12.010 Basic C: New Features

Pointers

( not in F77 )

1. int a; refers to value held at location a.
2. int *ptr_to_a; pointer to memory location where value is held.
3. ptr_to_a=&a; sets pointer to equal location in memory used by variable a.

Examples

main() {

float a; /* Floating point number */

float *ptr_to_a; /* Pointer to a floating point number */

a = 7.; /* Write 7. to memory location associated with a */

printf("Value in a == %f\n",a);

ptr_to_a = &a; /* Get the address of the memory location where */

/* assignments to a get written. */

printf("Memory address of a (in hexadecimal) == %X\n",ptr_to_a);

/* Now use pointer to read value stored at an address */

printf("Value stored at address %X == %f\n",ptr_to_a,*ptr_to_a);

/* Write a new value to an address in memory */

*ptr_to_a = 3.;

/* What value does a have now? */

printf("Value in a == %f\n",a);

/* In C arrays and pointers are the same thing! */

/* [0] is ptr_to_a + offset of 0*4 bytes */

/* [1] is ptr_to_a + offset of 1*4 bytes */

/* etc...... */

printf("Value stored at address %X == %f\n",ptr_to_a,ptr_to_a[0]);

}

Call by value

C #include <stdio.h> void afunc(int, int); main() { int i1, i2; i1 = 3; i2 = 4; printf("in main()"); printf(" i1 == %d, i2 == %d\n",i1,i2); afunc( i1, i2 ); printf("in main()"); printf(" i1 == %d, i2 == %d\n",i1,i2); } void afunc(int a, int b) { printf("in afunc()"); printf(" a == %d, b == %d\n",a,b); a = 7; b = 6; printf("in afunc()"); printf(" a == %d, b == %d\n",a,b); }

F77 PROGRAM MAIN INTEGER I1, I2 I1 = 3 I2 = 4 WRITE(6,'(A,I4,I4)') 'in AFUNC() I1,I2: ',I1,I2 CALL AFUNC(I1, I2) WRITE(6,'(A,I4,I4)') 'in AFUNC() I1,I2: ',I1,I2 END SUBROUTINE AFUNC( A, B ) INTEGER A, B WRITE(6,'(A,I4,I4)') 'in AFUNC() A,B: ',A,B A = 7 B = 6 WRITE(6,'(A,I4,I4)') 'in AFUNC() A,B: ',A,B END

Call by reference

C #include <stdio.h> void afunc(int *, int *); main() { int i1, i2; i1 = 3; i2 = 4; printf("in main()"); printf(" i1 == %d, i2 == %d\n",i1,i2); afunc( &i1, &i2 ); printf("in main()"); printf(" i1 == %d, i2 == %d\n",i1,i2); } void afunc(int *a, int *b) { printf("in afunc()"); printf(" a == %d, b == %d\n",*a,*b); *a = 7; *b = 6; printf("in afunc()"); printf(" a == %d, b == %d\n",*a,*b); }

F77 PROGRAM MAIN INTEGER I1, I2 I1 = 3 I2 = 4 WRITE(6,'(A,I4,I4)') 'in AFUNC() I1,I2: ',I1,I2 CALL AFUNC(I1, I2) WRITE(6,'(A,I4,I4)') 'in AFUNC() I1,I2: ',I1,I2 END SUBROUTINE AFUNC( A, B ) INTEGER A, B WRITE(6,'(A,I4,I4)') 'in AFUNC() A,B: ',A,B A = 7 B = 6 WRITE(6,'(A,I4,I4)') 'in AFUNC() A,B: ',A,B END

structures and defined types

C #include <stdio.h> typedef struct { float cx; float cy; float cz; int color; } t_point; void point_print( t_point ); main() { t_point point; point.cx=3.; point.cy=3.; point.cz=2.; point.color=10; point_print(point); } void point_print(t_point point) { printf(" cx == %f, cy == %f\n",point.cx,point.cy); printf(" cz == %f, color == %d\n",point.cz,point.color); }

F77 PROGRAM MAIN REAL POINT_COORD(3) INTEGER POINT_COLOR POINT_COORD(1) = 3. POINT_COORD(2) = 3. POINT_COORD(3) = 2. POINT_COLOR = 10 CALL POINT_PRINT( POINT_COORD, POINT_COLOR ) END SUBROUTINE POINT_PRINT( POINT_COORD, POINT_COLOR ) REAL POINT_COORD(3) INTEGER POINT_COLOR WRITE(6,'(A,3F12.4,I4)') 'COORD(1,2,3): ', & POINT_COORD(1), & POINT_COORD(2), & POINT_COORD(3), & POINT_COLOR END

malloc()

#include < stdio.h >

typedef struct { float cx;

float cy;

float cz;

int color; } t_point;

void point_print( t_point );

main() {

int nel;

t_point *points;

nel=10;

points = (t_point *) calloc(nel, sizeof(*points) );

for(i=0;i < nel;++i) {

point_print(points[i]);

}

}

void point_print(t_point point) {

printf(" cx == %f, cy == %f\n",point.cx,point.cy);

printf(" cz == %f, color == %d\n",point.cz,point.color);

}