continues as before.)

Silly formatting trivia

The if-else structure need not be laden with curly braces Just as you can abbre­

heavy-viate an if statement to one line, you can also abbreviate else

program that illustrates examples this crudely:

if(tax1>tax2) {

printf(“You owe $%i in taxes.\n”,tax1*10);

} else { printf(“You owe $%i in taxes.\n”,tax2*10);

else printf(“You owe $%i in taxes.\n”,tax2*10);

This format keeps the indenting intact, which is one way to see what belongs to what (and also

to easily identify the if-else structure) The following format is also possible, though it makes the program hard to read:

if(tax1>tax2) printf(“You owe

$%i in taxes.\n”,tax1*10);

else printf(“You owe $%i in taxes.\n”,tax2*10);

Everything is scrunched up on two lines; the if

statement has its own line, and the elsehas its own line Both lines end with a semicolon, which is how this works as two statements in

braces — whenever only one statement appears with an if or else keyword If multi­

ple statements must be executed, you’re why I recommend them all the time: No sense

if(tax1>tax2) printf(“You owe $%i in taxes.\n”,tax1*10);

else { printf(“You owe $%i in taxes.\n”,tax2*10);

printf(“It pays to live where it’s cold!\n”);

}

Because two printfstatements belong to the preceding else, the curly braces are required

I don’t recommend it, which

is why I’m terribly brief and don’t ever show a

of the TAXES.C program: the

the C language But, look-it It’s gross! Please don’t write your programs this way

You can do this trick — eliminating the curly

required by law to use the curly braces That’s risking prison over brevity To wit:

Either-or conditions are the daily bread of the if-else duo Either way, one set of statements is executed and not the other, depending on the compari­

son made by if What about “one, two, or the third” types of decisions? For them, you need the miraculous and overly versatile else-if combination It really drives you batty, but it’s handy

The following program is a modification of the GENIE1.C source code, as shown earlier in this chapter This time, the else-if combination is used to allow the computer genie to accurately report whether the number is less than 5, equal to 5, or greater than 5:

#include <stdio.h>

#include <stdlib.h>

int main() {

char num[2];

int number;

printf(“I am your computer genie!\n”);

printf(“Enter a number from 0 to 9:”);

gets(num);

number=atoi(num);

if(number<5) {

printf(“That number is less than 5!\n”);

} else if(number==5) {

printf(“You typed in 5!\n”);

} else { printf(“That number is more than 5!\n”);

} printf(“The genie knows all, sees all!\n”);

return(0);

}

Start working on this source code by loading the GENIE1.C source code I show you how to create earlier in this chapter Make modifications so that the latter part of the program looks like the new, GENIE2.C source code that was just listed

Watch your indenting Pay attention to everything; two equal signs are in the

else-if comparison Pay attention to where semicolons go and where they don’t go

After inserting the new lines, save the file to disk as GENIE2.C

Compile GENIE2.C If the error monster rears its ugly head, reedit the source code and then recompile

Run the final result and see how much more clairvoyant the computer genie

has become Type 3 and see That number is less than 5! Type 9 and

see That number is more than 5! Type 5 and the genie knows: You typed in 5!

 The else-if comparison resembles combined else and if statements

The second if comes right after else and a space Then, it has its own comparison statement, which is judged either true or false

 In GENIE2.C, the else-if comparison is number==5, testing to see whether the value of the number variable is 5 Two equal signs are used for this comparison

 You can do else-if, else-if, else-if all day long, if you want How­

ever, the C language has a better solution in the select-case structure

I cover this topic in this book’s companion volume, C All-in-One Desk

Reference For Dummies (Wiley)

Bonus program! The really, really smart genie

A solution always exists If you wanted to, you could write a program that would if-compare any value, from zero to infinity and back again, and the

“computer genie” would accurately guess it But, why bother with if at all?

Okay, the if keyword is the subject of this section, along with if-else and

else-if and so on But, the following source code for GENIE3.C doesn’t use

if at all It cheats so that the genie always guesses correctly:

#include <stdio.h>

int main() {

char num;

printf(“I am your computer genie!\n”);

printf(“Enter a number from 0 to 9:”);

num = getchar();

printf(“You typed in %c!\n”,num);

printf(“The genie knows all, sees all!\n”);

return(0);

}

You can create this source code by editing either GENIE1.C or GENIE2.C Make the necessary modifications and then save the source code to disk

as GENIE3.C

Compile the program Fix any errors that you (hopefully) don’t get Run the result:

I am your computer genie!

Enter a number from 0 to 9:8 You typed in 8!

The genie knows all, sees all!

Run the program again and again with different numbers Hey! That genie knows exactly what you typed! I wonder how that happened? It must be your deftness with the C language Tame that computer!

 The problem with GENIE3.C? It doesn’t make a decision The genie isn’t smart at all — it’s just repeating what you already know The purpose behind if, else, and the others is that they allow you to make a deci­sion in your program

 Refer to Chapter 10 for more information about the getchar() function

What If C==C ?

In This Chapter

Comparing characters with if

Understanding standard input

Fixing the flaws of getchar()

Making a yes-or-no decision

Apologies are in order for the preceding chapter Yes, I definitely drift into

Number Land while introducing the virtues of the if command Computer programs aren’t all about numbers, and the judgment that the if keyword makes isn’t limited to comparing dollar amounts, ecliptic orbits, subatomic particle masses, or calories in various fudgy cookie snacks No, you can also compare letters of the alphabet in an if statement That should finally answer

the mystery of whether the T is “greater than” the S and why the dollar sign is

less than the minus sign This chapter explains the details

I ask you: How can one compare two letters of the alphabet? Truly, this sub­ject is right up there with “How many angels can dance on the head of a pin?” and “Would it be a traditional dance, gavotte, rock, or perhaps heavenly hokey-pokey?”

Yet, comparing letters is a necessary task If you write a menu program, you have to see whether the user selects option A, B, or C That’s done by using the handy if keyword For example:

if(key==’A’)

Here, key is a single-character variable holding input from the keyboard The comparison that if makes is to see whether the content of that variable is

equal to (two equal signs) the letter A, which is enclosed in single quotes

This comparison is legitimate Did the user type an A?

 To compare a single-character variable with a character — letter, number,

or symbol — you must enclose that character in single quotes Single character, single quotes

 When if tests to see whether two things are equal, two equal signs are used Think “is equal to” rather than “equals.”

 When you compare a variable — numeric or single character — to a con­stant, the variable always comes first in the comparison

 On a good day, 4.9E3 angels can dance on the head of a pin, given the rela­tive size of the pin and the size of the angels and whether they’re dancing all at once or taking turns

 When you compare two letters or numbers, what you’re ‘really compar­ing are their ASCII code values This is one of those weird instances when the single-character variable acts more like an integer than a letter, num­ber, or symbol

Is the T greater than the S? Alphabetically speaking, yes T comes after S But,

what about the dollar sign and the minus sign? Which of those is greater? And,

why should Q be lesser than U, because everyone knows that U always follows

Q? Hmmm

Computers and math (do I even have

to remind you to skip this stuff?)

with math The computer evolved from the cal­

is somehow related to human fingers and thumbs After all, working with numbers is called

computing, which comes from the ancient Latin

term computare That word literally means “hire

a few more accountants and we’ll never truly

Another word for compute is reckon, which is

popular in the South as “I reck’n.” Another word

for compute is figure, which is popular in the

!South (not-South) as “I figure.” Computers reckon Go figure

Sad to say, too much about computers does deal culator, which has its roots in the abacus, which

know what’s going on.”

To solve this great mystery of life, I list next the source code for a program, GREATER.C It asks you to enter two single characters These characters are compared by an if statement as well as by if-else The greater of the two

is then displayed Although this program doesn’t lay to rest the head-of-a-pin problem, it soothes your frayed nerves over some minor alpha­

angels-on-the-betic conundrums:

#include <stdio.h>

int main() {

char a,b;

printf(“Which character is greater?\n”);

printf(“Type a single character:”);

a=getchar();

printf(“Type another character:”);

b=getchar();

if(a > b) {

printf(“‘%c’ is greater than ‘%c’!\n”,a,b);

} else if (b > a) {

printf(“‘%c’ is greater than ‘%c’!\n”,b,a);

} else { printf(“Next time, don’t type the same character twice.”);

} return(0);

}

Enter the source code for GREATER.C into your editor Make sure that you enter all the proper curly braces, confirm the locations of semicolons, watch your double quotes, and pay attention to the other minor nuances (or nui­

sances) of the C language

Save the file to disk as GREATER.C

Compile GREATER.C

Run the final program You’re asked this question:

Which character is greater?

Type a single character:

Type a character, such as the $ (dollar sign) Press Enter after typing the

character

Uh-oh! What happened? You see something like this:

Type another character:’$’ is greater than ‘

‘$

What? What? What?

You bet! A problem Right in the middle of the chapter that talks about compar­ing single-character variables, I have to totally pick up the cat and talk about something else vital in C programming: properly reading single characters from the keyboard

The problem with

Despite what it says in the brochure, getchar() does not read a single char­

acter from the keyboard That’s what the getchar() function returns, but it’s

not what the function does

Internally, getchar() reads what’s called standard input It grabs the first char­

acter you type and stores it, but afterward it sits and waits for you to type something that signals “the end.”

Two characters signal the end of input: The Enter key is one The second is the EOF, or end-of-file, character In Windows, it’s the Ctrl+Z character In the Unix-like operating systems, it’s Ctrl+D

Run the GREATER program, from the preceding section When it asks for

input, type 123 and then press the Enter key Here’s what you see for output:

Which character is greater?

Type a single character:123 Type another character:’2’ is greater than ‘1’

When you’re first asked to type a single character, you provide standard input for the program: the string 123 plus the press of the Enter key

The getchar() function reads standard input First, it reads the 1 and places

it in the variable a (Line 9 in GREATER.C) Then, the program uses a second

getchar() function to read again from standard input — but it has already been supplied; 2 is read into the variable b (refer to Line 11)