R – OCaml – For Loop inside If Statement

for-loopif statementocaml

Coming from a Java and C background, grasping some concepts of Ocaml has been quite interesting. One such is, getting a/multiple statement to run inside a for loop.

let test_method (x:vector list)(vec:vector) = 
    if List.length x != 0 then
          {a=0.;b=0.} (* Return a vector of 0,0 *)
       else 
          for i = 0 to List.length x do
                        let key =  {a=(List.nth x i).a;b=(List.nth x i).b} 
                done;;

the error that i get is done;; is an unexpected token. Why is this happening? this happens even if i do this

let test_method (x:vector list)(vec:vector) = 
    if List.length x != 0 then
          {a=0.;b=0.} (* Return a vector of 0,0 *)
       else 
          for i = 0 to List.length x do
                        let key =  {a=(List.nth x i).a;b=(List.nth x i).b} in
                        let value = vec_neg key;
                done;;

where vec_neg is a method that works fine that negates the vector type.

Any help would be greatly appreciated.

Best Answer

let expressions must have a body. (i.e. let var = val in body) (except let definitions at the top level of a module, which implicitly use the rest of the module as the body) let creates a local binding which is in scope inside the body. What's the point of doing that if you're not using it (i.e. don't have a body)? Plus every expression in the language has to evaluate to a value. A let expression evaluates to whatever the body evaluates to. That's why it needs a body.

Related Solutions

Javascript – Why is using “for…in” for array iteration a bad idea

The reason is that one construct:

var a = []; // Create a new empty array.
a[5] = 5;   // Perfectly legal JavaScript that resizes the array.

for (var i = 0; i < a.length; i++) {
    // Iterate over numeric indexes from 0 to 5, as everyone expects.
    console.log(a[i]);
}

/* Will display:
   undefined
   undefined
   undefined
   undefined
   undefined
   5
*/

can sometimes be totally different from the other:

var a = [];
a[5] = 5;
for (var x in a) {
    // Shows only the explicitly set index of "5", and ignores 0-4
    console.log(x);
}

/* Will display:
   5
*/

Also consider that JavaScript libraries might do things like this, which will affect any array you create:

// Somewhere deep in your JavaScript library...
Array.prototype.foo = 1;

// Now you have no idea what the below code will do.
var a = [1, 2, 3, 4, 5];
for (var x in a){
    // Now foo is a part of EVERY array and 
    // will show up here as a value of 'x'.
    console.log(x);
}

/* Will display:
   0
   1
   2
   3
   4
   foo
*/

Javascript – How to loop through or enumerate a JavaScript object

You can use the for-in loop as shown by others. However, you also have to make sure that the key you get is an actual property of an object, and doesn't come from the prototype.

Here is the snippet:

var p = {
    "p1": "value1",
    "p2": "value2",
    "p3": "value3"
};

for (var key in p) {
    if (p.hasOwnProperty(key)) {
        console.log(key + " -> " + p[key]);
    }
}

For-of with Object.keys() alternative:

var p = {
    0: "value1",
    "b": "value2",
    key: "value3"
};

for (var key of Object.keys(p)) {
    console.log(key + " -> " + p[key])
}

Notice the use of for-of instead of for-in, if not used it will return undefined on named properties, and Object.keys() ensures the use of only the object's own properties without the whole prototype-chain properties

Using the new Object.entries() method:

Note: This method is not supported natively by Internet Explorer. You may consider using a Polyfill for older browsers.

const p = {
    "p1": "value1",
    "p2": "value2",
    "p3": "value3"
};

for (let [key, value] of Object.entries(p)) {
  console.log(`${key}: ${value}`);
}

Best Answer

Related Solutions

Javascript – Why is using “for…in” for array iteration a bad idea

Javascript – How to loop through or enumerate a JavaScript object

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