Arrays in C/C++
More on usage
Copyright 1998-2007 Curt Hill
Tracing
• Somewhat problematic
• Since one name has many values we
have to be careful to keep track of
them properly
• Thus our tracing chart widens
Copyright 1998-2007 Curt Hill
Voting Machine Again
// Properties in the class
int counts[20];
...
// these set to zero
for(int i=0;i<20;i++)
counts[i] = 0;
...
void action () {
int vote;
cin >> vote;
if(vote>=0 && vote<=20){
counts[vote]++;
}
else // invalid vote
}
Copyright 1998-2007 Curt Hill
Technique
• In the vote example we knew in
advance how many candidates
before we got started
– At compile time
• What if we do not know?
– Two possibilities
• Make array large and do not fill it
• C/C++ also has dynamic arrays
– Any pointer may refer to a simple item
or an array of items
Copyright 1998-2007 Curt Hill
UnFull Array Technique
• Consider this declaration:
int used = 0,
vals[1000];
• There are now two integers
associated with vals:
– used is the number that are used
– 1000 is the maximum size
• Each time a value is added:
if(used<1000) vals[used++]=a;
Copyright 1998-2007 Curt Hill
Technique Continued
• The only time 1000 is used is to see if a
new item may be added
• All other references refer to used, such
as a display:
for(int i=0;i<used;i++)
cout << i << “ “ << vals[i]
<<“\n”;
Copyright 1998-2007 Curt Hill
Discussion
• In other languages the used variable
could either refer to:
– The last used value
– The first unused value
• In the C family of languages it is
almost always the latter
– This makes it similar to array size usage
– Thus the for starts at zero and has an
end conditional of strictly less than
Copyright 1998-2007 Curt Hill
More Discussion
• A function is completely unable to
determine the size of the array at
run-time
• This kills array bounds checking
– A compiler that can not check
something all the time will generally not
do it ever
• It also does not matter whether we
pass the function the size of the
array or the number of items used
Copyright 1998-2007 Curt Hill
Example: Statistics
• Consider that we wish to make a
simple statistics program
• It should allow the entry of values
through a window
• It should also allow display and
compute the average
• As time permits compute median or
mode
Copyright 1998-2007 Curt Hill
Data Structure
• The central data structure is the array:
double data [MAX];
int count = 0;
• Adding an item to the array:
if(count < MAX)
data[count++] = val;
• Now some functions
Copyright 1998-2007 Curt Hill
Find the average
double avg(double d[], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero
division
return sum/size;
else
return 0;
} // end of avg
Copyright 1998-2007 Curt Hill
Tracing
double avg(double d [], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero division
return sum/size;
else
return 0;
size
} // end of avg
4
Copyright 1998-2007 Curt Hill
d
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1.2
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4.0
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3.3
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-3.1
9.1
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Tracing
double avg(double d [], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero division
return sum/size;
else
return 0;
sum
size
} // end of avg
4
0
Copyright 1998-2007 Curt Hill
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1.2
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3.1
-3.1
9.1
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Tracing
double avg(double d [], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero division
return sum/size;
else
return 0;
sum i size
} // end of avg
0
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0
Copyright 1998-2007 Curt Hill
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9.1
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Tracing
double avg(double d [], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero division
return sum/size;
else
return 0;
sum i size
} // end of avg
0
4
0
Copyright 1998-2007 Curt Hill
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9.1
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Tracing
double avg(double d [], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero division
return sum/size;
else
return 0;
sum i size
} // end of avg
0
4
0
1.2
Copyright 1998-2007 Curt Hill
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9.1
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Tracing
double avg(double d [], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero division
return sum/size;
else
return 0;
sum i size
} // end of avg
0
4
0
1.2 1
Copyright 1998-2007 Curt Hill
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-3.1
9.1
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Tracing
double avg(double d [], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero division
return sum/size;
else
return 0;
sum i size
} // end of avg
0
4
0
1.2 1
Copyright 1998-2007 Curt Hill
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9.1
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Tracing
double avg(double d [], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero division
return sum/size;
else
return 0;
sum i size
} // end of avg
0
4
0
1.2 1
4.3
Copyright 1998-2007 Curt Hill
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9.1
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Tracing
double avg(double d [], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero division
return sum/size;
else
return 0;
sum i size
} // end of avg
0
4
0
1.2 1
4.3 2
Copyright 1998-2007 Curt Hill
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1.2
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3.1
-3.1
9.1
…
Tracing
double avg(double d [], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero division
return sum/size;
else
return 0;
sum i size
} // end of avg
0
4
0
1.2 1
4.3 2
8.3
Copyright 1998-2007 Curt Hill
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1.2
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3.1
-3.1
9.1
…
Tracing
double avg(double d [], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero division
return sum/size;
else
return 0;
sum i size
} // end of avg
0
4
0
1.2 1
4.3 2
8.3 3
Copyright 1998-2007 Curt Hill
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1.2
3.1
4.0
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0.1
9.1
3.3
3.1
-3.1
9.1
…
Tracing
double avg(double d [], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero division
return sum/size;
else
return 0;
sum i size
} // end of avg
0
4
0
1.2 1
4.3 2
8.3 3
13.7
Copyright 1998-2007 Curt Hill
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1.2
3.1
4.0
5.4
0.1
9.1
3.3
3.1
-3.1
9.1
…
Tracing
double avg(double d [], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero division
return sum/size;
else
return 0;
sum i size
} // end of avg
0
4
0
1.2 1
4.3 2
8.3 3
13.7 4
Copyright 1998-2007 Curt Hill
d
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1.2
3.1
4.0
5.4
0.1
9.1
3.3
3.1
-3.1
9.1
…
Tracing
double avg(double d [], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero division
return sum/size;
else
return 0;
sum i size
} // end of avg
0
4
0
1.2 1
4.3 2
8.3 3
13.7 4
Copyright 1998-2007 Curt Hill
d
0
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…
1.2
3.1
4.0
5.4
0.1
9.1
3.3
3.1
-3.1
9.1
…
Tracing
double avg(double d [], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero division
return sum/size;
else
return 0;
sum i size
} // end of avg
0
4
0
1.2 1
4.3 2
8.3 3
13.7 4
Copyright 1998-2007 Curt Hill
d
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…
1.2
3.1
4.0
5.4
0.1
9.1
3.3
3.1
-3.1
9.1
…
Tracing
double avg(double d [], int size){
double sum = 0;
for(int i = 0;i<size;i++)
sum += d[i];
if(size>0) // prevent zero division
return sum/size;
else
return 0;
sum i size
} // end of avg
0
4
0
1.2 1
4.3 2
8.3 3 result
13.7 4 3.425
Copyright 1998-2007 Curt Hill
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1.2
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4.0
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3.3
3.1
-3.1
9.1
…
Parameter Passage
• When a function receives a value
parameter as an actual parameter, it
receives a copy
• No matter what the method does the
formal parameter is not changed
• Since array names are pointers a
function receives a pointer
• The pointer cannot be changed but
the array to which it refers can be
Copyright 1998-2007 Curt Hill
Tracing Again
• Consider the following worthless code:
void m1(int ar[], int s, int x){
for(int i = 0;i<s;i++)
ar[i] = ar[i]/2 + 1;
x = -100;
}
…
int z[]={10,8,4}, x=12;
m1(z,3,x);
Copyright 1998-2007 Curt Hill
Tracing
void m1(int ar[], int s, int x){
for(int i = 0;i<s;i++)
ar[i] = ar[i]/2 + 1;
x = -100;
}
…
int z[]={10,8,4}, x=12;
m1(z,3,x);
x
12
z
10
8
4
Copyright 1998-2007 Curt Hill
Tracing
void m1(int ar[], int s, int x){
for(int i = 0;i<s;i++)
ar[i] = ar[i]/2 + 1;
x = -100;
}
…
int z[]={10,8,4}, x=12;
m1(z,3,x);
x
12
ar
z
x
12
10
8
4
Copyright 1998-2007 Curt Hill
Tracing
void m1(int ar[], int s, int x){
for(int i = 0;i<s;i++)
ar[i] = ar[i]/2 + 1;
x = -100;
}
…
int z[]={10,8,4}, x=12;
m1(z,3,x);
x
12
ar
z
10
8
4
Copyright 1998-2007 Curt Hill
x
i
12
0
Tracing
void m1(int ar[], int s, int x){
for(int i = 0;i<s;i++)
ar[i] = ar[i]/2 + 1;
x = -100;
}
…
int z[]={10,8,4}, x=12;
m1(z,3,x);
x
12
ar
z
6
8
4
Copyright 1998-2007 Curt Hill
x
i
12
0
Tracing
void m1(int ar[], int s, int x){
for(int i = 0;i<s;i++)
ar[i] = ar[i]/2 + 1;
x = -100;
}
…
int z[]={10,8,4}, x=12;
m1(z,3,x);
x
12
ar
z
6
8
4
Copyright 1998-2007 Curt Hill
x
i
12
1
Tracing
void m1(int ar[], int s, int x){
for(int i = 0;i<s;i++)
ar[i] = ar[i]/2 + 1;
x = -100;
}
…
int z[]={10,8,4}, x=12;
m1(z,3,x);
x
12
ar
z
6
5
4
Copyright 1998-2007 Curt Hill
x
i
12
1
Tracing
void m1(int ar[], int s, int x){
for(int i = 0;i<s;i++)
ar[i] = ar[i]/2 + 1;
x = -100;
}
…
int z[]={10,8,4}, x=12;
m1(z,3,x);
x
12
ar
z
6
5
4
Copyright 1998-2007 Curt Hill
x
i
12
2
Tracing
void m1(int ar[], int s, int x){
for(int i = 0;i<s;i++)
ar[i] = ar[i]/2 + 1;
x = -100;
}
…
int z[]={10,8,4}, x=12;
m1(z,3,x);
x
12
ar
z
6
5
3
Copyright 1998-2007 Curt Hill
x
i
12
2
Tracing
void m1(int ar[], int s, int x){
for(int i = 0;i<s;i++)
ar[i] = ar[i]/2 + 1;
x = -100;
}
…
int z[]={10,8,4}, x=12;
m1(z,3,x);
x
12
ar
z
6
5
3
Copyright 1998-2007 Curt Hill
x
i
12
3
Tracing
void m1(int ar[], int s, int x){
for(int i = 0;i<s;i++)
ar[i] = ar[i]/2 + 1;
x = -100;
}
…
int z[]={10,8,4}, x=12;
m1(z,3,x);
x
12
ar
z
6
5
3
Copyright 1998-2007 Curt Hill
x
i
12
3
Tracing
void m1(int ar[], int s, int x){
for(int i = 0;i<s;i++)
ar[i] = ar[i]/2 + 1;
x = -100;
}
…
int z[]={10,8,4}, x=12;
m1(z,3,x);
x
12
ar
z
x
-100
6
5
3
Copyright 1998-2007 Curt Hill
Tracing
void m1(int ar[], int s, int x){
for(int i = 0;i<s;i++)
ar[i] = ar[i]/2 + 1;
x = -100;
}
…
int z[]={10,8,4}, x=12;
m1(z,3,x);
x
12
z
6
5
3
Copyright 1998-2007 Curt Hill
Discussion
• A value parameter passed to a
function or method just passes a
copy
– The original cannot be changed
• An array just passes the pointer
– The array may changed but not the
pointer
• Arrays may thus have their values
changed
Copyright 1998-2007 Curt Hill
Partial Arrays
• Beside using the unfull array
technique we may also process
partial arrays
• A partial array is a subset of the full
array
• This can be accomplished in two
ways
– Passing a smaller length, which we
have seen
– Passing something other than the
beginning address
Copyright 1998-2007 Curt Hill
Lets Not Start at Beginning
• Since
– an array name is just a pointer
– the bracket is an address modification
operator
– we may use both to pass a fragment of
an array to a function
• Suppose:
int fun(int d[], int size);
int a[1000];
...
a[0] = fun(&a[10],100);
Copyright 1998-2007 Curt Hill
Discussion
• The expression a[10] is just an int
• The address of operator may then
obtain its address
• This is then passed to the function
• Of course the function believes that
the subscript range starts at zero
• So d[0] actually corresponds to
a[10]
• Of course, &a[i] works in the same
way
Copyright 1998-2007 Curt Hill
Conclusions
• Arrays are a valuable means to
structure data
• They require a subscript (or index)
to access individual elements
• Only parameter passage uses an
array without an index
• Methods may change array elements
• Multiple dimension arrays are also
possible but will be discussed later
Copyright 1998-2007 Curt Hill