Chapter 3 Control Flow Ku-Yaw Chang [email protected] Assistant Professor, Department of Computer Science and Information Engineering Da-Yeh University

Chapter 3Chapter 3

Control FlowControl Flow

Ku-Yaw ChangKu-Yaw [email protected]@mail.dyu.edu.tw

Assistant Professor, Department of Assistant Professor, Department of Computer Science and Information EngineeringComputer Science and Information Engineering

Da-Yeh UniversityDa-Yeh University

22Ku-Yaw ChangKu-Yaw Chang Control FlowControl Flow

3.1 Statements and Blocks3.1 Statements and Blocks

An An expressionexpression such as such as x = 0 orx = 0 or i++ ori++ or printf(…)printf(…)

becomes a becomes a statementstatement when it is followed by a when it is followed by a semicolonsemicolon x = 0;x = 0; i++;i++; prinft(...);prinft(...);

In C, the semicolon is a statement terminator.In C, the semicolon is a statement terminator.


3.1 Statements and Blocks3.1 Statements and Blocks

Braces { and } are used to group declarations Braces { and } are used to group declarations and statements together into a and statements together into a compound compound statementstatement, or , or blockblock.. Syntactically equivalent to a single statementSyntactically equivalent to a single statement No semicolon after the right brace that ends a blockNo semicolon after the right brace that ends a block


3.2 If-Else3.2 If-Else

The if-else statement is used to express The if-else statement is used to express decisions.decisions.

if (expression)if (expression)

statementstatement11

elseelse

statementstatement22

The expression is evaluatedThe expression is evaluatedTrue : expression has a non-zero valueTrue : expression has a non-zero valueFalse: expression is zeroFalse: expression is zero

The else part is optionalThe else part is optional


3.2 If-Else3.2 If-Else

Coding shortcutsCoding shortcutsif (expression)if (expression)

if (expression != 0)if (expression != 0)

Ambiguity when an else is omitted from a nested Ambiguity when an else is omitted from a nested if sequenceif sequenceifif (n > 0)(n > 0)

if (a > b)if (a > b)

z = a;z = a;

elseelse

z = b;z = b;

ifif (n > 0) {(n > 0) {if (a > b)if (a > b)

z = a;z = a;}}elseelse

z = b;z = b;


3.3 Else-If3.3 Else-If

A multi-way decisionA multi-way decisionif (expression)if (expression)

statementstatement

else if (expression)else if (expression)

statementstatement


statementstatement


statementstatement

elseelse

statementstatement

The last else partThe last else part Non of the above or Non of the above or

default casedefault case Can be omittedCan be omitted To catch an impossible To catch an impossible

conditioncondition


3.4 Switch3.4 Switch

A multi-way decisionA multi-way decisionTest whether an expression matches one of a Test whether an expression matches one of a number of number of constantconstant integer values, and integer values, and branches accordingly.branches accordingly.

switch (switch (expressionexpression) {) {case case const-exprconst-expr: : statementsstatementscase case const-exprconst-expr: : statementsstatementsdefault: default: statementsstatements

}}


3.4 Switch3.4 Switch

switch (n) {switch (n) {case 1:case 1:

printf(“The number is 1\n.”);printf(“The number is 1\n.”);break;break;

case 2:case 2:printf(“The number is 2\n.”);printf(“The number is 2\n.”);break;break;

default:default:printf(“The number is not 1 or 2\n.”);printf(“The number is not 1 or 2\n.”);break;break;

}}


Exercise 3-1Exercise 3-1

Write a program to count the occurrences Write a program to count the occurrences of each digits, while space, and all other of each digits, while space, and all other characters.characters.


3.5 Loops – While and For3.5 Loops – While and For

WhileWhilewhile (while (expressionexpression))

statementstatement The expression is evaluatedThe expression is evaluated

Non-zero: statement is executed and expression is re-Non-zero: statement is executed and expression is re-evaluatedevaluated

The cycle continues until expression becomes zeroThe cycle continues until expression becomes zero


3.5 Loops – While and For3.5 Loops – While and For

ForForfor (for (exprexpr11; ; exprexpr22; ; exprexpr33))

statementstatement

is equivalent tois equivalent toexprexpr11;;

while (while (exprexpr22) {) {

statementstatement

exprexpr33;;

}}

Any of the three parts can Any of the three parts can be omittedbe omitted

Semicolons must remainSemicolons must remain

An infinite loop:An infinite loop:for (;;) {for (;;) {

……

}} break or return break or return



Write a program that calculates and prints Write a program that calculates and prints the sum of the even integers from 2 to 30.the sum of the even integers from 2 to 30.



Write a function reverse(s) that reverses Write a function reverse(s) that reverses the string s in place.the string s in place. void reverse(char s[])void reverse(char s[])


3-6 Loops – Do-while3-6 Loops – Do-while

The syntax of the do isThe syntax of the do is

dodo

statementstatement

while (while (expressionexpression););

The body is always executed at least once.The body is always executed at least once. do-while is much less used than while and for.do-while is much less used than while and for.


3.6 Loops – Do-while3.6 Loops – Do-while

/* itoa: convert n to characters in s *//* itoa: convert n to characters in s */void itoa(int n, char s[])void itoa(int n, char s[]){{

int i, sign;int i, sign;if ((sign=n) < 0)if ((sign=n) < 0) /* record sign *//* record sign */

n = -n;n = -n; /* make n positive *//* make n positive */i = 0;i = 0;do {do { /* generate digits in reverse order *//* generate digits in reverse order */

s[i++] = n % 10 + ‘0’;s[i++] = n % 10 + ‘0’; /* get next digit *//* get next digit */} while ((n /= 10) > 0);} while ((n /= 10) > 0); /* delete it *//* delete it */if (sign < 0)if (sign < 0) s[i++] = ‘-’;s[i++] = ‘-’;s[i] = ‘\0’;s[i] = ‘\0’;reverse(s);reverse(s);

}}


3.7 Break and Continue3.7 Break and Continue

The break statement provides an early exit from The break statement provides an early exit from for, while, and do, just as from switch.for, while, and do, just as from switch.

A break causes the innermost enclosing loop or A break causes the innermost enclosing loop or switch to be exited immediately.switch to be exited immediately.



/* trim: remove trailing blanks, tabs, newlines *//* trim: remove trailing blanks, tabs, newlines */int trim(char s[])int trim(char s[]){{

int n;int n;

for (n = strlen(s)-1; n >= 0; n--)for (n = strlen(s)-1; n >= 0; n--)if (s[n] != ‘ ‘ && s[n] != ‘\t’ && s[n] != ‘\n’)if (s[n] != ‘ ‘ && s[n] != ‘\t’ && s[n] != ‘\n’)

break;break;s[n+1] = ‘\0’;s[n+1] = ‘\0’;return n;return n;

}}



The continue statement causes the next iteration The continue statement causes the next iteration of the enclosing for, while, or do loop to begin.of the enclosing for, while, or do loop to begin.

An example: skip negative valuesAn example: skip negative values

for ( i = 0; i < n; i++ ) {for ( i = 0; i < n; i++ ) {

if (a[i] < 0) /* skip negative elements */if (a[i] < 0) /* skip negative elements */

continue;continue;

… … /* do positive elements *//* do positive elements */

}}

The EndThe End

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Chapter 3 Control Flow Ku-Yaw Chang [email protected] Assistant Professor, Department of Computer Science and Information Engineering Da-Yeh University