CS A331 Programming Language Concepts

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1 CS A331 Programming Language Concepts Lecture 9 Subroutines (Continued) March 17, 2014 Sam Siewert

2 Exploration Questions from Before Why Does ANSI C not support Nested Functions? Why does GCC support anyway? Why does GCC also not support in C++ too? Does Java Support Nested Functions? C#? Python? Ada? Others? Why are Nested Functions Useful? Sam Siewert 2

3 Parameter Passing Mapping of Actual to Formal Arguments By Value C Default By Value/Result (Copying), Scripting Languages References (Aliases) Using Pointers or Access/Ref Operators in Strongly Typed PLs Closure/name Sam Siewert 3

4 Pass by Value vs. Reference Two Most Common Methods (can be confusing) By Value Actual is assigned to Formal and After, the Two are Independent By Reference A new name for corresponding Actual (alias assuming actual name is visible) and change to either name changes both! Common Use in Popular PLs C is Pass by Value (and Pointers/Address are used for Reference) C++ introduces a reference in addition to pointers, but default is still pass by value Java and Ada prefer to abstract pointers Java uses object references, however is still pass by value Ada allows specification of either using IN, OUT, IN OUT Sam Siewert 4

5 C and C++ Swap with Reference #include <stdio.h> void swapnum(int &i, int &j) int temp = i; i = j; j = temp; int main(void) int a = 10; int b = 20; C++ printf("a=%d and B=%d\n", a, b); swapnum(a, b); printf("a=%d and B=%d\n", a, b); return 0; C #include <stdio.h> void swapnum(int *i, int *j) int temp = *i; *i = *j; *j = temp; int main(void) int a = 10; int b = 20; printf("a=%d and B=%d\n", a, b); swapnum(&a, &b); printf("a=%d and B=%d\n", a, b); return 0; Sam Siewert 5

$? void badswap(int i, int j) int temp = i; i = j; j = temp; int main(void) int a = 10; int b = 20; printf("a=%d, B=%d\n", a, b); badswap(a, b); printf("a=%d, B=%d\n", a, b); return 0; public static$

6 C++ Bad Swap Must use Reference or Only Copies are Swapped #include <stdio.h> public class swapper Java?? void badswap(int i, int j) int temp = i; i = j; j = temp; int main(void) int a = 10; int b = 20; printf("a=%d, B=%d\n", a, b); badswap(a, b); printf("a=%d, B=%d\n", a, b); return 0; public static void main(string[] args) int a=1, b=2; System.out.format("a=%d b=%d\n", a, b); badswap(a,b); System.out.format("a=%d b=%d\n", a, b); private static void badswap(int a, int b) int temp = a; a = b; b = temp; Sam Siewert 6

7 Simple Java Swap Swap inside Object (privately!) public class swapper private static int a=1, b=2; public static void main(string[] args) System.out.format("a=%d and b=%d\n", a, b); badswap(a,b); System.out.format("a=%d and b=%d\n", a, b); goodswap(); System.out.format("a=%d and b=%d\n", a, b); private static void badswap(int var1, int var2) int temp = var1; var1 = var2; var2 = temp; private static void goodswap() int temp = a; a = b; b = temp; Sam Siewert 7

8 Subroutine Closures Create a Reference for a Subroutine and Pass as a Parameter Scope is where Function is Elaborated? Or where it is Invoked? Subroutines are Routinely Passed as Parameters in Functional Languages Let s Look at C qsort and function pointers to start void qsort(void *base, size_t nitems, size_t size, int (*compar)(const void *, const void*)) Before OO, this Method Used to Sort any Type Use of void * and function pointer for comparison Frowned upon today, but why? Sam Siewert 8

9 #include <stdio.h> #include <stdlib.h> C qsort old school ADT int values[] = 88, 56, 100, 2, 25 ; int cmpfunc (const void * a, const void * b) return ( *(int*)a - *(int*)b ); void main(void) int n; printf("before sorting the list is: \n"); for( n = 0 ; n < 5; n++ ) printf("%d ", values[n]); qsort(values, 5, sizeof(int), cmpfunc); printf("\nafter sorting the list is: \n"); for( n = 0 ; n < 5; n++ ) printf("%d ", values[n]); Sam Siewert 9

10 Alternative to Function Pointers are Common Algorithm Generic or Template Parametric Instantiation with a Given Type to Operate On Generally Cleaner and Less Confusing the void * and function pointers Avoids replication of Code for each Type Algorithms is Applied to (ADT) Sam Siewert 10

CS A331 Programming Language Concepts

CS A331 Programming Language Concepts Lecture 9 Subroutines March 17, 2014 Sam Siewert Purpose of a Subroutine Control Abstraction perform a well defined operation E.g. transform a color image into a graymap