Just another update

Task/Resistor-mesh/Python/resistor-mesh-2.py

@@ -0,0 +1,71 @@
+import sys, copy
+from fractions import Fraction
+
+def gauss(a, b):
+    n, p = len(a), len(a[0])
+    for i in range(n):
+        t = abs(a[i][i])
+        k = i
+        for j in range(i + 1, n):
+            if abs(a[j][i]) > t:
+                t = abs(a[j][i])
+                k = j
+        if k != i:
+            for j in range(i, n):
+                a[i][j], a[k][j] = a[k][j], a[i][j]
+            b[i], b[k] = b[k], b[i]
+        t = 1/a[i][i]
+        for j in range(i + 1, n):
+            a[i][j] *= t
+        b[i] *= t
+        for j in range(i + 1, n):
+            t = a[j][i]
+            for k in range(i + 1, n):
+                a[j][k] -= t*a[i][k]
+            b[j] -= t * b[i]
+    for i in range(n - 1, -1, -1):
+        for j in range(i):
+            b[j] -= a[j][i]*b[i]
+    return b
+
+def resistor_grid(p, q, ai, aj, bi, bj):
+    n = p*q
+    I = Fraction(1, 1)
+    v = [0*I]*n
+    a = [copy.copy(v) for i in range(n)]
+    for i in range(p):
+        for j in range(q):
+            k = i*q + j
+            if i == ai and j == aj:
+                a[k][k] = I
+            else:
+                c = 0
+                if i + 1 < p:
+                    c += 1
+                    a[k][k + q] = -1
+                if i >= 1:
+                    c += 1
+                    a[k][k - q] = -1
+                if j + 1 < q:
+                    c += 1
+                    a[k][k + 1] = -1
+                if j >= 1:
+                    c += 1
+                    a[k][k - 1] = -1
+                a[k][k] = c*I
+    b = [0*I]*n
+    k = bi*q + bj
+    b[k] = 1
+    return gauss(a, b)[k]
+
+def main(arg):
+    r = resistor_grid(int(arg[0]), int(arg[1]), int(arg[2]), int(arg[3]), int(arg[4]), int(arg[5]))
+    print(r)
+    print(float(r))
+
+main(sys.argv[1:])
+
+# Output:
+# python grid.py 10 10 1 1 7 6
+# 455859137025721/283319837425200
+# 1.6089912417307297