Switches and Loops

Today we will cover switch statements. The use of switch statements is basically to simplify overly long and tedious if-else statements. For example, if you had the following code in your program:

...
printf("Enter a month in digits:");
month = GetInteger();
if (month == 1) {
	printf("January\n");
} else if (month == 2) {
	printf("February\n");
} else if (month == 3) {
	printf("March\n");
...
} else if (month == 12) {
	printf("December\n");
} else {
	printf("No such month!\n");
}
...

A switch statement can simplify the above code into something like below:

...
printf("Enter a month in digits:");
month = GetInteger();
switch (month) {
	case 1:	printf("January\n");
		break;
	case 2: printf("February\n");
		break;
	...
	case 12: printf("December\n");
		 break;
	default: printf("No such month!\n");
		 break;
}
...

Remember the 3 rules of switch statements from the lecture. In short, they are:

1. Remember break statements at the end of each case. Generally this is what you want to do. In some rare cases you may want certain case to continue on to the statements for the next case, but for this class, most of the time you won't.
2. The variable used in a switch must be integer-compatible. This means that the variable must be an int, a char or a long. You will probably be using int or char a lot for switch statements in this class. The reason that char is integer-compatible is that a character is actually a number, only represented by a symbol (or a character) when it's processed to display to you.
3. case labels must be constants. This means you may not use a variable for a case. Also, only one constant may be used per case keyword.

Your boss tells you one day that he wants you to make a program that reminds him of his job duties. He tells you that on Mondays he goes to New York and has a meeting at 6 pm. On Tuesdays he goes to Taiwan and has a meeting on Wednesday at 3 pm. On Thursdays he has to visit his Mother in law and takes phone calls all day. And on Friday he needs to transfer money from his Swiss accounts to his Paypal so that he could shop at Ebay. (Hint: use a switch statement!!)

Loops allow you to repeat sections of code in your program. There are three ways to produce a loop in C.

for loops
• while loops
• do while loops

The general syntax of a for loop:

for (begin point; end point; incrementation) {
	statements to be repeated;
}

A working example of this is:

for (i = 1; i <= 10; i++){
	printf("%d\n", i);
}

This code will print out numbers from 1 to 10.

Loops can be nested. One for loop can go into another.

Write a program that adds up the numbers 1, 2, 3, ... , 200 and prints out the answer in the end.

Write a program that produces 5 sets of multiplication tables. (Use nested for loops and increment operators) Sample output:

1 x 1 = 1
1 x 2 = 2
...
1 x 10 = 10
...
2 x 1 = 2
...
5 x 10 = 50

while loops let you repeat statements based on certain conditions. It means that the number of repetitions you need is dependent on certain conditions and cannot be determined at the time of developing the program. The syntax and an example:

while (expression) {
	statements;
}

while (x < 99) {
	x++;
}

Using a while loop write a program that finds the greatest common divisor of two positive integers. The greatest common divisor is the largest number that goes into both numbers evenly. For example, the GCD of 51 and 85 can be found doing the following:

dividend/divisor    85/51         51/34          34/17           17/0
remainder           rem 34        rem 17         rem 0

The for and the while loops both perform the conditional test before the loop is executed. Sometimes, however, you are going to want to perform the test at the end of the loop. This means the body of the loop is done at least once. The do while loop looks like the following:

do {
	statements;
} while (expression);

do {
	i++;
} while (i < 10);

Write a program that prints out a positive integer backwards. (Use a do while loop) Sample output:

Please enter a positive integer: 146
The integer written backwards is: 641

Your homework assignment is to finish the classwork programs that you did not finish in class; and to read Chapter 5: Functions.