Update diff tests.

Author: zoziha

CHANGELOG.md

@@ -22,6 +22,7 @@ Features available from the latest git source
   - new procedures `arg`, `argd` and `argpi`
     [#498](https://github.com/fortran-lang/stdlib/pull/498)
   - new procedure `diff`
+    [#605](https://github.com/fortran-lang/stdlib/pull/605)
 
 Changes to existing modules
 

src/stdlib_math_diff.fypp

@@ -19,6 +19,11 @@ contains
         integer :: n_, i
 
         n_ = optval(n, 1)
+        
+        if (size(X) <= n_) then
+            allocate(Y(0))
+            return
+        end if
 
         Y = X
         do i = 1, n_
@@ -44,6 +49,11 @@ contains
             dim_ = 1
         end if
 
+        if (size(X, dim_) <= n_) then
+            allocate(Y(0, 0))
+            return
+        end if
+        
         Y = X
         if (dim_ == 1) then
             do i = 1, n_

src/tests/math/test_stdlib_math.fypp

@@ -378,6 +378,7 @@ contains
         type(error_type), allocatable, intent(out) :: error
         ${t1}$ :: x(6) = [${t1}$ :: 0, 5, 15, 30, 50, 75]
         ${t1}$ :: A(1, 3) = reshape([${t1}$ :: 1, 3, 5], [1, 3])
+        ${t1}$ :: B(2) = [${t1}$ :: 1, 2]
 
         call check(error, all_close(diff(x), [${t1}$ :: 5, 10, 15, 20, 25]), &
             "diff(x) in test_diff_real_${k1}$ failed")
@@ -391,6 +392,10 @@ contains
             "diff(x, n=1, dim=2) in test_diff_real_${k1}$ failed")
         if (allocated(error)) return
 
+        call check(error, size(diff(B, 2)), 0, "size(diff(B, 2)) in test_diff_real_${k1}$ failed")
+        if (allocated(error)) return
+        call check(error, size(diff(B, 3)), 0, "size(diff(B, 3)) in test_diff_real_${k1}$ failed")
+        
     end subroutine test_diff_real_${k1}$
     #:endfor
 
@@ -399,6 +404,7 @@ contains
         type(error_type), allocatable, intent(out) :: error
         ${t1}$ :: x(6) = [${t1}$ :: 0, 5, 15, 30, 50, 75]
         ${t1}$ :: A(1, 3) = reshape([${t1}$ :: 1, 3, 5], [1, 3])
+        ${t1}$ :: B(2) = [${t1}$ :: 1, 2]
 
         call check(error, all(diff(x) == [${t1}$ :: 5, 10, 15, 20, 25]), &
             "diff(x) in test_diff_int_${k1}$ failed")
@@ -412,6 +418,10 @@ contains
             "diff(A, n=1, dim=2) in test_diff_int_${k1}$ failed")
         if (allocated(error)) return
 
+        call check(error, size(diff(B, 2)), 0, "size(diff(B, 2)) in test_diff_real_${k1}$ failed")
+        if (allocated(error)) return
+        call check(error, size(diff(B, 3)), 0, "size(diff(B, 3)) in test_diff_real_${k1}$ failed")
+        
     end subroutine test_diff_int_${k1}$
     #:endfor