ch2-Array precedence

Chapter_2     Exercise_2-10     Arguments_precedence

ch2-Exercises     Chapter_3

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arrp.c     K&R, p. 53         download

#include <stdio.h> // for printf(), putchar()
// array precedence
int main()
{
int i, a[10];

for (i = 0; i < 10; i++)
{a[i] = -10;} // initialize a[]
i = -1;
while (i < 9)
{
a[i] = ++i;
}
printf("a[i] = ++i, i = -1..8\n");
for (i = 0; i < 10; i++)
{printf("a[%d] = %d,%c", i, a[i], (i % 5 == 4) ? '\n' : ' ');}
putchar('\n');

for (i = 0; i < 10; i++)
{a[i] = -10;} // reset a[]
i = -1;
while (i < 9)
{
a[i] = i++;
}
printf("a[i] = i++, i = -1..8\n");
for (i = 0; i < 10; i++)
{printf("a[%d] = %d,%c", i, a[i], (i % 5 == 4) ? '\n' : ' ');}
putchar('\n');

for (i = 0; i < 10; i++)
{a[i] = -10;} // reset a[]
i = -1;
while (i < 9)
{
a[++i] = i;
}
printf("a[++i] = i, i = -1..8\n");
for (i = 0; i < 10; i++)
{printf("a[%d] = %d,%c", i, a[i], (i % 5 == 4) ? '\n' : ' ');}
putchar('\n');

for (i = 0; i < 10; i++)
{a[i] = -10;} // reset a[]
i = 0;
while (i < 10)
{
a[i++] = i;
}
printf("a[i++] = i, i = 0..9\n");
for (i = 0; i < 10; i++)
{printf("a[%d] = %d,%c", i, a[i], (i % 5 == 4) ? '\n' : ' ');}
putchar('\n');

for (i = 0; i < 10; i++)
{a[i] = -10;} // reset a[]
i = -2;
while (i < 8)
{
a[++i] = ++i, a[i+1] = i+1; // test value of i
}
printf("a[++i] = ++i, a[i+1] = i+1, i = -2..6 (step 2)\n");
for (i = 0; i < 10; i++)
{printf("a[%d] = %d,%c", i, a[i], (i % 5 == 4) ? '\n' : ' ');}
putchar('\n');

for (i = 0; i < 10; i++)
{a[i] = -10;} // reset a[]
i = -2;
while (i < 8)
{
a[++i] = i++, a[i+1] = i+1; // test value of i
}
printf("a[++i] = i++, a[i+1] = i+1, i = -2..6 (step 2)\n");
for (i = 0; i < 10; i++)
{printf("a[%d] = %d,%c", i, a[i], (i % 5 == 4) ? '\n' : ' ');}
putchar('\n');

for (i = 0; i < 10; i++)
{a[i] = -10;} // reset a[]
i = -1;
while (i < 9)
{
a[i++] = ++i, a[i] = i+1; // test value of i
}
printf("a[i++] = ++i, a[i] = i+1, i = -1..7 (step 2)\n");
for (i = 0; i < 10; i++)
{printf("a[%d] = %d,%c", i, a[i], (i % 5 == 4) ? '\n' : ' ');}
putchar('\n');

for (i = 0; i < 10; i++)
{a[i] = -10;} // reset a[]
i = -1;
while (i < 9)
{
a[i++] = i++, a[i] = i+1; // test value of i
}
printf("a[i++] = i++, a[i] = i+1, i = -1..7 (step 2)\n");
for (i = 0; i < 10; i++)
{printf("a[%d] = %d,%c", i, a[i], (i % 5 == 4) ? '\n' : ' ');}

return 0;
}
/*
gcc arrp.c -o arrp
./arrp
a[i] = ++i, i = -1..8
a[0] = 0, a[1] = 1, a[2] = 2, a[3] = 3, a[4] = 4,
a[5] = 5, a[6] = 6, a[7] = 7, a[8] = 8, a[9] = 9,
// ++i is executed before the assignment and before the subscript is chosen

a[i] = i++, i = -1..8
a[0] = -1, a[1] = 0, a[2] = 1, a[3] = 2, a[4] = 3,
a[5] = 4, a[6] = 5, a[7] = 6, a[8] = 7, a[9] = 8,
// i++ is executed after the assignment, but before the subscript is chosen

a[++i] = i, i = -1..8
a[0] = 0, a[1] = 1, a[2] = 2, a[3] = 3, a[4] = 4,
a[5] = 5, a[6] = 6, a[7] = 7, a[8] = 8, a[9] = 9,
// ++i is executed before the subscript is chosen and before the assignment

a[i++] = i, i = 0..9
a[0] = 1, a[1] = 2, a[2] = 3, a[3] = 4, a[4] = 5,
a[5] = 6, a[6] = 7, a[7] = 8, a[8] = 9, a[9] = 10,
// i++ is executed after the subscript is chosen, but before the assignment

a[++i] = ++i, a[i+1] = i+1, i = -2..6 (step 2)
a[0] = 0, a[1] = 1, a[2] = 2, a[3] = 3, a[4] = 4,
a[5] = 5, a[6] = 6, a[7] = 7, a[8] = 8, a[9] = 9,
// both ++i before both operations

a[++i] = i++, a[i+1] = i+1, i = -2..6 (step 2)
a[0] = -2, a[1] = 1, a[2] = 0, a[3] = 3, a[4] = 2,
a[5] = 5, a[6] = 4, a[7] = 7, a[8] = 6, a[9] = 9,
// ++i before subscript, before assignment (but after i++), so after assignment,
// i++ before subscript, after assignment

a[i++] = ++i, a[i] = i+1, i = -1..7 (step 2)
a[0] = 1, a[1] = 2, a[2] = 3, a[3] = 4, a[4] = 5,
a[5] = 6, a[6] = 7, a[7] = 8, a[8] = 9, a[9] = 10,
// i++ after subscript, before assignment, ++i before both operations

a[i++] = i++, a[i] = i+1, i = -1..7 (step 2)
a[0] = -1, a[1] = 2, a[2] = 1, a[3] = 4, a[4] = 3,
a[5] = 6, a[6] = 5, a[7] = 8, a[8] = 7, a[9] = 10,
// first i++ after subscript, before assignment (but after i++), so after assignment,
// second i++ before subscript, after assignment
*/

Notes:

Within while() we have to set all values of a[], so we have to use proper indices and assignments.
For one incrementation (the first 4 cases), i varies from -1 to 8 or from 0 to 9 (10 values), corresponding to a[0]..a[9].
For two incrementations (the last 4 cases) we need an extra assignment because i is increased by 2 at each step. Here i varies from -2 to 6 (step 2) or from -1 to 7 (step 2), 5 values in total, corresponding to a[0], a[2], a[4], a[6], a[8]. For the other 5 values, we use the extra assignment a[i+1] = i+1 or a[i] = i+1, depending on the value of i.
After each case, we reset a[] to values -10 to be sure the next case sets all the values of a[] (unset values will display -10).

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Chapter_2     Exercise_2-10     Args_prec

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