Difference between Expression and Statement in C

/* 
 * diff_regptr_and_ptr2const1.c -- Program shows what happens when regular
 * pointer is assigned addresses of non-constant and constant data 
 */
#include <stdio.h>
 
int main(void)
{
    float pass_per = 60.9;        /* pass_per is non-constant float */
    const float fare = 31.35;     /* fare is constant float */
    float *fp = &pass_per;        /* fp, regular pointer to float */
 
    printf("\nIn the exp. \"float *fp = &pass_per\":\n");
    printf("fp is a regular pointer to float pass_per, value of "
               "pass_per is %f\n", *fp);
    printf("Now, we try modify pass_per using regular pointer fp...\n");
 
    *fp = 70.9;
 
    printf("O key, now, in exp. \"*fp = 70.9\", pass_per is %f\n", *fp);
    printf("\nNow, let's see what happens when fp is assigned address of"
           " some \"constant data\"\n");
 
    fp = &fare;	        /* assigned fp address of constant fare */
 
    printf("\nIn the exp. \"fp = &fare\":\n");
    printf("fp is a regular pointer to constant float \"fare\", value of"
           " fare is %f\n", *fp);
    printf("Now, we try to modify constant fare using fp...\n");
 
    *fp = 62.70;
 
    printf("O key, now, in the exp. \"*fp = 62.70\", value of constant "
           "data fare is %f\n\n", *fp);
 
    return 0;
}

test1.c: In function ‘main’:
test1.c:20:8: warning: assignment discards ‘const’ qualifier from pointer 
target type [enabled by default]
[root@localhost ch06_Pointers]# ./a.out 
 
In the exp. "float *fp = &pass_per":
fp is a regular pointer to float pass_per, value of pass_per is 60.900002
Now, we try modify pass_per using regular pointer fp...
O key, now, in exp. "*fp = 70.9", pass_per is 70.900002
 
Now, let us see what happens when fp is assigned address of some "constant 
data"
 
In the exp. "fp = &fare":
fp is a regular pointer to constant float "fare", value of fare is 31.350000
Now, we try to modify constant fare using fp...
O key, now, in the exp. "*fp = 62.70", value of constant data fare is 
62.700001

/* 
 * diff_regptr_and_ptr2const2.c -- Program shows what happens when pointer
 * to const is assigned address of non-constant and constant data
 */
#include <stdio.h>
 
int main(void)
{
    int holidays = 69;            /* non-constant holidays */
    const int teachers = 98;
    /* teachers declared and initialized to be constant */
 
    const int *cip = &holidays;
    /* cip is a pointer-to-constant-integer */
 
    printf("\nFor Non-Constant Integer \"holidays\":\n");
    printf("In exp. \"const int *cip = &holidays\", no of holidays in 2014"
           " %d\n", *cip);
 
    printf("\nO key, now we try to modify non- constant integer "
           "\"holidays\" \nusing pointer to constant integer \"cip\"\n");
    *cip = 100;                   /* modifying holidays using cip */
 
    printf("In exp. \"*cip = 100\", we tried to modify no of holidays in "
           "2014 from 69 to %d\n", *cip);
 
    cip = &teachers;
    /* assigning pointer-to-constant-int address of const data 'teachers' */
 
    printf("\nFor Constant Integer \"teachers\":\n");
    printf("In exp. \"cip = &teachers\", there are %d teachers in a school."
           "\n", *cip);
 
    printf("\nO key, now we try to modify constant integer \"teachers\"\n"
           "using pointer-to-constant-integer \"cip\"\n");
    *cip = 100;                   /* modifying teachers using cip */
 
    printf("In exp. \"*cip = 100\", we tried to modify no of teachers in a "
           "school from 98 to %d\n", *cip);
 
    return 0;
}

[root@localhost ch06_Pointers]# gcc diff_regptr_and_ptr2const2.c 
diff_regptr_and_ptr2const2.c: In function ‘main’:
diff_regptr_and_ptr2const2.c:14:2: error: assignment of read-only location 
‘*cip’
diff_regptr_and_ptr2const2.c:24:2: error: assignment of read-only location 
‘*cip’

/* adv_ptr2const.c -- Program displays advantages of pointer-to-constant */
#include <stdio.h>
#define FAM_MEM 5
#define SUB 5
void modify_const_data(const float *, int);            /* fun. prototype */
 
int main(void)
{
    const float expenses[FAM_MEM] = {360.00, 437.12, 45.65, 338.34, 98.987};
                                             /* array of constant floats */
    float marks[SUB] = {34.03, 56.98, 66.78, 68.0, 80.0};    
                                          /*array of non-constant floats */
 
    modify_const_data(marks, SUB);
    modify_const_data(expenses, FAM_MEM);
 
    return 0;
}
 
void modify_const_data(const float *base, int val)          /* fun. def. */
{
    int i, FLAG = 1;
    const float *copy;
 
    for (i = 0; i < val; i++) {
        copy = base++;
        if (FLAG) {
            printf("Now, we try to modify elements of array...\n");
            FLAG = 0;
        }
        *copy += 10.0;  /* increment each array element by 10.0 */
    }
}

[root@localhost ch06_Pointers]# gcc adv_ptr2const.c 
adv_ptr2const.c: In function ‘modify_const_data’:
adv_ptr2const.c:30:9: error: assignment of read-only location ‘*copy’