Lect 15P' 1

1
Arrays and Pointers

• Lecture 15

2
Arrays and Pointers

• So far, we have looked at pointer variables which
  were declared with statements like
• FILE fptr
• int aptr
• We have also used pointer constants when we sent
  the address of a variable to a function. We did
  this by placing an ampersand before the variable
  name. For instance, the address of variable a is
  a, which is a pointer constant.

3
Arrays and Pointers

• The name of an array without a subscript (index
  number) is also a pointer constant. When the
  name is used without the subscript it references
  the address of element 0 of the array.
• int myarray10 / Declare an array /
• int myptr / Declare a pointer (not
  initialized)/
• printf (d\n, myarray) / print address of
  myarray0 /
• scanf (d, myarray) / get value from
  keyboard and
• store in myarray0 /
• scanf (d,myarray0) / same thing as above
  /

4
Arrays and Pointers

• myptr myarray2 / Assign the address of
  the third
• element of myarray to myptr /
• printf(d, myptr) / Print the value of what
  myptr is
• pointing to, i.e., the value of
• myarray2 /
• Note that the in front of myptr de-references
  the pointer. That is, it says use the value in
  the address that is pointed to.
• The following shows a small program and its
  output.

5
Arrays and Pointers

• / Printing array values and addresses /
• include ltstdio.hgt
• int main ( )
• int k
• float a4 1000, 2000, 3000, 4000
• printf ("k a ak ak\n")
• for (k0 klt4 k)
• printf ("d ld ld f\n", k, a, ak,
  ak)

6
Running the Program

• r1tel () millerm 52gt a.out
• k a ak ak
• 0 2147462916 2147462916 1000.000000
• 1 2147462916 2147462920 2000.000000
• 2 2147462916 2147462924 3000.000000
• 3 2147462916 2147462928 4000.000000
• r1tel () millerm 53gt

7
Arrays and Pointers

• We have seen how to pass a pointer for a single
  valued variable to a function.
• Sometimes we want to pass an entire array to a
  function. The name of an array without a
  subscript is a pointer constant that contains the
  address of element 0 of the array.
  Therefore, if we pass the array name with no
  subscript to a function, we are passing a pointer
  to that array.
• The following program illustrates this.

8
Arrays and Pointers

• / Passing an array to a function /
• include ltstdio.hgt
• include ltstring.hgt
• void myfunct (int , char )
• int main ( )
• char name20 "Michael J. Miller"
• myfunct (20, name)

9
Arrays and Pointers

• void myfunct (int len , char text )
• int k
• printf ("d\n", strlen(text))
• for (k 0 k lt len k) printf ("c",
  textk)
• /Program Output /
• 17
• Michael J. Miller

10
Arrays and Pointers

• Given the declaration int a3 1, 2, 3
• a is a pointer to (the address of) a0
• a0 is a pointer to a0
• a0 is the value 1 ( a is also the value 1)
• a1 is a pointer to a1
• a1 is the value 2
• a2 is a pointer to a2
• a2 is the value 3
• a01 is a pointer to a1
• a02 is a pointer to a2

11
Arrays and Pointers

• Given the declaration
• int b33 1,3,5, 7,9,11,
  13,15,17
• b is a pointer to b00
• b0 is also a pointer to b00
• b1 is a pointer to b10
• b2 is a pointer to b20
• b is a pointer to b0 (special case)
• b1 is the value of b10 (which is 7)
• b2 1 is the value of b20 1 (which is
  14)

12

• / Double subscript arrays and
• user-written functions /
• include ltstdio.hgt
• void printarray (int 7, int)
• int main( )
• int calendar571,2,3,4,5,6,7,
• 8, 9, 10,11,12,13,14,
• 15,16,17,18,19,20,21,
• 22,23,24,25,26,27,28,
• 29,30,31,32,33,34,35
• printarray (calendar , 5)

13

• void printarray (int cal7, int j)
• int k, n
• for (k 0 k lt j k)
• for (n 0 n lt 7 n)
• printf ("3d ", calkn)
• printf ("\n")

14

• / Double subscript arrays, user-written
• functions and pointer arithmetic /
• include ltstdio.hgt
• void printarray (int , int , int)
• int main ( )
• int calendar57 1,2,3,4,5,6,7, 8, 9,
  10,11,12,13,14,
• 15,16,17,18,19,20,21,
• 22,23,24,25,26,27,28,
• 29,30,31,32,33,34,35
• printarray (calendar0 , 5 , 7)

15

• void printarray (int cal, int j, int m)
• for (k 0 k lt jm k m)
• for (n 0 n lt m n)
• printf ("3d ", (calkn))
• printf ("\n")

16
Assignment G13

• A data file named g13.dat exists in the class
  "common area" on the UNIX system.
• The file contains actual data from one test of an
  instrumented bicycle from an engineering hands-on
  lab experiment.
• You should look at the file on the screen (with a
  "more" command), but DO NOT print it out, as it
  contains several thousand (but less than 12,000)
  lines of data.

17
Assignment G13

• You will note that at the beginning of the file
  there are several lines of "header" information
  followed by many lines of data in four columns.
• Count by hand the number of lines from the
  beginning of the file until you get to a line
  that has the actual data in the four columns.
  (You will need this number in Step 2 later.)
• The fourth column is the raw data (voltage)
  values from the lab experiment.

18
Steps for G13

• Write a complete C program, (say, g13.cpp), which
  does the following
• Opens the data file for input.
• Input the correct number of header lines one by
  one, display each one on the screen and print
  each one to a result file (say, g13res.dat), and
  then discard the information.

19
Steps for G13

• For opening the data file and the output file,
  just use the usual fopen routine.
• For reading the header lines, it might be rather
  convenient to read the complete line into a
  character array or string.
• How long should this string be?
• What routine might be used?

20
Steps for G13

• Input each of the lines of data arranged in the
  four columns, discarding the data values from
  each of the first three columns and storing only
  the data from the fourth column in a
  one-dimensional array.
• For skipping over the columns with unwanted
  data, you will need to use the assignment
  suppression operator, , in the scanf format.
• Your program will need to detect the end-of-file
  (EOF) to know when to stop inputting data. Close
  the input file when you reach the EOF.

21
Data from g13.dat

• "
• "Source File C\PROGRA1\PSLOG\195RIDE.PL1
• "ID IE Group 4 Bike Stress friday XR440
  12 bi
• "Ch1 lbl/scl Start/Stop /A
• "Ch2 lbl/scl input voltage signal /-5.000
  05.000
• "Ch3 lbl/scl Off /C
• "Ch4 lbl/scl Off /C
• "Rate (mins) 0.0000833333324 Bat 8.8
• "First Fri 20-Nov-1998 103847am
• "Last Fri 20-Nov-1998 103940am
• "Transferred Fri 20-Nov-1998 104237am
• "Eq. PC time Fri 20-Nov-1998 104238AM

22
Data from g13.dat

• "Rate (mins) 0.0000833333324 Bat 8.8
• "First Fri 20-Nov-1998 103847am
• "Last Fri 20-Nov-1998 103940am
• "Transferred Fri 20-Nov-1998 104237am
• "Eq. PC time Fri 20-Nov-1998 104238AM
• "
• Date,Time,Ch1Deg F,Ch2
• 11/20/1998 103847.000 -4.989 0.238
• 11/20/1998 103847.005 -4.989 0.231
• 11/20/1998 103847.010 -4.989 0.228
• 11/20/1998 103847.015 -4.989 0.231
• 11/20/1998 103847.020 -4.989 0.228

23
Processing the G13 Data File

• For skipping unwanted columns, we need to use the
  "assignment suppression operator" in the format
  specification
• fscanf (infile, "sssf", data_val i )
  
• Remember, to check for EOF, you could use the
  feof function.

24
Assignment G13

• Find the largest value in the array and the
  smallest value in the array.
• Display the results on the screen, and also write
  the results to the output file, g13result.dat.
  The results to be displayed and printed are
• The total number of data points in the file
• The maximum voltage and time at which it occurred
• The minimum voltage and time at which it occurred
• The elapsed time between the maximum and minimum
  values

25
Assignment G13

• The change in the output voltage is related to
  the strain of the strain gage by
• where
• ?Vout V in A Sge
• V in 5.0 Volts e is the strain
• A 500 (amplification) ?Vout is the change in
  voltage
• Sg 2.085 (gage factor)
• The equation can be solved to give the strain as
  a function of the output voltage

26
Assignment 13

• Stress-Strain Hookes Law
• The stress(s) in the bicycle fork can be
  calculated from the strain (e), by using Hookes
  Law
• s E e
• Where E is the Modulus of Elasticity
• For the bike fork material E 29.0 x 106 psi
• The yield stress 36,000 psi