# Field Projection

rascal-0.34.0

#### Synopsis​

Projection of tuple.

#### Syntax​

Exp < Field₁, Field₂ ... >

#### Description​

Exp should evaluate to a tuple or relation, and Fieldᵢ should be a field name or an integer constant that refers to elements in the order in which they occur in the original value (counting from 0).

#### Examples​

Suppose we have a relation with traffic information that records the name of the day, the day number, and the length of the traffic jams at that day.

rascal>rel[str day, int daynum, int length] Traffic =
>>>>>>>{<"mon", 1, 100>, <"tue", 2, 150>, <"wed", 3, 125>,
>>>>>>> <"thur", 4, 110>, <"fri", 5, 90>};
rel[str day,int daynum,int length]: {
<"thur",4,110>,
<"mon",1,100>,
<"fri",5,90>,
<"wed",3,125>,
<"tue",2,150>
}
rascal>Traffic<length,daynum>;
rel[int length,int daynum]: {
<110,4>,
<125,3>,
<90,5>,
<150,2>,
<100,1>
}
rascal>Traffic<2,day>;
rel[int length,str day]: {
<110,"thur">,
<125,"wed">,
<90,"fri">,
<150,"tue">,
<100,"mon">
}

Field projection thus selects parts from a larger value that has a fixed number of parts. The selection is based on position and not on value and can be used to completely reorder or remove the parts of a larger value.