Imperative Programming

Imperative Programming

Imperative Programming • Heart of program is assignment statements • Aware that memory contains instructions and data values • Commands: variable declarations, loops, conditionals, procedural abstraction • Commands make a flowchart

Imperative Languages • FORTRAN • COBOL • BASIC • Pascal • C

Pascal Example program Loops;vari: Integer;begin for i := 1 to 10 do beginWriteln('Hello');Writeln('This is loop ',i); end;end.

C Example /*Hello, world” */ #include <stdio.h> main() { printf(“Howdy, earth! \n”); return 0; }

C Printing • Print strings directly • Other types must be converted, because they are really numbers: char c = ‘A’; printf(“Print char c: %c.\n”,c); int n = 6; printf(“Print int n: %d.\n”,n);

Increment/Decrement(P.S. Same in Java) int x,y,z; x=y=z=1; y = x++; x=1; z = ++x; printf("x++ gives %d\n",y); printf("++x gives %d\n",z);

C-isms • 0 is false, 1 is true (or non-zero) • Same logical operators, &&, ||, ! • But there are also bitwise operators &, |, ~ • Other bitwise operators: ^, >>, << • x ? y : z means “if x, y, else z”(Same as Java) char c = x > 0? ‘T’ : ‘F’

Memory Addresses (C) • Pointers are addresses of where values are stored in memory. • &variable means “address of variable” • What happens? int a = 13; printf("a is %d\n",a); printf("&a is %p\n",&a);

Pointers in C int* ptr; -> ptr is a pointer to an int (with no pointee!) What does it do? int *ptr; int num = 4; ptr = &num;

More Pointer Fun in C int *ptr; int num = 4; ptr = &num; *ptr = 5;

Pointers in Pascal program Pointers;var p: ^integer;begin new(p); p^ := 3;writeln(p^); dispose(p);end.

C Arrays intarInt[5]; int arInt2[5] = {1,4,3,2,1}; intar[2][3] = {{1,2,3},{4,5,6}};

C Strings char str[] = “I like C.”; char *ptrStr = “I love pointers.”; Last character in String is ‘\0’

C Strings – Example:Compute length of string intmyStrLen(const char* s) { int count = 0; while (*s != ‘\0’){ count++; s++; } return count; }

Preprocessor Directives • #define – substitute before compiling: #define MAX 1000 if(x > MAX)…

C Scope Modifiers • static: don’t delete from memory - permanent duration • register: suggest use of register for variable. Can’t ask for address of the variable • const: make a variable constant. When pointers are const, their “pointees” are constant

Memory • malloc: allocates memory. Must give number of bytes wanted. malloc function returns a pointer to the memory. (Null ptr if memory error). • free: frees memory. • char *ptr; • char str[] = “String!”; • ptr = malloc(strlen(str) + 1); • free(ptr);

Defining new types • Struct (record in other langs) typedef struct{ char* word; int frequency; int size; } wordInfo; wordInfo wordOne; wordOne.word = “apple”; wordOne.frequency = 0; wordOne.size = 0;

Pointers to Structs • We often use pointers to structs (do you know why? wordInfo *wiPtr; • Can dereference explicitly: wordInfo wi = *wiPtr; • Or shortcut straight to the fields: int f = wiPtr->frequency;

What C Lacks • Iterators • Exception Handling • Overloading • Generics

Parameter Passing • Pass by value vs Pass by reference • What’s it like in Java?

Parameter Passing Mechanisms • By value • By reference • By value-result • By name

Pass by Value • Compute the value of the argument at the time of the call and assign that value to the parameter. • So passing by value doesn’t normally allow the called function to modify an argument’s value. • All arguments in C and Java are passed by value. • But references can be passed to allow argument values to be modified. E.g., void swap(int *a, int *b) { … }

A Look at Swapping - Java 1 • void swap(int a, int b){ • int t = a; • a = b; • b = a; • } • int x = 3; • int y = 4; • swap(x,y); • System.out.println("x value: "+x+" y value: "+y);

A Look at Swapping - Java 2 • void swap2(int[] array, int a, int b){ • int t = array[a]; • array[a] = array[b]; • array[b] = t; • } • int[] arr = {0,1}; • swap2(arr, 0,1); • System.out.println("array: "+arr[0]+” "+arr[1]);

A Look at Swapping - Java 3 • void swap3(Double a, Double b){ • Double t = a; • a = b; • b = t; • } • Double a = new Double(3.0); • Double b = new Double(4.0); • swap3(a,b); • System.out.println("a value: "+a.doubleValue()+" b value: "+b.doubleValue());

A Look at Swapping - C1 • void swap(int a, int b){ • int t = a; • a = b; • b = t; • } • int main(void){ • int a = 3; • int b = 4; • swap(a,b); • printf("a value is %d and b value is %d", a, b); • return 0; • }

A Look at Swapping - C2 • void swap2(int* a, int* b){ • int temp = *a; • *a = *b; • *b = temp; • } • int main(void){ • int a = 3; • int b = 4; • swap2(&a,&b); • printf("a value is %d and b value is %d", a, b); • return 0; • }

C++ Swapping - Pass by Reference/Value voidswap(int a, int b){ int temp = a; a = b; b = temp; } voidswap(int& a, int& b){ int temp = a; a = b; b = temp; } • intmain(void){ • int a = 3; • int b = 4; • swap(a,b); • printf("a value is %d and b value is %d", a, b); • return 0; • }

Pass by Reference int h, i; void B(int* w) { int j, k; i = 2*(*w); *w = *w+1; } void A(int* x, int* y) { bool i, j; B(&h); } int main() { int a, b; h = 5; a = 3; b = 2; A(&a, &b); } Compute the address of the argument at the time of the call and assign it to the parameter. Example Since h is passed by reference, its value changes during the call to B.

Pass by Value-Result • Pass by value at the time of the call and copy the result back to the argument at the end of the call. • E.g., Ada’s in out parameter can be implemented as value-result. • Value-result is often called copy-in-copy-out. • Reference and value-result are the same, except when aliasing occurs.

Aliasing • 2 Names for same thing: • Java example: • Object o = new Object… • Object x = o; • Can happen other ways as well: • Pass variable as parameter and refer to it globally • Pass variable by reference twice

When Pass-by-Value-Result gives different answers… • (Ada:) • procedure f (x,y: in out Integer) is • begin • x := x + 1;y := y + 1; • end f; • f(a,a) increments a by ONE only.

More on Pass by Value-Result • -Sometimes the order of copying makes a difference! Different with different implementations of language. • Ex: • f(i,a[i]) -> change i first or a[i] first? • -All actual parameters must be l-values - things that can be assigned. (Same in pass-by-reference!) • Ex: • f(a+1) -> How would it be copied back in?

Pass by Name • Textually substitute the argument for every instance of its corresponding parameter in the function body. • Originated with Algol 60 (Jensen’s device), but was dropped by Algol’s successors -- Pascal, Ada, Modula. • Exemplifies late binding, since evaluation of the argument is delayed until its occurrence in the function body is actually executed. • Associated with lazy evaluation in functional languages

When Pass by Name Gives different results Call swap(i, a[i]). Method becomes: begin integer t; t := i; i := a[i]; a[i]:= t end; • Algol 60: • procedure swap(a,b); • integer a,b; • begin integer t; • t := a; • a := b; • b:= t • end; i = 0, a=[3,0,0,0]. Want: i=3, a = [0,0,0,0]. What really happens? What happens if I call swap(a[i],i) instead?

Imperative Programming