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C‐Programming,
continued Functions Arrays
The^ if^ statement
(and^ comparisons)
•^ The^ following
variety^ might
be^ used
-^ if^ (i^ <^ 2) –^ if^ (i^ <=^ 2) –^ if^ (i^ >=^ -1) –^ if^ (i^ ==^ 4)^
//^ note^ difference
between^ ==^ and^ =
-^ if^ (x^ ==^ 1.0) –^ if^ (fabs(x)^ <^ 10.0) –^ if^ (i^ <^8 &&^ i^
>^ -5)^ //
&&^ =^ and
-^ if^ (x^ >^ 10.0^ ||
x^ <^ -10.0)^ //
||^ =^ or
•^ Remember
to^ double^ up
==,^ &&,^ ||
Functions
do^ stuff
#include^ <stdio.h>#include^ <math.h>double^ gauss(double
x,^ double^ amplitude,
double^ center,^ double
sigma);
int^ main(){double^ gaussval1,gaussval2;double^ xval=-1.4,ampl=100.0,ctr=0.1,sig=2.0;gaussval1^ =^ gauss(1.5,10.0,0.0,2.0);gaussval2^ =^ gauss(xval,ampl,ctr,sig);printf("Gaussval
=^ %f;^ Gaussval^
=^ %f\n",gaussval1,gaussval2); return^ 0;} double^ gauss(double
x_val,^ double^ amplitude,
double^ center,^ double
sigma)
{return^ amplitudeexp(-0.5pow((x_val-center)/sigma,2));}
Functional
Notes
•^ In^ the^ previous
program: – we have a function^ declaration^ before
main(),^ specifying
the
argument^ types,^ with
temporary^ (descriptive
is^ good)^ names
-^ not^ strictly^ necessary,
but^ aids^ in^ checking^ errors
during^ compilation
-^ we^ can^ pass^ either
numerical^ arguments
or^ variables^ (or^ a^ mix)
-^ names^ don’t^ have
to^ match^ from^ declaration
to^ use^ in^ main()
to^ names^ in^ function()^ •^ but^ have^ to^ match
within^ function^ (note
x^ vs.^ xval^ vs.^ x_val
-^ can^ pass^ any^ number
of^ arguments^ of^ any
type^ into^ function
-^ limited^ to^ a^ single
value^ out
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Arrays
•^ We^ can^ hold^ more
than^ just^ one^ value
in^ a^ variable
-^ but^ the^ program
needs^ to^ know^ how
many^ places^ to^ save
in
memory • Examples: – we^ can^ either^ say^ how
many^ elements^ to
allow^ and^ leave
them^ unset;^ say^ how
many^ elements^ and
initialize^ all
elements^ to^ zero;^ leave
out^ the^ number^ of
elements^ and
specify^ explicitly;^ specify
number^ of^ elements
and^ contents
-^ character^ arrays
are^ strings
-^ strings^ must^ end
in^ ‘\0’^ to^ signal^
the^ end
-^ must^ allow^ room:
char^ name[4]=“Bob” • fourth element is^ ‘\0’^ by^ default
int^ i[8],^ j[8]={0},
k[]={9,8,6,5,4,3,2,1,0}; double^ x[10],^ y[10000]={0.0},
z[2]={1.0,3.0}; char^ name[20],^ state[]=“California”;
Indexing^ Arrays • Index array integers,^ starting^ with^ zero • Sometimes initialize^ in^ loop^ (z[]^ above) • String assignment^ awkward^ outside^ of
declaration^ line
-^ #include <string.h>
provides^ “useful”^ string
routines
int^ i,j[8]={0},k[]={2,4,6,8,1,3,5,7};double^ x[8]={0.0},y[2]={1.0,3.0},z[8];char^ name[20],state[]="California";for^ (i=0;^ i<8;^ i++){z[i]^ =^ 0.0;printf(”j[%d]^ =
%d,^ k[%d]^ =^ %d\n",i,j[i],i,k[i]); }name[0]='T';name[1]='o';name[2]='m';name[3]^ =^ '\0';printf("%s^ starts
with^ %c^ and^ lives
in^ %s\n",name,name[0],state);
Memory^
Allocation
in^ Arrays
•^ state[]=“California”;
•^ name[11]=“Bob”;
-^ empty^ spaces^ at
the^ end^ could^ contain
any^ random^ garbage
•^ int^ i[]^ = {9,8,7,6,5,4,3,2};
-^ indexing^ int[8]
is^ out^ of^ bounds,^ and
will^ either^ cause^ a
segmentation^ fault
(if^ writing),^ or^ return
garbage^ (if^ reading)
C^ a^ l^ i^ f^
o^ r^ n^ i^ a^
\
B^ o^ b^ \0^9 8 7 6
#define^ to
ease^ the^
coding
-^ #define^ comes
before^ the^ function^
definitions,^ up^ with^ the
#include^ statements^ –^ note^ no^ semi‐colons^ –^ just^ a^ text^ replacement
process:^ any^ appearance
of^ NPOINTS^ in^ the
source^ code^ is^ replaced
by^10
-^ Convention^ to^ use^ all
CAPs^ to^ differentiate^ from
normal^ variables^ or
commands – Now^ to^ change^ the^ number
of^ points^ processed^ by
that^ program,^ only
#define^ NPOINTS^10 #define^ NDIMS^3 int^ main(){int^ shots[NPOINTS],hits[NPOINTS],flag[NDIMS];double^ coords[NDIMS][NPOINTS],time_hit=[NPOINTS];…}^ have^ to^ modify^ one^ line
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Notes^ on^
mm3x
•^ The^ function
is^ made^ to^ deal
with^1 ‐d^ instead
of^2 ‐d^ arrays^ –^9 elements^
instead^ of^ 3x3 – it could have^ been^ done^ either
way
•^ There^ is^ a^ pointer,
*cptr^ being
used
–^ by^ specifying
cptr^ as^ a^ double
pointer,^ and
assigning^ its^ address
(just^ cptr)^ to
c,^ we^ can
stock^ the^ memory
by^ using^ “pointer
math”
–^ cptr^ is^ the^ address;
*cptr^ is^ the^
value^ at^ that
address – just^ like^ &x_val
is^ an^ address,^
while^ x_val
contains^ the^ value
Using^ mm3x
•^ passing^ just
the^ names^ (addresses)
of^ the
arrays^ –^ defining^ a^ and
b,^ just^ making
space^ for^ c
#include^ <stdio.h>void^ mm3x3(double^ a[],^ –^ note function declaration before main
double^ b[],^ double
c[]); int^ main(){double^ a[]={1.0,
2.0,^ 3.0,^ 4.0,^ 5.0,
6.0,^ 7.0,^ 8.0,^ 9.0};
double^ b[]={1.0,^
2.0,^ 3.0,^ 4.0,^ 5.0,
4.0,^ 3.0,^ 2.0,^ 1.0};
double^ c[9];mm3x3(a,b,c);printf("c^ =^ %f^ %f
%f\n",c[0],c[1],c[2]); printf("^ %f^ %f
%f\n",c[3],c[4],c[5]); printf("^ %f^ %f
%f\n",c[6],c[7],c[8]); return^ 0;}
Another^ way
to^ skin^ the
cat
•^ Here,^ we^ define
the^ arrays^ as
2 ‐d,^ knowing
that^ in^ memory
they^ will^ still
be^1 ‐d
–^ we^ will^ get^ compiler
warnings,^ but^
the^ thing
will^ still^ work – not^ a^ recommended
approach,^ just
presented
here^ for^ educational
purposes
Note that we could replace
a^ with^ &a[0][0]
double^ a[3][3]={{1.0,
2.0,^ 3.0},{4.0, 5.0,^ 6.0},{7.0, 8.0,^ 9.0}};
double^ b[3][3]={{1.0,
2.0,^ 3.0},{4.0, 5.0,^ 4.0},{3.0, 2.0,^ 1.0}};
double^ c[3][3];mm3x3(a,b,c);
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