Here’s a brain puzzle for you: Write a function that accepts a numeric array and returns the high and low values from that array. Can it be done?
\n
\nOf course it can be done! I’ve yet to encounter a programming puzzle that can’t be cracked. Doing it well or doing it sloppy is the only issue.<\/p>\n
What’s sloppy? Global variables.<\/p>\n
Then how can you return two values from a function?<\/p>\n
Sure, you could write a function that fills a structure with both high and low elements of the array. To make a structure work with a function, however, you have to declare it as a global variable. Sloppy.<\/p>\n
The real solution is to use pointers. The function accepts three arguments: The array, of course, and then two addresses, one for the high value and the other for the low value. Here’s your sample code:<\/p>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n#include <time.h>\r\n\r\n#define SIZE 25 \/* size of the array *\/\r\n\r\nvoid hilo(int *a, int *h, int *l);\r\nvoid fill_array(int *a);\r\n\r\nint main()\r\n{\r\n int array[SIZE];\r\n int x,high,low;\r\n\r\n fill_array(array); \/* assign random values to the array *\/\r\n\r\n for(x=0;x<SIZE;x++) \/* display array *\/\r\n printf(\"%d\\t\",array[x]);\r\n putchar('\\n');\r\n \/* Get high and low values *\/\r\n hilo(array,&high,&low);\r\n printf(\"The highest value is %d.\\n\",high);\r\n printf(\"The lowest value is %d.\\n\",low);\r\n\r\n return(0);\r\n}\r\n\r\n\/* Fetch the highest and lowest values in the array *\/\r\n\r\nvoid hilo(int *a, int *h, int *l)\r\n{\r\n int x = 0; \/* initalize x to zero *\/\r\n\r\n *h = *l = a[x]; \/* Initialize high\/low values *\/\r\n for(;x<SIZE;x++) \/* x is already initialized *\/\r\n {\r\n if(a[x] < *l) \/* new low value? *\/\r\n *l = a[x]; \/* set new low value *\/\r\n if(a[x] > *h) \/* new high value? *\/\r\n *h = a[x]; \/* set new high value *\/\r\n }\r\n}\r\n\r\n\/* Put random values 1 through 100 into the array *\/\r\n\r\nvoid fill_array(int *a)\r\n{\r\n int x;\r\n\r\n srandom((unsigned)time(NULL)); \/* Seed the randomizer *\/\r\n\r\n for(x=0;x<SIZE;x++)\r\n {\r\n a[x] = random() % 100; \/* Get random int value 0 to 99 *\/\r\n a[x]++; \/* add one to equal 1 to 100 *\/\r\n }\r\n}<\/pre>\nHere’s sample output:<\/p>\n
74\t71\t28\t5\t21\t52\t86\t67\t12\t94\r\n58\t58\t74\t10\t98\t95\t29\t82\t31\t76\r\n99\t21\t37\t57\r\nThe highest value is 99.\r\nThe lowest value is 5.\r\n<\/code><\/pre>\nThe meat of the code is the hilo()<\/em> function, which uses dratted pointers to “return” the high and low values. Let me explain how that works.<\/p>\nint<\/code> variables high<\/code> and low<\/code> are declared at Line 13. They’re regular int<\/code> variables, not pointers. When those variables are passed to the hilo()<\/em> function at Line 21, however, only their addresses (memory locations) are sent, thanks to the &<\/code> prefix operator.<\/p>\nRemember: A pointer is a variable that stores a memory location<\/em>. So the effect of passing the &high<\/code> and &low<\/code> memory locations is that the function receives those variables as pointers. The function definition shows the variables passed defined that way:<\/p>\nvoid hilo(int *a, int *h, int *l)<\/code><\/p>\nWithin the hilo()<\/em> function, both variables h<\/code> and l<\/code> represent the memory locations of the high<\/code> and low<\/code> variables. Those variables must be prefixed by the *<\/code> operator to access the variables’ contents. The end effect is that the values assigned to *h<\/code> and *l<\/code> within hilo()<\/em> directly affect the original variables in main()<\/em>. There’s no need to “return” anything.<\/p>\nThe a[x]<\/code> variable doesn’t need to be prefixed by *<\/code> within the hilo()<\/em> function because it’s an array. The compiler is just being nice to you by letting you use array notation. Internally, it sees *(a+x)<\/code>.<\/p>\nThe method of returning a value by using a pointer explains why some functions specify pointers as arguments, such as scanf()<\/em>. That way the function directly affects a variable’s value and doesn’t need to return something. Or, in the case of hilo()<\/em>, it returns multiple values.<\/p>\n","protected":false},"excerpt":{"rendered":"Functions in C return only one value, but it’s possible to have them return many values. The trick is to use pointers. Yeah, those scary, nasty pointers. Continue reading