Lecture 13: Arrays, Pointers, Code examples

B Burlingame

2 Dec 2015

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

Recall: what is an array?

Index No. [0] [1] [2] [3 [4] [5] [6] [7] [8] [9]int nums [10];

Element No. 1 2 3 4 5 6 7 8 9 10

int nums [10]; 10 element array of integers

Element no. 3 is accessed by:nums [2]

because indexing begins at 0

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 <stdio.h>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]); }}

How do pointers and arrays relate?

Index No. [0] [1] [2] [3 [4] [5] [6] [7] [8] [9]int nums [10];

Element No. 1 2 3 4 5 6 7 8 9 10Offset +0 +1 +2 +3 +4 +5 +6 +7 +8 +9

Address 0x0 0x4 0x8 0xC 0x10 0x14 0x18 0x1C 0c20 0x24

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

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 <stdio.h>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); }}

Pointers and Arrays An array is basically a static pointer to the

head (element 0) of an arrayvoid 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) ); }}

Pointers and Arrays An array is basically a static pointer to the

head (element 0) of an arrayvoid 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 );}

Examples

Problem Statement

Prompt the user for a string Convert the string to lower case Display the lower case version of the

string

Calling program#include <stdio.h>

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;}

Convert to lower casevoid 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

Problem

Compare two strings for equivalence Equivalence means they are the same

length with the exact same characters

Calling program#include <stdio.h>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;}

Comparing two strings//compare two strings, return 1 if they are equivalent, //else return 0int 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; }}

Comparing two string, alternateint 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 }}

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

Calling Program#include <stdio.h>

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);}

Generate Statsvoid 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)

Alternate#include <stdio.h>#include <stdlib.h>

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

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;}

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;}

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}