1. Input and Output: I/O Finish reading chapters 1 and 2 of the text
Data into the Memory
e.g.
from the keyboard (via scanf)
assignment statement
...
Output:
Data out of the Memory
e.g.
printing
displaying on a monitor
Examples:
printf(“Enter the distance in miles”);
scanf(“%lf”, &miles);
printf(“This is %f kms\n”,1.609*miles);
output
input
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2. Display Input and Output
Use printf and scanf
printf(“control string”,list of expressions);
output format
what to output
input format
what to input
scanf(“control string”,&variable list);
do not
forget
for printf
placeholders (%d,...)
int i;
double pi;
i=2;
pi = 3.14;
printf(“%d times %f is %f\n”,i,pi,i*pi);
newline
must match in number, order and type
2 times is
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3. Formatting Output to control the output format in printf double val;
printf(“%4.2f”,val);
to control the output format in printf
3.14
4 characters (2 after the decimal point)
_ can have scientific or decimal notation
e (or E) f
4.28e+01
42.8
_ other placeholders
%i or %d for an int
%c for a char
check manual or text
142 D -3

4. printf(“pay, hours, and age? ”); scanf(“%lf%lf%i”,&pay,&hours,&age);
for scanf
double pay, hours;
int age;
printf(“pay, hours, and age? ”);
scanf(“%lf%lf%i”,&pay,&hours,&age);
placeholders
must match in number, order and type
lf for a double
i for an int
Notes
When reading numbers (e.g double, int…),
scanf skips spaces, tabs and newlines:
'\t','\n'
user can type spaces, tabs, newlines
between numbers
But NOT skipped when reading char
scanf(“%c”,&letter);
Try it out and learn from your mistakes!
142 D -4

5. Initializing variables
giving something a value for the
first time
double radius;
printf(“Radius of the circle? ”);
scanf(“%lf”,&radius);
We have seen: or
double radius;
radius = 2.14; or
double radius = 2.14;
Potential source of errors:
forgetting to initialize a variable
the value of the variable is random
double pay, hours;
pay = 26.5 * hours;
the result is unpredictable!
142 D-5

6. Errors when programmingC O M P I L E R L I N K E R
executable
program
.obj file
.c file
your
program
library (ANSI)
header
(stdio.h)
debugger
syntax errors:
e.g. omitting ;
check warnings
e.g. variables
not
initialized
link errors:
I can’t find
the functions
you want!
e.g. using
printf
without
#include
<stdio.h>
run time errors:
error, e.g.
_ bad input
(divide by 0,
forgot & in
scanf)
_ bad program
logic
142D-6

7. Programming Style A program is a document:
The compiler reads some of it
(comments are ignored)
Some people have to read ALL of it
(update, corrections…)
A program MUST be easy to read by another
programmer
Use comments, spacing, indentation,
meaningful names
142D-7

8. Identifiers Identifiers: names used throughout the program
for variables and functions
(see later, like printf, scanf…)
Reminder:
_ contains letters, underscores, numbers
_ do not begin with a number or underscore
_ not a key word, e.g. double
_case sensitive (var and Var are different
variables)
_ all capital lettered names are for #define
constants (see next slide)
_ use meaningful names that say what the
variable is for.
NOT m but miles
142D-8

9. #define allows you to define constants. Think of it as
a text replacement command
(but it can do more).
#define PI
no ;
no =
everywhere we write PI, the preprocessor will
write
Then, the file is sent to the compiler.
Style convention: use only uppercase letters
for these constants.
142D-9

10. Other examples #define LENGTH 100 #define WIDTH 150
#define HALF_PERIMETER (WIDTH+LENGTH)
parentheses are important
(without, can lead to computational errors)
Centralize changes
No magic numbers
avoid accidental assignments to constants
PI= ; /* Error */
142D-10

11. Clarity Avoid obscure statements: NO: x = (y=x) + 1; YES: y = x;
x = x + 1;
Be clever for a GOOD reason and say why
in a comment
int i;
i = 1; …
/* toggle the value of i
between 0 and 1*/
i = (i+1)%2;
142D-11