1. The scanf Function
The scanf function reads input from the standard input device into one or more variables
Example: scanf(“%lf”, &miles);
Reads a real number that (by default) the user has typed in into the variable miles
Each variable must be preceded by the ampersand (&) address-of operator

2. The scanf Function
We can read values into multiple variables with a single scanf as long as we have correpsonding format strings for each variable
Example: scanf(“%c%c%c”, &letter_1, &letter_2, &letter_3);
The user must press the return or enter key to cause the value(s) to be read

3. The scanf Function
For numeric values (associated with a %lf or %d placeholder) blank spaces are skipped
For character values (associated with a %c placeholder) blank spaces are not skipped (since a space is a character!)

4. The scanf Function Example: scanf(“%c%c%c”,&x,&y,&z);
input: cat
result:
input: c a t
Example: scanf(“%d%d”,&n1,&n2);
input: 12 3

5. The return Statement
The statement- return(0); - transfers control from your program to the OS
It should be the last statement in your program. (Why?)
0 is the result of the execution of function main and signifies that the program completed without error

6. General Form of a C Program
At this point, we have learned enough of the elements of a C program to be able to combine them into a working program
Figure 1.9 shows an example of a working C program. Note the order in which the elements occur in this program

7. General Form of a C Program

8. Program Style
It is important to develop a consistent approach to programming style
For starters, follow the style illustrated in the text
Use spaces and indentation consistently
Use meaningful comments
A header section given info. on the programmer and program should be used

9. Arithmetic Expressions
Arithmetic expressions are formed of operators and operands
The C arithmetic operators are: +, -, *, /, and % (modulo or remainder)
The operands in an arithmetic expression can be either numeric literals (numbers) or numeric variables

10. Arithmetic Expressions
The division operator (/) can be applied to integers values, real values, or a combination
When applied to reals or mixed reals and integers, the result is a real value
When applied to two integers, the result is the integral part of the result
Example: 7/2 is 3
7.0/2 is 3.5

11. Arithmetic Expressions
The modulo operator can be applied to integers and it gives the remainder of integer division
Example: 7 % 2 is 1
Example: 8 % 2 is 0
Division by zero will cause a run-time error (execution of the program will be halted at that point)

12. Arithmetic Expressions
C allows mixed-type expressions in which the two operands are of different types
C defines numerical data types int and double (among others)
If both operands are of type int, the result is also an int
Otherwise, the result is a double

13. Arithmetic Expressions
An expression may involve multiple arithmetic operators
Operators may be either unary or binary
Unary operators are unary + and -
Examples:
x = -y;
p = +x * y;

14. Arithmetic Expressions
What is the value of * 2? How about 10 / 5 * 2
In order to know, we must examine how C evaluates expressions
There are three rules C uses to evaluate expressions
If there are parentheses, evaluate the subexpression within the parentheses first

15. Arithmetic Expressions
Example 3 * (5 + 2)
Parentheses can also be nested 2 * ( 5 * (2 + 1))
In this case, evaluate the most deeply nested subexpression first
The second rule involves operator precedence
We evaluate the operator which has highest precedence first
C precedence: (unary) + and - have highest precedence; *, /, and % are next; (binary) + and - have the lowest precedence

16. Arithmetic Expressions
If we have multiple operators with the same precedence we use associativity rules
Unary + and - are evaluated using right associativity (from right to left)
The binary operators are evaluated using left associativity (from left to right)
In order to better understand evaluation of complex expressions, use evaluation trees

17. Formatting Numbers in Output
Unless otherwise instructed, C displays numeric values in printf statements using a default notation
To change the default notation for integers, add a numeric field width between the ‘%’ and ‘d’
printf(“%3d %4d\n”, 11, 1); /* Note the use of a numeric literal in the print list! */

18. Formatting Numbers in Output
Real numbers can be formatted by specifying both a field width and the number of decimal places to be displayed
Printf(“%5.2f %3.2f %4.1f\n”, 3.14,3.14,3.14);
Note that if the field width isn’t wide enough for the numeric value, it will be expanded
This doesn’t work with character variables!

19. Interactive and Batch Modes
So far, we have seen programs which employ interactive mode
In this mode, input comes from the keyboard and output goes to the monitor
In batch mode, input and output are to and from files
To work in this mode, we need to redirect input, output or both

20. Interactive and Batch Modes
In DOS and UNIX, redirect input by following the name of your program with a ‘<‘ and the name of the file you want to read data from (one line for each scanf)
prog1 < myinput.dat
Redirect output by following the name of your program with a ‘>‘ and the name of the file you want to write data to
prog1 > myoutput.dat

21. Types of Errors There are three categories of programming errors
Syntax errors are discovered by the compiler
Run-time errors cause execution of the program to be halted
Logic errors cause the program to give incorrect results