Selection Statements (2)
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Selection Statements (2) Program Design (I) 2021 Fall Fu-Yin Cherng Dept. CSIE, National Chung Cheng University 1
Transcript of Selection Statements (2)
Selection Statements (2)Program Design (I)
2021 Fall Fu-Yin Cherng
Dept. CSIE, National Chung Cheng University
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Outline
● Conditional Expression● Boolean Value● The switch statement
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Conditional Expressions
● C’s conditional operator allows an expression to produce one of two values depending on the value of a condition.
● The conditional operator consists of two symbols (? and :), which must be used together
● A bit hard to understand, but a shorter and useful expression○ can achieve same result by using if statement○ still be used in some modern languages, for example, R and Python
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expr1 ? expr2 : expr3
Conditional Expressions
● “if expr1 then expr2 else expr3”● The expression is evaluated in stages● expr1 is evaluated first; ● if its value isn’t zero (true), then expr2 is evaluated, and its value (expr2) is
the value of the entire conditional expression.● If the value of expr1 is zero (false), then the value of expr3 is the value of
the conditional expression
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expr1 ? expr2 : expr3
nonzero (true)
zero (false)
Conditional Expressions
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int i = 1, j = 2, k;
k = i > j ? i : j; /* k is now __(1)__ */
k = (i >= 0 ? i : 0) + j; /* k is now __(2)__*/compare the value of i and j, if i > j is true, then return value of i. If i is less than j, then return j. k is used to store the returned value
What is the value of (1) ?
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What's the value of (1)?
ⓘ Start presenting to display the poll results on this slide.
Conditional Expressions
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int i = 1, j = 2, k;
k = i > j ? i : j; /* k is now 2 */
k = i >= 0 ? i : 0; + /* k is now __(2)__*/
What is the meaning of this condition expression ?
What is the value of (2) ?
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What's the value of (2)?
ⓘ Start presenting to display the poll results on this slide.
Conditional Expressions
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int i = 1, j = 2, k;
k = i > j ? i : j; /* k is now 2 */
k = (i >= 0 ? i : 0) + j; /* k is now 3 */
parentheses are necessary; precedence of conditional operators is less than other operators (except the assignment operator; store the value after finishing all computation)
Conditional Expressions
● Conditional expressions are often used in return statements○ return i > j ? i : j;
● Calls of printf can sometimes benefit from condition expressions
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if (i > j)
printf("%d\n", i);
else
printf("%d\n", j);
printf("%d\n", i > j ? i : j); //shorter way
Boolean Values
● True (1), False (0) => Boolean Values● You can actually declare variable which type is Boolean● This type of variable can be used to represent the true or false● What’s the meaning of Boolean?
○ name after George Boole○ mathematician, philosopher, and logician (1815 - 1864)○ creator of Boolean algebra○ fundations for the modern information and computer sicence
11https://en.wikipedia.org/wiki/George_Boole
Boolean Values
● There are two ways to declare variable with Boolean type● _Bool: is an integer type, so a _Bool variable is really just an integer
variable in disguise.● however, a _Bool variable can only be assigned 0 or 1
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_Bool flag;
if (flag) statement;
// tests whether flag is 1 (true)
Boolean Values
● The second way is to use <stdbool.h> header which makes it easier to work with Boolean values.
● supplies macros named true and false, which stand for 1 and 0,
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_Bool flag;
if (flag) statement;
// tests whether flag is 1 (true)
#include <stdio.h>
#include <stdbool.h>
...
bool flag;
flag = false; //0
flag = true; //1
How to print Boolean values?
14https://stackoverflow.com/questions/17307275/what-is-the-printf-format-specifier-for-bool
The switch Statement
● When there is a series of values/conditions you need to match/compared, you might used casecaded if statment
● C provides an alternative which is easier to read and faster
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if (grade == 4)
printf("Excellent");
else if (grade == 3)
printf("Good");
else if (grade == 2)
printf("Average");
else if (grade == 1)
printf("Poor");
else if (grade == 0)
printf("Failing");
else
printf("Illegal grade");
The switch Statement
● The switch statement ● switch statement will test the
value of grade variable ● If match 4, executed the
statements in case 4○ print “Excellent”○ break statement makes the
execution flow jump out of the switch statement
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switch (grade) {
case 4: printf("Excellent");
break;
case 3: printf("Good");
break;
case 2: printf("Average");
break;
case 1: printf("Poor");
break;
case 0: printf("Failing");
break;
default: printf("Illegal grade");
break;
}
...
if grade == 4
The switch Statement
● The switch statement ● switch statement will test the
value of grade variable ● If match 4, executed the
statements in case 4○ print “Excellent”○ break statement makes the
execution flow jump out of the switch statement
● If don’t match any number from 0 to 4, go to default
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switch (grade) {
case 4: printf("Excellent");
break;
case 3: printf("Good");
break;
case 2: printf("Average");
break;
case 1: printf("Poor");
break;
case 0: printf("Failing");
break;
default: printf("Illegal grade");
break;
}
...
if grade is out of the range from 0 to 4
The switch Statement
● Common form of the switch statement
● switch○ must be followed by an controlling
expression in parentheses● controlling expression
○ integer expression (e.g., grade)○ Can’t use floating-point numbers
(float grade; ) and strings (“hello”)
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switch ( controlling expression ) {
case constant-expression : statements
…
case constant-expression : statements
default : statements}
The switch Statement
● Case labels○ Each case begins with a label
(constant-expression)● Constant-expression
○ ordinary expression except that it can’t contain variables or function calls.
○ expressions only contain constant● For example
○ 5, 5 + 1○ n + 10 (not constant expression)
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switch ( expression ) {
case constant-expression : statements
…
case constant-expression : statements
default : statements}
The switch Statement
● Statements○ After each case label comes any
number of statements○ No braces ({ }) are required around
the multiple statements (rare case in C)
○ The last statement in each group is normally break.
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switch ( expression ) {
case constant-expression : statements
…
case constant-expression : statements
default : statements}
The switch Statement
● Duplicate case labels aren’t allowed.○ can’t have two case 4 in a switch
● The order of the cases doesn’t matter, and the default case doesn’t need to come last.
● Several case labels may precede a group of statements
○ when grade is 1, 2, 3, or 4, will always print Passing
● If default is missing, just continue run the next statement after switch
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switch (grade) { case 4: case 3: case 2: case 1: printf("Passing"); break; case 0: printf("Failing"); break; default:
printf("IllegalGrade"); break;
}
The Role of the break Statement
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switch (grade) { //if grade is 3
case 4: printf("Excellent");
case 3: printf("Good");
case 2: printf("Average");
case 1: printf("Poor");
case 0: printf("Failing");
default: printf("Illegal grade");
}
// if grade is 3, printed message is:
// GoodAveragePoorFailingIllegal grade
● break helps the program to “break” out of the switch statement after finish the statements in the matched case
○ Continue to execute next statement after the switch
● What happen without break?○ continue run the statements of next
case!
The Role of the break Statement
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switch (grade) { //if grade is 3
case 4: printf("Excellent");
case 3: printf("Good");
case 2: printf("Average");
case 1: printf("Poor");
case 0: printf("Failing");
default: printf("Illegal grade");
}
// if grade is 3, printed message is:
// GoodAveragePoorFailingIllegal grade
● Forgetting to use break is a common error
● Always remember to put one after each case (including default) in switch statement
Summary
● Conditional Expression○ expr1 ? expr2 : expr3
● Boolean Value○ _Bool○ <stdbool.h>, boll
● The switch statement○ general form○ introduction of each component○ break statement
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