Will if-else statements nest without brackets? - c++

I want to write something utterly ridiculous that calls for a great depth of conditional nesting. The least disorienting way to write this is to forgo brackets entirely, but I have not been able to find any info on if nesting single-statement if-else guards is legal; the non-nested version causes people enough problems it seems.
Is it valid to write the following? (In both C and C++, please let me know if they differ on this.)
float x = max(abs(min), abs(max));
uint32 count = 0u;
// divides and conquers but, tries to shortcut toward more common values
if (x < 100'000.f)
if (x < 10.f)
count = 1u;
else
if(x < 1'000.f)
if (x < 100.f)
count = 2u;
else
count = 3u;
else
if (x < 10'000.f)
count = 4u;
else
count = 5u;
else
... // covers the IEEE-754 float32 range to ~1.0e+37 (maybe 37 end branches)
--skippable lore--
The underlying puzzle (this is for fun) is that I want to figure out the number of glyphs necessary to display a float's internal representation without rounding/truncation, in constant time. Counting the fractional part's glyph count in constant time was much neater/faster, but unfortunately I wasn't able to figure out any bit-twiddling tricks for the integer part, so I've decided to just brute-force it. Never use math when you can use your fists.

From cppreference.com:
in nested if-statements, the else is associated with the closest if that doesn't have an else
So as long as every if has an else, nesting without brackets works fine. The problem occurs when an else should not be associated with the closest if. For example:
if ( condition1 ) {
if ( condition2 )
DoSomething();
} // <-- This is needed so the else goes with the intended if.
else
DoOtherThing();
A quick scan of your code looks like it's fine.

Related

If else condition precedence in Verilog

I have noticed that there is a precedence of assignment while using if-else conditionals in Verilog. For example as in the code below:
if(counter < 6)
z <= 1;
else if(counter < 12)
z <= 2;
else
z <= 3;
I noticed that until the counter is less than 6, the value of z is assigned the value of 1 (z <= 1) and as soon as the value of the counter exceeds 6 and is less than 12, z is assigned the value 2 (z <= 2).
What if there are different variables used inside the conditionals as in the code below?
if(wire1_is_enabled)
z <= 1;
else if(wire2_is_enabled)
z <= 0;
What happens when both conditions are true? What is the behaviour of the assignment operator here?
I believe this is poor programming habit.
Yes, nested if-else branching statements naturally assume a priority in the order of execution. Think about using a case statement instead of deeply nesting if statements which are much more readable.
There is nothing wrong with this coding style unless you have code like:
if(counter < 6)
z <= 1;
else if(counter < 2)
z <= 2; // unreachable
else
z <= 3;
Then the statement z <= 2; becomes unreachable because when the first condition is false, the second condition can never be true. Fortunately there are a number of tools that can flag this problem for you.
Both if and case statements assume priority logic. However, you have an option to explicitly add a priority or unique keyword before the if or case keywords to declare and check your intent. See sections 12.4 and 12.5 in the IEEE 1800-2017 SystemVerilog LRM.
The 2 if/else statements behave the same way; the first condition to be true has the highest priority. Once a condition evaluates to true, all the following else clauses are ignored. Therefore, z <= 1 if both wire1_is_enabled and wire2_is_enabled are true. This is easy to prove to yourself with a simple simulation.
This is not a poor coding habit. This situation is common in Verilog. When you say programming, perhaps you are thinking of software languages. Keep in mind that these are hardware signals instead of software variables.

Can I use return to return to the start of an if statement?

I'm currently programming some homework, and we have to make a program that turns a hindu-arabic numeral into a roman numeral. I've already made the first part, where my teacher said we had to make sure the number is in between 1 and 3999. My algorithm so far is like this:
if (num-1000) > 0 {
add M to output
num -= 1000
return if
}
else {
(repeat for other digits, aka 500, 100, 50, and so on)
}
The problem is, I don't know if it's even possible. All the Stack Overflow pages I've seen say that I should use while statements for this, but since we haven't tackled while statements yet (even though I've self-learned it) we can't use while loops. So, can I use return to return to the start of an if statement?
What you are describing is a while. You can also achieve that with a go to but using that at this stage of learning would inspire some very very bad habits so I wholeheartedly discourage it. Another way you could do this is with recursion but that is an even more advance topic.
Given your restrictions (no loops, a number between 1 and 3999) I think you are supposed to use a bunch of ifs. Something like this pseudocode:
if (n >= 3000)
add 'M'
else if (n >= 2000)
add 'MM'
else if (n >= 1000)
add 'MMM'
n = n % 1000;
if (n >= 900)
add 'CM'
// and so on

Multiply numbers which are divisible by 3 and less than 10 with a while loop in c++?

In C++, I should write a program where the app detects which numbers are divisible by 3 from 1 till 10 and then multiply all of them and print the result. That means that I should multiply 3,6,9 and print only the result, which is 162, but I should do it by using a "While" loop, not just multiplying the 3 numbers with each other. How should I write the code of this? I attached my attempt to code the problem below. Thanks
#include <iostream>
using namespace std;
int main() {
int x, r;
int l;
x = 1;
r = 0;
while (x < 10 && x%3==0) {
r = (3 * x) + 3;
cout << r;
}
cin >> l;
}
Firstly your checking the condition x%3 == 0 brings you out of your while - loop right in the first iteration where x is 1. You need to check the condition inside the loop.
Since you wish to store your answer in variable r you must initialize it to 1 since the product of anything with 0 would give you 0.
Another important thing is you need to increment the value of x at each iteration i.e. to check if each number in the range of 1 to 10 is divisible by 3 or not .
int main()
{
int x, r;
int l;
x = 1;
r = 1;
while (x < 10)
{
if(x%3 == 0)
r = r*x ;
x = x + 1; //incrementing the value of x
}
cout<<r;
}
Lastly I have no idea why you have written the last cin>>l statement . Omit it if not required.
Ok so here are a few hints that hopefully help you solving this:
Your approach with two variables (x and r) outside the loop is a good starting point for this.
Like I wrote in the comments you should use *= instead of your formula (I still don't understand how it is related to the problem)
Don't check if x is dividable by 3 inside the while-check because it would lead to an too early breaking of the loop
You can delete your l variable because it has no affect at the moment ;)
Your output should also happen outside the loop, else it is done everytime the loop runs (in your case this would be 10 times)
I hope I can help ;)
EDIT: Forget about No.4. I didn't saw your comment about the non-closing console.
int main()
{
int result = 1; // "result" is better than "r"
for (int x=1; x < 10; ++x)
{
if (x%3 == 0)
result = result * x;
}
cout << result;
}
or the loop in short with some additional knowledge:
for (int x=3; x < 10; x += 3) // i know that 3 is dividable
result *= x;
or, as it is c++, and for learning purposes, you could do:
vector<int> values; // a container holding integers that will get the multiples of 3
for (int x=1; x < 10; ++x) // as usual
if ( ! x%3 ) // same as x%3 == 0
values.push_back(x); // put the newly found number in the container
// now use a function that multiplies all numbers of the container (1 is start value)
result = std::accumulate(values.begin(), values.end(), 1, multiplies<int>());
// so much fun, also get the sum (0 is the start value, no function needed as add is standard)
int sum = std::accumulate(values.begin(), values.end(), 0);
It's important to remember the difference between = and ==. = sets something to a value while == compares something to a value. You're on the right track with incrementing x and using x as a condition to check your range of numbers. When writing code I usually try and write a "pseudocode" in English to organize my steps and get my logic down. It's also wise to consider using variables that tell you what they are as opposed to just random letters. Imagine if you were coding a game and you just had letters as variables; it would be impossible to remember what is what. When you are first learning to code this really helps a lot. So with that in mind:
/*
- While x is less than 10
- check value to see if it's mod 3
- if it's mod 3 add it to a sum
- if not's mod 3 bump a counter
- After my condition is met
- print to screen pause screen
*/
Now if we flesh out that pseudocode a little more we'll get a skeletal structure.
int main()
{
int x=1//value we'll use as a counter
int sum=0//value we'll use as a sum to print out at the end
while(x<10)//condition we'll check against
{
if (x mod 3 is zero)
{
sum=x*1;
increment x
}
else
{
increment x
}
}
//screen output the sum the sum
//system pause or cin.get() use whatever your teacher gave you.
I've given you a lot to work with here you should be able to figure out what you need from this. Computer Science and programming is hard and will require a lot of work. It's important to develop good coding habits and form now as it will help you in the future. Coding is a skill like welding; the more you do it the better you'll get. I often refer to it as the "Blue Collar Science" because it's really a skillset and not just raw knowledge. It's not like studying history or Biology (minus Biology labs) because those require you to learn things and loosely apply them whereas programming requires you to actually build something. It's like welding or plumbing in my opinion.
Additionally when you come to sites like these try and read up how things should be posted and try and seek the "logic" behind the answer and come up with it on your own as opposed to asking for the answer. People will be more inclined to help you if they think you're working for something instead of asking for a handout (not saying you are, just some advice). Additionally take the attitude these guys give you with a grain of salt, Computer Scientists aren't known to be the worlds most personable people. =) Good luck.

Range checking for my remove function in c++

So I have a method in c++ that takes an array and removes a certain number of values in the array. The method removes the range of values from the starting value all the way up to but not including the end value. void dynamic_array::remove(int start, int end) {
The only problem I'm having is with the range checking. So I've set up a way to check to make sure the start and end values are not in the incorrect places however whenever I test the code, it appears that it doesn't catch the range exception. Here's the code that's supposed to check the exception:
if (not (0 <= ((start <= (end < size))))){
throw exception(SUBSCRIPT_RANGE_EXCEPTION);
}
you cannot use the notation 1 < x < 2 in c++ (or most languages). So you have to do each comparison separately. ie. (1<x) && (x<2) (brackets not really necessary here).
If you are interested, you actually can use the notation, but it means something different than you might think. It means that you first compare 1<x which gives either true (1) or zero(0) and then you compare this 1 or 0 with two.
It should be written
if(!(0 <= start && start <= end && end < size)){
throw exception
}
As i know, C++ can't understand the way you write it.
C++ does not work this way. The result of a single logical comparison is a boolean value. For example, the first comparison:
end < size
If this comparison is true, the result becomes a true value, which is for all practical purposes is 1. So, your expression now becomes, for all practical purposes:
if (not (0 <= ((start <= 1)))){
Which is already pretty much nonsensical, not to mention that there isn't a not operator in C++. Things pretty much roll downhill, from that point on.
You just need to make two logical comparisons: start < end, and end <= size. If you spend a few moments to think about it, you would realize this is all you need:
if (!(start < end && end <= size))

Generate Block Compile Time If-Else Parameterized

I'm looking to be able to parametric some behavioral level Verilog using the generate block. The module is for a re-configurable readout and FIFO block, mainly so we can code this up one and just use a parameter at the top level.
Lets say we have:
always #(posedge write_out_clk or posedge RESETN)
begin
if (RESETN)
SENSE_ADDR <= 0;
else if (enb[0] == 1)
SENSE_ADDR <= 1; // for example but may be some other wire/bus etc
else if (enb[1] == 2)
SENSE_ADDR <= 1; // for example but may be some other wire/bus etc
else
SENSE_ADDR <= SENSE_ADDR;
end
end
This is behavioral so the specifics of implementation are left to the compiler with user given timing constraints etc. This works for 'n' else-if statements within the block if I hard code them, currently synthesis and simulation are both working for 16 statements.
My question however is how to parameterise this using generate? Clearly if 'n=8' its not too much of a big deal to hard code it. What if 'n=64' or 'n=128' etc. Seems a shame to hard code it if the rest of the module is fully parameterized using the generate for 'n'...
I have tried doing something like:
genvar elseif_generate;
generate
for (elseif_generate=0; elseif_generate<FIFO_SUB_BLOCKS; elseif_generate=elseif_generate+1)
begin: elseif_generate_logic
always #(posedge write_out_clk or posedge RESETN)
begin
if (RESETN)
SENSE_ADDR <= 0;
else if (enb[elseif_generate] == 1)
SENSE_ADDR <= some_wire[elseif_generate];
else
SENSE_ADDR <= SENSE_ADDR;
end
end
endgenerate
This however leads to Multi-source errors for the output wire 'SENSE_ADDR'. This leads me to the further question. Clearly a generate block is not suitable here but how would I go about implementing parameterised code replication for this block? Basically I want the functionality of the behavioral, hard coded if-else always block in a parameterised form...
Does this serve your needs? No generate required.
module mux #(
parameter WIDTH = 5,
parameter NUM = 2,
parameter NUMLG = $clog2(NUM)
) (
input [NUMLG -1:0] sel,
input [WIDTH - 1:0] in [0:NUM-1],
output [WIDTH - 1:0] out
);
assign out = in[sel];
endmodule
If your simulator doesn't support SystemVerilog that well you'll have to modify this to blow out the input array but the concept is the same.
You don not need a generate block. Add a combination always block to calculate next_SENSE_ADDR that will be flopped to SENSE_ADDR.
always #(posedge write_out_clk or posedge RESETN)
begin
if (RESETN)
SENSE_ADDR <= 0;
else
SENSE_ADDR <= next_SENSE_ADDR;
end
integer idx;
always #* begin // #(SENSE_ADDR or enb or some_wire)
next_SENSE_ADDR = SENSE_ADDR; // default, value if enb is all 0
// count down because lsb has higher priority
for ( idx=FIFO_SUB_BLOCKS-1; idx>=0; idx-- ) begin
if ( enb[idx] )
next_SENSE_ADDR = some_wire[idx];
end
end