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

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

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!)

The scanf Function

Example: scanf(“%c%c%c”,&x,&y,&z);– input: cat

• result:

– input: c a t• result:

Example: scanf(“%d%d”,&n1,&n2);– input: 12 3

• result:

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

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

General Form of a C Program

preprocessor directivesmain function heading{

declarationsexecutable statements

}

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

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

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

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)

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

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;

Arithmetic Expressions

What is the value of 5 + 7 * 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

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

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

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! */

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!

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

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

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