- (void) setField: (NSInteger) fieldno asString: (NSString*) string asDouble: (double) value;

Function setField() puts values to a field in memory before writing a record to a disc.
There are different versions of setField: you can pass a double value, a string or both. In last case setField will use a value which is more suitable for this type of field.

    // your code
    [dbf clearRecord];
    [dbf setField:0 asString:@"string value"];
    [dbf setField:1 asDouble:123.45];
    [dbf setField:2 asString:@"the string" asDouble:666.66];
    [dbf writeRecord:0];
    // your code