Part
– 5: Selection using Switch Statements
in previous lesson we saw how we can use
"if" statement in programs when they need to choose an option from
among several alternatives. There is an alternative C programming command which
in some cases can be used as an alternate way to do this. “Switch…case” command
is more readable nd
easier language structure.
"Switch
... case" structure
We can use "if" statement yet but
it is better to use "switch" statement which is created for
situations that there are several choices.
switch(...)
{
case ... : command;
command;
break;
case ... : command;
break;
default:
command;
}
In the above switch command we will be able
to run different series of commands with each different case.
Example
5-1: example5-1.c
Rewrite example 4-5 of previous lesson and
use switch command instead of "if" statement.
#include
#include
main()
{
int choice;
while(1)
{
printf("\n\nMenu:\n");
printf("1- Math Program\n2- Accounting
Program\n");
printf("3- Entertainment Program\n4-
Exit");
printf("\n\nYour choice -> ");
scanf("%d",&choice);
switch(choice)
{
case 1 : printf("\nMath Program Runs.
!");
break;
case 2 : printf("\nAccounting Program
Runs. !");
break;
case 3 : printf("\nEntertainment Program
Runs. !");
break;
case 4 : printf("\nProgram Ends.
!");
exit(0);
default:
printf("\nInvalid choice");
}
}
}
In "switch…case" command, each
“case” acts like a simple label. A label determines a point in program which
execution must continue from there. Switch statement will choose one of “case”
sections or labels from where the execution of the program will continue. The
program will continue execution until it reaches “break” command.
"break" statements have vital rule
in switch structure. If you remove these statements, program execution will
continue to next case sections and all remaining case sections until the end of
"switch"
block will be executed (while most of the
time we just want one “case” section to be run).
As we told, this is because each “case” acts
just as a label. The only way to end execution in switch block is using break
statements at the end of each “case” section.
In one of case sections we have not used
"break". This is because we have used a termination command
"exit(0)" (which terminates program execution) and a break statement
will not make any difference.
"default" section will be executed
if none of the case sections match switch comparison. Parameter inside “switch”
statement must be of type “int or char” while using a variable in “case
sections” is not allowed at all. This means that you are not allowed to use a
statement like below one in your switch block.
case i: something;
break;
Break
statement
We used "break" statement in
“switch...case” structures in previous part of this lesson. We can also use
"break" statement inside loops to terminate a loop and exit it (with
a specific condition).
Example
5-2:example5-2.c
while (num<20)
{
printf("Enter score : ");
scanf("%d",&scores[num]);
if(scores[num]<0)
break;
}
In above example loop execution continues
until either num>=20 or entered score is negative. Now see another example.
Example
5-3: example5-3.c
#include
#include
main()
{
int choice;
while(1)
{
printf("\n\nMenu:\n");
printf("1- Math Program\n2- Accounting
Program\n");
printf("3- Entertainment Program\n4-
Exit");
printf("\n\nYour choice -> ");
scanf("%d",&choice);
switch(choice)
{
case 1 : printf("\nMath Program Runs.
!");
break;
case 2 : printf("\nAccounting Program
Runs. !");
break;
case 3 : printf("\nEntertainment Program
Runs. !");
break;
case 4 : printf("\nProgram Ends.
!");
break;
default:
printf("\nInvalid choice");
}
if(choice==4) break;
}
}
In above example we have used a break
statement instead of exit command used in previous example. Because of this
change, we needed a second break statement inside while loop and outside switch
block.
If the choice is 4 then this second “break
command” will break while loop and we reach the end of main function and when
there is no more statements left in main function program terminates
automatically.
getchar()
and getch()
getchar() function is an alternative choice
when you want to read characters from input. This function will get single
characters from input and return it back to a variable or expression in our
program.
ch=getchar();
There is a function for sending characters to
output too.
putchar(ch);
Example
5-4: example5-4.c
#include
#include
main()
{
char ch;
while(ch!='.')
{
ch=getchar();
putchar(ch);
}
system("pause");
}
First look at output results and try to guess
the reason for results. Console Screen:
test <--This is typed by us
test <--output of putchar after we pressed
enter
again.testing <--Typed string includes a
'.' character
again. <--Loop terminates when it reaches
'.' char
Above program reads any character that you
have typed on keyboard until it finds a '.' character in input characters. Each
key press will be both shown on console and added to a buffer. This buffer will
be delivered to 'ch' variable after pressing enter key (not before this). So
input characters are buffered until we press enter key and at that moment
program execution continues running statements that follow getchar() function
(These are loop statements).
If there is a '.' character in buffered
characters, loop execution continues sending characters to console with
putchar() function until it reaches '.' and after that stops the loop and comes
out of while loop. If it does not encounter '.' in characters it returns to the
start of loop, where it starts getchar() function again and waits for user
input.
First 'test' string appeared in output is the
result of our key presses and second 'test' is printed by putchar statement
after pressing enter key.
In some operating systems and some C
Programming language compilers, there is another character input function
"getch". This one does not buffer input characters and delivers them
as soon as it receives them. (in Borland C you need to include another header
file to make this new function work).
Example
5-5: example5-5.c
#include
main()
{
char ch;
while(ch!='.')
{
ch=getch();
putch(ch);
}
system("pause");
}
Testing. <--program terminates immediately
after we use the
character ‘.', also characters are sent to
output once. With this function we can also check validity of each key press
before accepting and using it. If it is not valid we can omit it. Just pay
attention that some compilers do not support getch(). (again Borland c needs
the header file conio.h to be included)
Look at below example.
Example
5-6: example5-6.c
#include
#include
main()
{
char choice;
while(1)
{
printf("\n\nMenu:\n");
printf("1- Math Program\n2- Accounting
Program\n");
printf("3- Entertainment Program\n4-
Exit");
printf("\n\nYour choice -> ");
choice=getch();
switch(choice)
{
case '1' : printf("\nMath Program Runs.
!");
break;
case '2' : printf("\nAccounting Program
Runs. !");
break;
case '3' : printf("\nEntertainment
Program Runs. !");
break;
case '4' : printf("\nProgram Ends.
!");
exit(0);
}
}
}
In above example we have rewritten example
5-1. This time we have used getch() function instead of scanf() function. If
you test scanf based example you will see that it does not have any control on
entered answer string. If user inserts an invalid choice or string (a long junk
string for example), it can corrupt the screen. In getch function, user can insert
one character at a time. Program immediately gets the character and tests it to
see if it matches one of the choices.
In this example we have omitted optional
"default" section in “switch...case”. Pay careful attention that in
scanf based example we used to get an integer number as the user choice while
here we get a
character input. As a result previously we
were using integer variable in “switch section” and integer
numbers in “case sections”. Here we need to
change the “switch … case” to match the character type
data. So our switch variable is of char type
and values being compared are used like '1', '2', '3', '4' because they are
character type data ('1' character is different from the integer value 1).
If user presses an invalid key while loop
will continue without entering any of "case" sections. Invalid key
press will be omitted and only valid key presses will be accepted.
"continue"
statement
Continue statement can be used in loops. Like
break command "continue" changes flow of a program. It does not
terminate the loop however. It just skips the rest of current iteration of the
loop and returns to starting point of the loop.
Example
5-7: example5-7.c
#include
main()
{
while((ch=getchar())!='\n')
{
if(ch=='.')
continue;
putchar(ch);
}
system("pause");
}
