1. Lecture 13: Arrays, Pointers, Code examples B Burlingame 2 Dec 2015

2. Announcements Final exam: Dec 10, 9:45 AM Homework 8 & Final Lab due Dec 10 with the final  Final review Extra credit opportunity  Find an academic paper related to this class in some way  Read it and write a one page synopsis  Turn in the synopsis and the paper it relates to with the final

3. Recall: what is an array? int nums [10];  10 element array of integers Element no. 3 is accessed by: nums [2] because indexing begins at 0

4. Passing an Array to a Function Prototype and function header need:  data type  array name  [ ] Function call with actual name of array Note: in the function prototype and function header char *array would also work #include void PrintArray(int elements, char array[]); int main() { /* initialize the array */ char test[]={'M','E','3','0'}; /* get the size of the array */ int num_elem=sizeof(test)/sizeof(char); /* pass array to function */ PrintArray(num_elem, test); return 0; } /* PrintArray() function definition */ void PrintArray(int num_elem, char array[]) { int i=0; for(i=0; i<num_elem; i++) { printf("test[%d]==%c",i,array[i]); printf(" @ 0x%p\n",&array[i]); }

5. How do pointers and arrays relate? int nums [10];  10 element array of integers Element no. 3 is accessed by: nums [2] -or - Element no. 3 can be access by: *(num + 2) Where num is the head address of the array and 2 is the offset

6. Passing an Array Pointer to a Function Prototype and function header need:  data type  array name (as pointer) Function call with actual name of array #include void PrintArray(int elements, char *array); int main() { /* initialize the array */ char test[]={'M','E','3','0'}; /* get the size of the array */ int num_elem=sizeof(test)/sizeof(char); /* pass array to function */ PrintArray(num_elem, test); return 0; } /* PrintArray() function definition */ void PrintArray(int num_elem, char *array) { int i=0; for(i=0; i<num_elem; i++) { printf("test[%d]==%c",i, *(array + 1)); printf(" @ 0x%p\n", array + 1); }

7. Pointers and Arrays An array is basically a static pointer to the head (element 0) of an array void show_array( int x[], int *y, int size ); int main( void ) { int a[10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }; //since arrays are already storing addresses, no & necessary show_array( a, a, 10 ); return 0; } void show_array( int x[], int *y, int size ) { int i = 0; for( i = 0; i < size; ++i ) { //note how the array and pointer notation looks printf( "%d\t%d\n", x[i], *(y + i) ); }

8. Pointers and Arrays An array is basically a static pointer to the head (element 0) of an array void show_elem( int *x, int *y ); int main( void ) { int a[10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }; //an element of an array, still stores a value; therefore an & is //necessary. Using the pointer offset method, however, doesn’t show_elem( a + 3, &a[3] ); return 0; } void show_elem( int *x, int *y ) { printf( “%d\t%d\n”, *x, *y ); }

9. Examples

10. Problem Statement Prompt the user for a string Convert the string to lower case Display the lower case version of the string

11. Calling program #include void lc(char s[]); // Convert a string to all lowercase int main(void) { char string1[100] = ""; // Declare a sizeable string char buffer[100] = ""; // Declare a buffer printf("Enter a string: "); // Prompt the user for a string fgets(buffer, sizeof(buffer), stdin); // Read string as a line from the sscanf(buffer, "%s", string1); // keyboard (assumed as stdin) lc(string1); // Convert the string to lower case printf("%s is lower cased\n", string1); // print the string to the display return 0; }

12. Convert to lower case void lc(char s[]) { unsigned int i = 0; int utoc = 'A' - 'a'; // recall that characters are just numbers for (i = 0; s[i] != '\0'; ++i) // iterate over entire string { if (s[i] >= 'A' && s[i] <= 'Z') // if the letter is upper case { s[i] = s[i] - utoc; // subtract the difference between the } // upper and lower case values } } // recall that arrays and strings are // effectively pointers, so the change // is permanent

13. Problem Compare two strings for equivalence Equivalence means they are the same length with the exact same characters

14. Calling program #include int strcmp(char str1[], char str2[]); int main(void) { char string1[100] = ""; char string2[100] = ""; char buffer[100] = ""; printf("Enter a string: "); // Obtain two strings from the user fgets(buffer, sizeof(buffer), stdin); sscanf(buffer, "%s", string1); printf("Enter a string: "); // Obtain second string fgets(buffer, sizeof(buffer), stdin); sscanf(buffer, "%s", string2); if (strcmp(string1, string2)) { // Compare the strings for equivalence printf("%s and %s are equal\n", string1, string2); } else { printf("%s and %s are NOT equal\n", string1, string2); } return 0; }

15. Comparing two strings //compare two strings, return 1 if they are equivalent, //else return 0 int strcmp( char str1[], char str2[] ) { unsigned int i = 0; // go through each string, line by line for( i = 0; str1[i] == str2[i] && str1[i] != '\0' && str2[i] != '\0‘; ++i ) //continue if the chars are equal and the ends haven’t been found { // do nothing in the loop } if( str1[i] == str2[i] ) { return 1; } else { return 0; } }

16. Comparing two string, alternate int strcmp( char *str1, char *str2 ) { unsigned int i = 0; // go through each string, line by line while( *(str1 + i) == *(str2 + i) && *(str1 + i) != '\0' && *(str2 + i) != '\0' ) { ++i } // check the final character for return values if( *(str1 + i) == *(str2 + i) ) { return 1; // true, the strings are the same } else { return 0; // false, the strings are not the same }

17. Problem Statement Read through a file of numbers  One floating point number per line Add the numbers Count the numbers Find the average Display all three

18. Calling Program #include void stats(FILE *fh, int *count, double *sum, double *average); int main(void) { FILE *infile = fopen("c:/temp/data.dat", "r"); // Open the file int count = 0; double sum = 0.0; double average = 0.0; if (infile == NULL) // Ensure the file { // opened correctly fprintf(stderr, "Error opening c:/temp/data/dat.\n"); // report an error, if return 1; // it didn’t } stats(infile, &count, &sum, &average); // Generate the stats printf("Count: %d\n", count); // Report the results printf("Sum: %6.3lf\n", sum); printf("Average: %6.3lf\n", average); }

19. Generate Stats void stats(FILE *fh, int *count, double *s, double *ave) { double value = 0.0; // Storage for the current value while (fscanf(fh, "%lf", &value) != EOF) // Read until end of file (EOF) is { // found ++(*count); // increment the counter *s += value; // add to the sum } // sum is a type of accumulator *ave = *s / *count; // calculate the arithmetic mean } // (the average)

20. Alternate #include FILE* open_input_file(char path[]); void generate_stats(FILE *fh, int *count, double *sum, double *average); void display_stats(int count, double sum, double average); #define PATH "c:/temp/data.dat" int main(void) { int count = 0; double sum = 0.0; double average = 0.0; FILE *infile = open_input_file(PATH); // Note: the pseudocode is generate_stats(infile, &count, &sum, &average); // written in the function names display_stats(count, sum, average); // open input files } // generate stats // display stats

21. FILE* open_input_file(…) FILE* open_input_file(char path[]) // simply open the file { // and confirm that it FILE *fh = fopen(path, "r"); // opened correctly if (fh == NULL) { fprintf(stderr, "Error opening c:/temp/data/dat.\n"); exit(1); // premature program } // termination, from // stdlib.h return fh; }

22. void generate_stats(…) void generate_stats(FILE *fh, int *count, double *s, double *ave) { double value = 0.0; // This is the same as stats while (fscanf(fh, "%lf", &value) != EOF) // from the previous rendition { ++(*count); *s += value; } *ave = *s / *count; }

23. void display_stats(…) void display_stats(int count, double sum, double average) { printf("Count: %d\n", count); // Very simple though useful routine printf("Sum: %6.3lf\n", sum); // It puts all of the output in one printf("Average: %6.3lf\n", average); // clearly defined location }