Transcript Getting Started - WalterMilner.com

Starting C
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An introduction to programming in ANSI C
This material is intended to get people started in writing C
code in reasonable style
It is intended to be cross-platform and not compilerspecific
However there are a few slides about getting started in
Visual Studio ( VC++ 6)
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :1
Software needed - a C compiler
· Microsoft Visual C++ or MS Visual Studio or Watcom or
CodeWarrior
· Turbo C v.2.01, free from
http://community.borland.com/museum/ (just 3 floppies,
works in DOS)
· Turbo C++ v. 4.5, free on a magazine CD-ROM ( Windows)
The GNU C compiler, free from
http://www.gnu.ai.mit.edu/software/gcc/ .This is a
command line compiler which you may find tricky to set up
and use to start with
If you use Linux or another version of Unix, you will find
you already have a C compiler, either installed or still on
the CD.
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :2
Books
Computer Science A Structured Programming Approach Using
C by Forouzan and Gilberg publishers Brooks/Cole
The C Programming Language by Kernighan and Ritchie
second edition published by Prentice Hall
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :3
Using MS Visual C++ IDE - 1
1.
Start MS VC++
2.
Click File..New
3. Set up the dialog
box like this
a console
application
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :4
project
name
where it is
saved
Using MS Visual C++ IDE - 2
1.
A wizard starts
– choose
‘Empty Project’
2.
Click OK
3. You will see this
select
FileView
not
ClassView
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :5
Using MS Visual C++ IDE - 3
1.
From the
menu, click
Project.. Add to
Project.. New..
2.
You will see
this..
A filename ending in .c
We are
actually
adding a C
source
code file
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :6
Writing the program
to run it
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :7
A first program
/* A first program */
int main()
{
int x, y;
x = 5;
y = x + 7;
return 0;
}
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :8
Output
/* a program with output */
#include <stdio.h>
int main()
{
int x,y;
x = 5;
y = x + 7;
printf(" y is %i \n ", y );
return 0;
}
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :9
1. Enter the first program into an editor /* a program with output */
#include <stdio.h>
int main()
{
int x,y;
x = 5;
y = x + 7;
printf(" y is %i \n ", y );
return 0;
}
Save it and compile it. Correct any syntax errors reported. Run the program. If you do not get the answer
12, fix it!
2. Write a program to work out the area of a rectangle. Have 3 integer variables, called width, length
and area. Make the width 5 and the length 10. Calculate the area and use printf to output it.
3. Use printf to output a Christmas tree, like
*
***
*****
*******
*
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :10
What compilers do
1: int main()
2: {
00401010 push ebp
00401011 mov ebp,esp
00401013 sub esp,8
3: int x,y;
4: x = 5;
00401016 mov dword ptr [x],5
5: y = x + 7;
0040101D mov eax,dword ptr [x]
00401020 add eax,7
00401023 mov dword ptr [y],eax
6: return 0;
00401026 xor eax,eax
7: }
00401028 mov esp,ebp
0040102A pop ebp
0040102B ret
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :11
Integral data types
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Variables must be one of a small set of data
types. All data is represented internally as
binary patterns, but different types are
encoded into binary in different ways
The standard data type for an integer (whole
number) is int
Often, an int is stored in 2 bytes = 16 bits. In
that case, ints are restricted to the range
±32000
but the range of all data types in C is
implementation-dependent ie not defined by
C - the opposite to Java
limits.h contains the ranges of these types
and will contain different values in different
versions
the type short is an integer which is uses the
same or less memory than int
long uses at least as much as an int. Often
long uses 4 bytes = 32 bits, giving a range of
about ±2 500 000 000
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :12
#include <stdio.h>
#include <limits.h>
int main()
{
int i;
short s;
long x;
printf("Biggest integer =
%i\n",INT_MAX);
/* get 32767 */
i = 255;
x = 128000L;
printf("i = %i and x = %li\n",i,x);
/* get 255 and
128000 */
return 0;
}
Floating point types
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Floating point numbers contain decimal parts
Floating point is less precise and slower than
integer
The basic type is float which is often stored in 4
bytes
The most often used is double which is extended
precision and greater range
Details of ranges and precision (implementationdependent) is in <float.h>
The formatting sequence for a double or float in
printf is %f. A form like %5.13f indicates the
number of digits before and after the decimal
point. %e gives scientific notation display.
7 / 2 requests the division of 1 integer by
another, so integer division is used, giving the
result 3
7.0 / 2.0 divides one float constant by another, so
floating point division is used, and 3.5 is the
result
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :13
#include <stdio.h>
int main()
{
float f;
double d;
f = 1.23456789;
printf("%f\n",f);
d = 1.2345678912345;
printf("%5.13f \n",d);
f = 7 / 2;
/* f becomes 3.0 */
f = 7.0 / 2.0;
/* f becomes 3.5 */
}
Arithmetic operators
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The arithmetic operators are + - * / and %, which is the
remainder eg 11 % 9 = 2.
Each has a precedence level so that BODMAS works and brackets
may be needed eg 4 * 2 + 2 is 14, 4 * ( 3 + 2) is 20
There are floating point and integer versions of these,
depending on the types of the operands.
eg 7 / 3 gives 2
For expressions with more than one type, values are converted
to the ‘biggest’ type
eg in 3.5 + 7, the 7 is converted to 7.0 and floating point
addition is used.
= is also an operator, and the value is the right hand side - for
example
x = 7;
does 2 things - it makes x to be 7 (a side-effect), and it
produces a value of 7.
So you can say for example
Set 1-2
y = x = 7;
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :14
1. Write a program which calculates and outputs the
circumference ( 2 r ) and area ( r2 ) of a circle. Use a variable
called radius of type double. Take  as 3.1416. Test your program
works.
2. Write a program which calculates the average of 3 numbers.
Use double type variables called x1, x2 and x3 as the three
numbers. Check it works.
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :15
Type casting
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An expression of one data type can be assigned to a variable of another
type
such as
int x;
long lo;
x = 3;
lo = x;
This is called a type cast
Casting a value to a ‘bigger’ type, such as char to int, or float to double,
produces no data loss - but the reverse potentially can
The example above is an implicit cast - it is up to the compiler to
identify that a cast is needed, and provide the code to do it
An explicit cast is like this
lo = (long) x;
where (long) tells the compiler to convert the type to long
Implicit casts are bad practice - they imply that the programmer is
unaware of the type of the data involved
Compiler settings should be such as to generate warnings on implicit
casts - then the programmer can write explicit casts where appropriate
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :16
Conditional - if
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The idea is that the machine evaluates an
expression and takes different actions depending
on whether the result is true or false
Note the round brackets around the condition
The { and } form a block of statements to do. If
there is only one statement, they are not needed.
The else part can be missed out if it is not needed.
A value of 0 is 'false', while any non-zero value is
'true' - see later
the relational and logical operators can be used, so
that
if ( ( x == 4 ) || ( y < x + 7 ) ) ..
means if x is 4, or y is less than x+7, do
something..
Logical operators
Symbol
Means
&&
AND
||
OR
!
NOT
Idea
both things true
either thing true
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :17
if ( x > y )
{
x=7;
y = 55;
}
else
{
y = 9+x;
printf(“Hello”);
}
Relational
Symbol
>
<
==
>=
<=
!=
operators
means..
greater than
less than
equal to
Greater than or equal to
less than or equal to
not equal to
Input - scanf
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This is run-time input of data, normally from the keyboard
For example
int i;
scanf("%i", &i);
execution pauses at the scanf, and waits for the user to type in a
value for i
Note the &, the address operator, in front of the variable
scanf is prototyped in stdio.h
scanf is not usually used in real programs (not even DOS ones)
because of the difficulty with validation. In other words if the
user does not type in a valid value - its too late.. but scanf is OK
for initial teaching programs
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :18
1. Write a program to work out the area of a rectangle, using scanf to input data
at run-time. Have 3 integer variables, called width, length and area. Calculate
the area and use printf to output it. Can you find data to enter which you crash
the program?
2. Write a tax calculator, which inputs a person's salary then outputs the tax,
calculated by the following if they earn less than £10 000, the tax is zero
over £10 000, they pay tax at 20%
3. Write some code to input a person's age and validate it. If the age is less
than 0 or more than 120, output a message 'Age invalid' - otherwise output 'Age
OK'.
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :19
Increment, decrement and assignment operators
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The need to increase a variable by 1 is so common there
is an increment operator - for example
x++; is the same as
x=x+1;
Similarly there is a decrement operator
x--; is the same as
x=x-1;
For both these, there are pre and post versions. For
example, post-increment y = x++;
first makes y equal to x, then increases x by 1 but pre-increment
y = ++x;
first increases x by one, then makes y equal this new
value of x
Assignment operators are short-hand. For example +=
x = 7;
x += 9; same as x = x + 9 - so x becomes 16
More examples Operator
-=
Example
x -= 4 ;
Equivalent
x = x - 4;
*=
x *= y + 3; x = x * (y + 3);
%=
x %= 2;
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :20
x = x % 2;
Expressions and statements
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An expression is something which has a value. For example assuming the variables
have been declared and given values, all of the following are expressions 2+2
5*x+4
4+(y+3)/(x+2)
sin( 3 * x )
/* sin is a function prototyped in math. h
*/
38
x=4
/* remember = is an operator with a side-effect
*/
x++
/* post-increment, side-effect, value is the initial value of x */
An expression becomes a statement if it is followed by a ;
So
x + 2;
is a syntactically valid statement in C - but a good compiler would warn you that
'code has no effect'
maybe you meant to type
x = 2;
especially since + and = are on the same key!!!
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :21
1. Look at this piece of code int a, b, c;
a
b
c
b
c
= 4;
= ++a;
= a++;
*= a;
-= 3;
What should be the values of a b and c after this? Work it out on paper, then
write a program to check.
2. Classify each of the following as an expression, a statement, or as invalid -
1 + 1
x = 3;
x + 2
1 + 1;
x++
x++;
a = b = c;
x++2;
2++
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :22
if - again
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remember 0 is ‘false’, and any non-zero value is ‘true’
programmers often use this to compress code
eg
x = 3 * y + 5;
if ( x == 0 )……
would probably be coded as
if ( ! ( x = 3 * y + 5 ) ) ….
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this uses the side-effect of = to execute the assignment
and the value is the value of x
if x is 0, !x is 1, which is true, so the if is triggered
bug number 1..
y = ….something...;
x = y;
if (x=0)…..
many compilers will warn this
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :23
conditional - switch
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suppose you want to execute various alternative blocks of code, depending on the
value of a variable you might write
if (x==1)
switch (x)
…..
{
else if (x==2)
1:
…..
…..
else if (x==3)
……
…..
the switch construct caters for this
break;
switch (x)
2:
{
….
1: …..
…..
……
2: ….
break;
…..
3: …..
default: ….
...
…
default: ….
}
…
}
but..
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the 1, 2 ,3 etc must be constant integral expressions
when one case triggers, it falls through all other cases
so the usual form is..
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :24
conditional - the ternary operator
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if you want to get one value if an expression is
something, and an alternative if it is not..
such as
if ( x > 38 )
y = 47;
else
y = 32;
it is faster to write
y = ( x > 38) ? 47 : 38 ;
which is like
y is.. is x >38? if so, 47, else 38
Set 1-5
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :25
char type
/* char */
#include <stdio.h>
int main()
{
char c;
c = 'a';
printf("c = %c or %i\n",c,c);
/* a or 97 */
c = 'b';
c = c + 2;
printf("c = %c\n",c); /* gives d */
return 0;
}
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Introduction to C : set 1 © Walter Milner 2005 :::: Slide :26
char is to be thought of as a
character, or as a small integer
Traditionally, characters are
represented in a machine by codes
which are 1 byte long. Most PCs
use the ASCII code system, in
which A is 65, B is 66, C is 67,.. a
is 97, b is 98,.. 0 is 48, 1 is 49, a
SPACE is 32 and so on. There are
other character codes which are 1
byte long, such as EBCDIC
A char variable can be assigned a
value like 'a' single quotes 'like'
and NOT "double quotes", which
delimit strings
A char can be displayed using the
%c formatting sequence in printf
Since chars are small integers, you
can do arithmetic with them, like c
=c+2
1. Use switch to write a simple calculator which would run as
follows (sample user input is in italics ) -
Input first number:
2
Input second: 3
Choose operation ( + - * or /): +
Result = 5
Use scanf for input, and a char variable for the operation.
Note : if you try to input the char using scanf("%c",&c); it
picks up the previous carraige return – so say
scanf(" %c",&c); // space before %c skips over CR
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :27
loops - do..while
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use a loop if some process is to be repeated
there are 3 basic loop structures in C
the do while structure repeatedly executes a block of code
so long as some condition is true
for example to output the numbers 1 to 10..
x = 1;
x = 1;
do
do
{
printf(“%i \n”, x);
printf(“%i \n”, x++);
x++;
while (x<11);
}
while (x<11);
or..
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :28
loops - while
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a do..while loop will always execute the loop body at least
once, since the condition is checked at the end.
Sometimes this is inappropriate eg to delete all files in a
directory and the directory is empty
an alternative loop checks the condition at the start
for example to output the numbers 1 to 10..
x = 1;
while (x<11)
x = 11;
{
while (--x)
printf(“%i \n”, x);
printf("%i \n",x);
x++;
}
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or, to output 10 to 1..
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Introduction to C : set 1 © Walter Milner 2005 :::: Slide :29
Loop templates
Task:
Output a message 10 times
This kind of process just needs an ‘index variable’ which is used
to count the number of times the loop should repeat
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :30
Loop Template 1
In C:
#include <stdio.h>
In pseudo-code
initialise counter to 0
while ( counter < 10 )
output message
increase counter by 1
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :31
int main()
{
int counter;
counter = 0;
while (counter<10)
{
printf(“Hello\n”);
counter++;
}
return 0;
}
Loop template 2
Task:
•Add up the integers from 1 to 100
•There are 2 aspects to this –
•Go through the numbers 1 to 100
•Add them up
•We need a counter variable to represent the 1 to 100
•and another which is the “running total”
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :32
Loop template 2
Pseudo-code plan –
Initialise counter to 1
Initialise total to 0
while ( counter < ?? )
add counter into total
add 1 to counter
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :33
In C
#include <stdio.h>
int main()
{
int counter, total;
counter = 1;
total = 0;
while (counter<101)
{
total += counter;
counter++;
}
printf("Total = %d\n",total);
return 0;
}
Loop template 3
Task
Input integers from the keyboard, until a rogue value of –99 is entered
Output the average
Thoughts –
Input using scanf – need a ‘number’ variable
We have to add up the numbers – we need a total variable
We don’t know (at ‘compile time’) how many times we will loop
We have to count them – we need a counter variable
We must avoid adding in and counting the -99
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :34
Loop template 3
Pseudo-code
Initialise total to 0
Initialise counter to 0
Input number
while ( number not –99 )
Add number to total
Increment counter
Input number
average = total / counter
Output average
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :35
#include <stdio.h>
int main()
{
int counter, total, number;
double average;
counter = 0;
total = 0;
scanf("%d",&number);
while (number!= -99)
{
total += number;
counter++;
scanf("%d",&number);
}
average = (double)total / (double) counter;
printf("Average = %lf\n",average);
return 0;
}
1. Here is a program which outputs the integers 1 to 5 #include <stdio.h>
int main()
{
int stepper;
stepper = 1;
while (stepper<6)
{
printf("%d ",stepper);
stepper++;
}
return 0;
}
We will use this as a basis for several more programs. Type it in and
run it.
2. Change the program so it outputs integers from 1 to 99 (inclusive).
3. Change it so it outputs from -99 up to 99.
4. Change it so it goes up in steps of 2 ( so you get -99 -97 -95........97
99 ).
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :36
5. Change it so it starts at 100 and goes down to 1.
1. Use a do-while loop to output the even numbers from 2
to 100 inclusive.
2. Input an integer then output 'Hello' that number of
times. Include the possibility of 0 iterations.
3. Use a loop to output the sequence 3, 6, 9, 12,..300
4. Add up the integers from 1 to 100.
5. The user inputs a sequence of integers, terminated by a
'rogue value' of -99. Add up the sequence and output the
result Sample input 1, 2, 3, -99
Output 6
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :37
Arrays
An array is a set of data values – not just one
Think of an array as a row of boxes
Each box is an element
The elements have an index – a label for which box is which
array of 10 integers
The 10
elements
7
13 42 4
17 22 7
66 3
9
0
1
4
7
9
2
3
5
6
8
index of each
element
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :38
Arrays
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An array is a data structure – a way of arranging a set of data
items
An array is a set of numbered boxes, each box holding a
value
an array marks[20] could hold the exam marks of 20 students
This would be declared as
int marks[20];
< must use square brackets
The boxes are numbered 0 to 19
Some examples are
marks[0] = 35;
store 35 into the first box
marks[9] = 48;
store 48 into the 10th. box
x = marks[0] + marks[1]; add up first 2 boxes
k = 34;
marks[k] = 17;
put 17 into 35th. Box
marks[20] = 23;
!! Array bounds error !!
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :39
Arrays and loops
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To process arrays loops are often used to go through the different
elements
For example, this would declare an array, read values in from the
keyboard, then print them out again..
int index = 0;
int numbers[5];
while ( index < 5 )
{
printf("Enter a number..");
scanf("%i", &numbers[index] );
index++;
}
index = 0;
while ( index < 5 )
{
printf(“ %i %i \n ", index, numbers[ index ] );
index++;
}
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :40
Arrays - count zeros
Suppose we have an array called numbers, containing 5 integers,
and we want to count how many zeros there are zeroCount = 0;
index = 0;
while ( index < 5 )
{
if ( numbers [ index] == 0 )
zeroCount++;
index++;
}
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :41
Arrays - find largest element
biggest = 0;
index = 0;
while ( index < 5 )
{
if ( numbers [ index] > biggest )
biggest = numbers[index];
index++;
}
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :42
Arrays - find total
total = 0;
index = 0;
while (index < 5 )
total = total + numbers[index++];
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :43
Arrays of char
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Strings in C are stored as sequences of character codes,
and are terminated by a 0 - ASCIIZ
so “ABC” is stored in 4 bytes, with values 65 66 67 0
this can be done using arrays eg
char word[4] = “ABC”;
or
char word[4];
word[0]=‘A’;
word[1]=‘B’;
word[2]=‘C’;
word[3]=‘\0’;
not word[3]=‘0’;
or
char word[] = “a longer string”;
the problem with this is an array is fixed in length, so you
can’t store longer strings in the array. Consequently
pointers to chars are more often used..
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :44
1. Fill an array with 5 random integers ( use the function
rand() from stdlib.h ). Print them out, together with the
average of the 5 numbers.
2. Fill an array with 50 random integers. Print them out,
then reverse them, and print them out again
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :45
for loops
2 kinds of C loops – while( ) and do..while( )
The 3rd and last kind is a for loop
It looks like –
for (…………..)
{
..
..
}
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :46
There are 3 parts in this
bracket
This is the loop body –
the statements to repeat
for loops
Initialise
change at the end of each
repeat
for (
x=0
{
..
..
}
; x<5 ; x++
)
loop repeats so long as this is true
so this loops with x having the values
0 1 2 3 and 4
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :47
for loop – output 1 to 100 (nearly)
..
int x;
..
for ( x = 0; x < 100; x++)
printf(“%d\n”, x);
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :48
for loop – counting down
..
int x;
..
for ( x = 100; x > 0; x = x - 2)
printf(“%d\n”, x);
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :49
for loop – totalling an array
int data[100]
int index, total = 0;
..
for ( index = 0; index < 100; index++ )
total = total + data[index];
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :50
nested loops
int x;
int y;
y = 0;
012
012
..
for (y = 0; y<30; y = y + 10 )
{
for (x = 0; x<3; x++)
printf(“%d ”, x + y );
printf(“\n”);
}
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :51
10 11 12
20 21 22
1. Use a for loop to output 1 to 100 inclusive
2. Use a for loop to output the 4 times table like
1X4=4
2X4=8
..
12 X 4 = 48
3. Modify the program in 2 so it produces all the times tables
from 2 to 12
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :52
Functions - ideas







Long pieces of code are very hard to manage
It is more effective to structure programs into small pieces or
units
All C programs are made of units called functions
main() is a special function - where execution starts - so far the
only function in the program
Real programs are broken up into several functions
A function should not have more than around 30 lines of code or it should be broken up into other functions
This modularity means that






small functions are easier to write
they are easier to debug
they can be tested separately faster
they will not stop each other working
they can be written by teams of programmers
they can be re-used in other projects
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :53
Function call sequence
the definition
of function
func
a call of
func()
void func()
{
..
return;
}
..
..
int main()
{
..
func();
..
}
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :54
function example
#include <stdio.h>
void greeting()
{
printf("Hello\n");
return;
}
int main()
{
greeting();
return 0;
}
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :55
void means the function does not
‘return a value’
it just does something
•This doesn’t work –
#include <stdio.h>
prototypes
•Thisfunction
works –
int main()
#include <stdio.h>
{
greeting();
void greeting(); // function prototype
return 0;
}
int main()
void greeting()
{
{
greeting();
printf("Hello\n");
return 0;
return;
}
}
void greeting()
•because the compiler works
{
through program top to bottom, and
printf("Hello\n");
•it sees the call to greeting() before
return;
it has seen the definition of what
}
greeting is
•header files (like stdio.h) are mostly
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :56
prototypes
A value can be sent into a function to
specify what it does
function
This is called a function parameter
#include <stdio.h>
void greeting(int howManyTimes)
{
int i;
for (i=0; i<howManyTimes; i++)
printf("Hello\n");
return;
}
int main()
{
greeting(5);
return 0;
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :57
}
parameters
function with a parameter
#include <stdio.h>
void table( int );
int main()
{
table(5);
return 0;
}
void table( int n)
{
int i;
for (i=1; i<13; i++)
printf("%i X %i = %i\n", i,n,i*n);
return;
} Introduction to C : set 1 © Walter Milner 2005 :::: Slide :58
output…
1X5=5
2 X 5 = 10
3 X 5 = 15
4 X 5 = 20
5 X 5 = 25
6 X 5 = 30
7 X 5 = 35
8 X 5 = 40
9 X 5 = 45
10 X 5 = 50
11 X 5 = 55
12 X 5 = 60
int g;
global and local variables
void func()
{
This is a global or external
variable
int x;
..
}
int main()
{
int y;
..
} Introduction to C : set 1 © Walter Milner 2005 :::: Slide :59
This are local or
automatic
variables
External variables
this program is supposed to fill an
array with numbers, display them,
reverse them, and display them again
 so each function needs access to the
array, and fill() and reverse() need to
alter its contents
 This can be done by making it an
external variable
 These are declared outside functions –
they are ‘global variables’ – contrasted
with those declared at the start of a
function, which are automatic
variables
 External variables provide for
unrestricted interaction between
functions
 In particular a function can alter them
in invalid or accidental ways so..
 External variables should be avoided if
at all possible
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :60

int numbers[20];
void fill(void);
void reverse(void);
void display(void);
int main()
{
fill();
display();
reverse();
display();
return 0;
}
void fill()
{ /* define fill */
return;
}
void display()
{ /* define display */
return;
}
void reverse()
{ /* define reverse */
return;
}
Scope




The scope of a variable is the section of the
program within which the variable can be
accessed
The scope of an automatic variable (local
variable) is the function within which it is
declared
The scope of an external variable is from where
it is declared to the end of the file
Automatic variables with the same name in
different functions are unrelated
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :61
#include <stdio.h>
int ex = 0;
void foo1()
{
int a;
a = 1;
return ;
}
void foo2()
{
int a;
ex = 3;
a = 3;
return;
}
int main()
{
ex = 7;
foo1();
foo2();
return 0;
}
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :62
Scope example
ex is external and
has this scope.
It can be accessed
in foo1(), where it
will have value 7
(set in main() )
Workshop 12
functions returning values




This program uses a function to
calculate the average of two
numbers – it has to return the
answer
The protoype
double average(double, double );
tells the compiler the return type,
and the number and type of the
arguments
The prototype must be consistent
with the header of the function
definition
The names of the arguments in
the definition ( x and y ) are
matched with the values in the
function call ( a and b ) in order
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :63
#include <stdio.h>
double average(double, double );
int main()
{
double a,b,c;
a=1.0;
b=2.0;
c=average(a,b);
printf("%f\n",c);
return 0;
}
double average(double x, double y)
{
double r;
r = (x+y)/2.0;
return r;
}
call by value
void foo( int );
int main()
{
int i=1;
foo(i);
/* how big is i now? */
return 0;
}



void foo( int n)
{
n++;
/* n is 2 - but what is i ? */
return;
}
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :64
parameters are always ‘called
by value’
this means that a copy of the
value of a parameter is passed
into the function
the function can alter the
value of the copy, but not the
original
Functions
1. Write a function which finds the average of 3 numbers rather than 2.
Write a main() to test it.
2. Write a function bigger that takes 2 integer parameters, and returns
the larger one.
3. Write a function IsUpper, which takes a char argument, and returns 1 if
the argument is a capital letter, and 0 otherwise. For example,
isUpper('A')
returns 1
isUpper('x')
returns 0
hint: if (c>='A' && c<='Z') … then c is an upper case character.
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :65
1. Suppose you want to find the square root of x. Here is a
method set y to 1
repeat
replace y by (y + x / y) / 2
y will get closer and closer to the square root of x. Write a
function called root using this method.
2 The factorial of a number, written n!, is the product of all the
integers up to n. So for example 4! = 4 X 3 X 2 X 1 = 24. Write a
void function called PrintFactorial which has an integer
argument, and which outputs the factorial of it. Test it with 10! =
3 628 800
Introduction to C : set 1 © Walter Milner 2005 :::: Slide :66