Chapter 6. Section 6.3 The switch Statement. CS 50 Hathairat Rattanasook

Transcription

1 Chapter 6 Section 6.3 The switch Statement CS 50 Hathairat Rattanasook

2 switch-case The switch statement provides C programmers with a multiway decision statement. Depending on the result of a test on an expression any number of alternatives could be executed. The do-while loop has the following syntax: switch ( expression ) { case c1: statement1; case c2: statement2;... default: statement3; } CS 50 - Hathairat Rattanasook 1

3 switch-case The keywords are switch, case, and default. The expression is any legal C expression with one of the integer types. The switch statement is typically followed by one or multiple case statements and an optional default statement. A case statement has a constant. Each case can contain 0 n statements that are executed if the expression matches to the constant for that case. The default block can have 0 n statements which are executed if the expression does not match any of the defined cases. The default labeled statement is optional. CS 50 - Hathairat Rattanasook 2

4 Switch Execution 1. A switch statement first evaluates the control expression and tests if the value matches to any value of the cases. 2. When a match is found, the statements in the case with the matching value are executed. 3. Afterwards, all statements in the following cases are executed regardless of their case value. This includes the statements in the default label. 4. If that is not intended, a break; statement must be used at the end of a case or each case block. CS 50 - Hathairat Rattanasook 3

5 switch Example 1 1 int start = 3; 2 switch (start) { 3 case 3: printf("three, "); 4 case 2: printf("two, "); 5 case 1: printf("one, "); 6 default: printf("go!"); 7 } Output: three, two, one, go! The switch matches the value of start to one the cases. If a match is found, the statement(s) in that case is executed. The flow then continues to execute the following cases and default. If not match is found, the default block get executed. What is the output if start is 1? Output: one, go! What is the output if start is 7? Output: go! CS 50 - Hathairat Rattanasook 4

6 switch Example 2 1 int start = 7; 2 switch (start) { 3 default: printf("ready?, "); 4 case 3: printf("three, "); 5 case 2: printf("two, "); 6 case 1: 7 printf("one, "); 8 printf("go!"); 9 } Output: ready?, three, two, one, go! The default and case statements can be in any order! What is the output if start is 1? Output: one, go! What is the output if start is 2? Output: two, one, go! What is the output if start is 0? Output: ready?, three, two, one, go! CS 50 - Hathairat Rattanasook 5

7 switch (break) Example 3 1 int start = 3; 2 switch (start) { 3 case 3: printf("three"); break; 4 case 2: printf("two"); break; 5 case 1: printf("one"); break; 6 default: printf("go!"); break; 7 } Output: three The break statement stops executing the switch-case block and jumps to the end of the switch statement (line 7). What is the output if start is 2? Output: two What is the output if start is 8? Output: go! You will learn more about the break statement in a later section! CS 50 - Hathairat Rattanasook 6

8 switch (break) Example 4 1 int start = 3; 2 switch (start) { 3 case 3: printf("three"); 4 case 2: printf("two"); break; 5 case 1: printf("one"); 6 default: printf("go!"); break; 7 } Output: threetwo The break statement stops executing the switch-case block and jumps to the end of the switch statement (line 7). What is the output if start is 2? Output: two What is the output if start is 8? Output: go! What is the output if start is 1? Output: onego! You will learn more about the break statement in a later section! CS 50 - Hathairat Rattanasook 7

9 Question? Can you write the following if-else code as a switch case? 1 int i; 2 printf("enter a number: "); 3 scanf("%d ", &i); 4 if (i == 0) { 5 printf("zero"); 6 } else if (i == 1) { 7 printf("one"); 8 } else if (i == 2) { 9 printf("two"); 10 } else { 11 printf("other"); 12 } CS 50 - Hathairat Rattanasook 8

10 Answer 1 int i; 2 printf("enter a number: "); 3 scanf("%d ", &i); 4 switch (i) { 5 case 0: printf("zero"); break; 6 case 1: printf("one"); break; 7 case 2: printf("two"); break; 8 default: printf("other"); break; 9 } 1. The switch statement reads i and checks if it matches on of the cases. 2. If a match is found, the case statement is executed. The break; statement ensures that the flow does not continue executing the following statement but instead jumps to the end of the switch block. 3. If not match is found, the default block is executed. CS 50 - Hathairat Rattanasook 9