Indexing into Arrays (Using the C Math Library)

Using the C Math Library

Comparison of C and MATLAB Indexing Syntax

The table below summarizes the differences between the MATLAB and C indexing syntax. Although the MATLAB C Math Library provides the same functionality as the MATLAB interpreter, the syntax is very different. Refer to Assumptions for the Code Examples to look up the conventions used for the code within the table.

Note For the examples in the table, matrix X is set to the 2-by-2 matrix
[ 4 5 ; 6 7 ], a different value from the 3-by-3 matrix A in the previous sections.

Example Matrix X

4 5
6 7

Table 5-2: MATLAB/C Indexing Expression Equivalence
Description
MATLAB Expression
C Expression
Result

Extract 1,1 element
X(1,1)
mlfIndexRef(
   X,
   "(?,?)",
   mlfScalar(1),
   mlfScalar(1)
   )
4

Extract first element
X(1)
mlfIndexRef(
   X,
   "(?)",
   mlfScalar(1)
   )
4

Extract third element
X(3)
mlfIndexRef(
   X,
   "(?)",
   mlfScalar(3)
   )
5

Extract all elements into column vector
X(:)
mlfIndexRef(
   X,
   "(?)",
   mlfCreateColonIndex()
   )
4
6
5
7

Extract first row
X(1,:)
mlfIndexRef(
   X,
   "(?,?)",
   mlfScalar(1),
   mlfCreateColonIndex()
   )
4 5

Extract second row
X(2,:)
mlfIndexRef(
   X,
   "(?,?)",
   mlfScalar(2),
   mlfCreateColonIndex()
   )
6 7

Extract first column
X(:,1)
mlfIndexRef(
   X,
   "(?,?)",
   mlfCreateColonIndex(),
   mlfScalar(1)
   )
4
6

Extract second column
X(:,2)
mlfIndexRef(
   X,
   "(?,?)",
   mlfCreateColonIndex(),
   mlfScalar(2)
   )
5
7

Replace first element
with 9
X(1) = 9
mlfIndexAssign(
   &X,
   "(?)",
   mlfScalar(1),
   mlfScalar(9)
   );
9 5
6 7

Replace first row with
[11 12]
X(1,:) = [ 11 12 ]
mlfIndexAssign(
   &X,
   "(?,?)",
   mlfScalar(1),
   mlfCreateColonIndex(),
   mlfHorzcat(
      mlfScalar(11),
      mlfScalar(12),
      NULL)
   );
11 12
6 7

Replace element 2,1 with 9
X(2,1) = 9
mlfIndexAssign(
   &X,
   "(?,?)",
   mlfScalar(2),
   mlfScalar(1),
   mlfScalar(9)
   );
4 5
9 7

Replace elements 1 and 4 with 8 (one-dimensional indexing)
X([1 4]) = [ 8 8 ]
mlfIndexAssign(
   &X,
   "(?)",
   mlfHorzcat(
      mlfScalar(1),
      mlfScalar(4),
      NULL),
   mlfHorzcat(
      mlfScalar(8),
      mlfScalar(8),
      NULL)
   );
8 5
6 8

**Table 5-2: MATLAB/C Indexing Expression Equivalence**
Description	MATLAB Expression	C Expression	Result
Extract `1,1` element	`X(1,1)`	`mlfIndexRef(` `X,` `"(?,?)",` `mlfScalar(1),` `mlfScalar(1)` `)`	`4`
Extract first element	`X(1)`	`mlfIndexRef(` `X,` `"(?)",` `mlfScalar(1)` `)`	`4`
Extract third element	`X(3)`	`mlfIndexRef(` `X,` `"(?)",` `mlfScalar(3)` `)`	`5`
Extract all elements into column vector	`X(:)`	`mlfIndexRef(` `X,` `"(?)",` `mlfCreateColonIndex()` `)`	`4` `6` `5` `7`
Extract first row	`X(1,:)`	`mlfIndexRef(` `X,` `"(?,?)",` `mlfScalar(1),` `mlfCreateColonIndex()` `)`	`4 5`
Extract second row	`X(2,:)`	`mlfIndexRef(` `X,` `"(?,?)",` `mlfScalar(2),` `mlfCreateColonIndex()` `)`	`6 7`
Extract first column	`X(:,1)`	`mlfIndexRef(` `X,` `"(?,?)",` `mlfCreateColonIndex(),` `mlfScalar(1)` `)`	`4` `6`
Extract second column	`X(:,2)`	`mlfIndexRef(` `X,` `"(?,?)",` `mlfCreateColonIndex(),` `mlfScalar(2)` `)`	`5` `7`
Replace first element with `9`	`X(1) = 9`	`mlfIndexAssign(` `&X,` `"(?)",` `mlfScalar(1),` `mlfScalar(9)` `);`	`9 5` `6 7`
Replace first row with `[11 12]`	`X(1,:) = [ 11 12 ]`	`mlfIndexAssign(` `&X,` `"(?,?)",` `mlfScalar(1),` `mlfCreateColonIndex(),` `mlfHorzcat(` `mlfScalar(11),` `mlfScalar(12),` `NULL)` `);`	`11 12` `6 7`
Replace element `2,1` with `9`	`X(2,1) = 9`	`mlfIndexAssign(` `&X,` `"(?,?)",` `mlfScalar(2),` `mlfScalar(1),` `mlfScalar(9)` `);`	`4 5` `9 7`
Replace elements `1` and `4` with `8` (one-dimensional indexing)	`X([1 4]) = [ 8 8 ]`	`mlfIndexAssign(` `&X,` `"(?)",` `mlfHorzcat(` `mlfScalar(1),` `mlfScalar(4),` `NULL),` `mlfHorzcat(` `mlfScalar(8),` `mlfScalar(8),` `NULL)` `);`	`8 5` `6 8`

Deleting Elements from a Structure Array Calling Library Routines