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Arrays and Pointers

Arrays and Pointers. Lecture 15. Arrays and Pointers. So far, we have looked at pointer variables which were declared with statements like: FILE *fptr ; int *aptr ;

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Arrays and Pointers

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  1. Arrays and Pointers Lecture 15 Winter Quarter

  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. Winter Quarter

  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 myarray[10]; /* Declare an array */ int *myptr; /* Declare a pointer (not initialized)*/ printf(“%d\n”, myarray); /* print address of myarray[0] */ scanf (“%d”, myarray); /* get value from keyboard and store in myarray[0] */ scanf(“%d”,&myarray[0]); /* same thing as above */ Winter Quarter

  4. Arrays and Pointers myptr = &myarray[2]; /* 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 myarray[2] */ • 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. Winter Quarter

  5. Arrays and Pointers /* Printing array values and addresses */ #include <stdio.h> int main ( ) { int k; float a[4] = {1000, 2000, 3000, 4000}; printf("k a &a[k] a[k]\n"); for(k=0; k<4; k++) printf("%d %ld %ld %f\n", k, a, &a[k], a[k]); } Winter Quarter

  6. Running the Program r1tel (~) millerm 52> a.out k a &a[k] a[k] 0 2147462916 2147462916 1000.000000 1 2147462916 2147462920 2000.000000 2 2147462916 2147462924 3000.000000 3 2147462916 2147462928 4000.000000 r1tel (~) millerm 53> Winter Quarter

  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 withouta 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. Winter Quarter

  8. Arrays and Pointers /* Passing an array to a function */ #include <stdio.h> #include <string.h> void myfunct (int , char [ ]); int main ( ) { char name[20] = "Michael J. Miller"; myfunct (20, name); } Winter Quarter

  9. Arrays and Pointers void myfunct (int len , char text[ ]) { int k ; printf("%d\n", strlen(text)); for(k = 0 ; k < len ; k++)printf("%c", text[k]); } /*Program Output */ 17 Michael J. Miller Winter Quarter

  10. Arrays and Pointers • Given the declaration int a[3] = { 1, 2, 3 } ; a is a pointer to (the address of) a[0] &a[0] is a pointer to a[0] a[0] is the value 1 ( *a is also the value 1) &a[1] is a pointer to a[1] a[1] is the value 2 &a[2] is a pointer to a[2] a[2] is the value 3 &a[0]+1 is a pointer to a[1] &a[0]+2 is a pointer to a[2] Winter Quarter

  11. Arrays and Pointers Given the declaration int b[3][3] = {{1,3,5}, {7,9,11}, {13,15,17}}; b is a pointer to b[0][0] b[0] is also a pointer to b[0][0] b[1] is a pointer to b[1][0] b[2] is a pointer to b[2][0] *b is a pointer to b[0] (special case) *b[1] is the value of b[1][0] (which is 7) *b[2] + 1 is the value of b[2][0] + 1 (which is 14) Winter Quarter

  12. /* Double subscript arrays and user-written functions */ #include <stdio.h> void printarray (int [ ][7], int); int main( ) { int calendar[5][7]={{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 (calendar , 5); } Winter Quarter

  13. void printarray (int cal[][7], int j) { int k, n ; for(k = 0 ; k < j ; k++) { for(n = 0 ; n < 7 ; n++) printf("%3d ", cal[k][n]); printf("\n"); } } Winter Quarter

  14. /* Double subscript arrays, user-written functions and pointer arithmetic */ #include <stdio.h> void printarray (int * , int , int); intmain( ) { int calendar[5][7]= {{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 (calendar[0] , 5 , 7); } Winter Quarter

  15. void printarray (int *cal, int j, int m) { for(k = 0 ; k < j*m ; k += m) { for(n = 0 ; n < m ; n++) printf("%3d ", *(cal+k+n)); printf("\n"); } } Winter Quarter

  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. Winter Quarter

  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. Winter Quarter

  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. Winter Quarter

  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? Winter Quarter

  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 scanfformat. 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. Winter Quarter

  21. Data from g13.dat " "Source File: C:\PROGRA~1\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 10:38:47am "Last: Fri 20-Nov-1998 10:39:40am "Transferred: Fri 20-Nov-1998 10:42:37am "Eq. PC time: Fri 20-Nov-1998 10:42:38AM Winter Quarter

  22. Data from g13.dat "Rate (mins): 0.0000833333324 Bat: 8.8 "First: Fri 20-Nov-1998 10:38:47am "Last: Fri 20-Nov-1998 10:39:40am "Transferred: Fri 20-Nov-1998 10:42:37am "Eq. PC time: Fri 20-Nov-1998 10:42:38AM " Date,Time,Ch1:Deg F,Ch2: 11/20/1998 10:38:47.000 -4.989 0.238 11/20/1998 10:38:47.005 -4.989 0.231 11/20/1998 10:38:47.010 -4.989 0.228 11/20/1998 10:38:47.015 -4.989 0.231 11/20/1998 10:38:47.020 -4.989 0.228 Winter Quarter

  23. Processing the G13 Data File • For skipping unwanted columns, we need to use the "assignment suppression operator" in the format specification: fscanf (infile, "%*s%*s%*s%f", &data_val[ i ] ) ; • Remember, to check for EOF, you could use the feof function. Winter Quarter

  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 Winter Quarter

  25. Assignment G13 • The change in the output voltage is related to the strain of the strain gage by: • where: ΔVout = V in * A * Sg*ε V in= 5.0 Volts ε is the strain A = 500 (amplification) ΔVout 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 Winter Quarter

  26. Assignment 13 • Stress-Strain: Hooke’s Law • The stress(σ) in the bicycle fork can be calculated from the strain (ε), by using Hooke’s Law: σ = E * ε • Where E is the Modulus of Elasticity • For the bike fork material E = 29.0 x 106 psi • The yield stress = 36,000 psi Winter Quarter

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