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; var i: Integer; begin for i := 1 to 10 do begin Writeln('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

int arInt[5];int arInt2[5] = {1,4,3,2,1};int ar[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

int myStrLen(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

• 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;• }

void swap(int a, int b){

int temp = a;

a = b;

b = temp;

}

void swap(int& a, int& b){

int temp = a;

a = b;

b = temp;

}

Pass by Reference

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.

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);}

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

• Algol 60:• procedure

swap(a,b);• integer a,b;• begin integer t;• t := a;• a := b;• b:= t• end;

Call swap(i, a[i]). Method becomes:

begin integer t;

t := i;

i := a[i];

a[i]:= 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?