Python:How to create a constraint which includes a variable and a kind of index(two indices simultaneous)

Answered

Ching Chang

July 24, 2020 07:15

I would like to write the constraint code as the content of picture.

#index

J=['j1','j2','j3','j4','j5','j6','j7','j8','j9','j10','j11','j12','j13','j14','j15','j16','j17','j18','j19','j20', 'j21']

#variable of decision

F= m.addVars(J , vtype=GRB.BINARY,name="Fj")

#code

b=len(J)-1
c=0
for j in J:
c+=1
if c<=b:
m.addConstrs((F[j+1]<=F[j]) ,'c13')

#error report

I also have tried to create a letter to instead "j+1" but obviously the code shouldn't have two kinds of index.

HELP ME PLEASE

0

Comments

4 comments

Official comment
Simranjit Kaur

Gurobi Staff

June 23, 2025 03:09 Edited
This post is more than three years old. Some information may not be up to date. For current information, please check the Gurobi Documentation or Knowledge Base. If you need more help, please create a new post in the community forum. Or why not try our AI Gurobot?.
Jaromił Najman

Gurobi Staff

July 24, 2020 08:50 Edited
Hi,

you could use integers as indices, i.e.,

#index
```
J=[int(i)+1 for i in range(21)]
```
#code
```
b=len(J)-1
c=0
for j in J:
    c+=1
    if c<=b:
        m.addConstr((F[j+1]<=F[j]) ,'c'+str(j)
```
Please note that you have to use addConstr instead of addConstrs.

Best regards,
Jaromił
1
Ching Chang

July 27, 2020 02:46 Edited
Dear Jaromił

Thank you for solving my problem.

B.R.
1
Jaromił Najman

Gurobi Staff

July 27, 2020 07:02
Hi,

since you are using Python, you could make your code even shorter
```
J = range(1,22)
m.addConstrs( (F[j+1] <= F[j] for j in J[:-1]), name = 'c' )
```
Best regards,
Jaromił
1

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