Today i bought my first arduino uno and a hc-sr04 sensor so i'm a complete noob, i've search for example projects and i found this one
example with hc-sr04 connected directly
Today i didn't buy a breadboard i might buy it tomorrow,
Almost all of the other example projects i found require a breadboard, so i wonder, why bothering using a breadboard when you can simply connect it directly into the arduino like the example above?
Read this, I am sure you will get your answer.
https://learn.sparkfun.com/tutorials/how-to-use-a-breadboard
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It's fine to not use a breadboard for that project but when you start building more complex circuits with multiple components that would turn into a huge mess that's impossible to troubleshoot and easy to get shorts. It's very convenient to have a board to lay everything out on neatly. The breadboard also makes it convenient to make multiple connections to a single point. If you do it your way what happens when you need more ground or 5V connections?
I also bought this shield:
https://electronilab.co/tienda/ethernet-shield-con-wiznet-w5100-internet-para-arduino/
having no breadboard, how could i connect the sensor to the ethernet shield and the ethernet shield to the arduino uno?
Using the Digital I/O pins as a power supply for sensors isn't wise. The current requirements of most sensors exceeds the ability of the digital pin to source that current, and can damage it.
Hooking up the sensor to the 5V/3V pins is the safest course of action, and they have limits as well. Burning up the Uno so early in the game won't help you keep interest. Get a breadboard and some jumper wires. There are shields that have little breadboards on them if you want something small.
Not having a breadboard makes people do silly things like using LEDs without resistors.
Cobbling together power buses to power multiple devices with bits of wire increases your chances of a short that will kill many things.
You need a breadboard more than you need the few bucks it would cost.
Mmedina_student:
having no breadboard, how could i connect the sensor to the ethernet shield and the ethernet shield to the arduino uno?
Actually in this case it's no problem. The way shields work is they plug directly into the Uno. You just need to be careful not to use any of the pins that the shield is using (4, 10, 11, 12, 13) for the sensor.
Even so, if you're planning to continue to work with Arduino you should definitely buy a breadboard and a selection of jumper wires in different sizes in male to male and male to female (and female to female too if you have the budget for it) for the reasons already given. You can get them super cheap from Aliexpress or eBay but the shipping from China takes a while so it's a good idea to order one before you desperately need it.
pert:
Actually in this case it's no problem. The way shields work is they plug directly into the Uno. You just need to be careful not to use any of the pins that the shield is using (4, 10, 11, 12, 13) for the sensor.Even so, if you're planning to continue to work with Arduino you should definitely buy a breadboard and a selection of jumper wires in different sizes in male to male and male to female (and female to female too if you have the budget for it) for the reasons already given. You can get them super cheap from Aliexpress or eBay but the shipping from China takes a while so it's a good idea to order one before you desperately need it.
I'll get the jumper wires tomorrow but i want to experiment today so just to be sure i understood, the pins i can use for the sensor without a breadboard are pins 10,11,12,13?
Mmedina_student:
so just to be sure i understood, the pins i can use for the sensor without a breadboard are pins 10,11,12,13?
No!
When you plug the Ethernet shield into the Uno most of the pins just pass right through from the male pins on the bottom of the shield to the female header on the top of the shield and are unaffected by the presence of the shield.
However, the Ethernet controller chip and the SD card on the shield are connected to some of the pins on the Uno. The Ethernet controller is connected to the SPI bus: pins 11, 12, 13. The CS pin of the Ethernet controller is pin 10. The SD card is also connected to the SPI bus pins (SPI devices can share the SPI bus, they just need unique CS pins), the CS pin of the SD is pin 4.
If you connect the sensor to pins 4, 10, 11, 12, or 13 then your hardware won't work correctly because. You need to chose other pins if you have the Ethernet shield attached to the Uno. If you're wondering how I came upon this knowledge, it's clearly stated in the Ethernet library documentation:
as well as the Ethernet Shield documentation:
https://www.arduino.cc/en/Main/ArduinoEthernetShieldV1
The other pins that you should generally avoid using on your Uno whether or not you have the Ethernet shield attached. are pins 0 and 1. Those pins are Serial, which is used for communication with your computer over USB, including uploading sketches to the Uno and using the Serial Monitor. If you connect things to those pins there can be problems.
Not meaning to hijack but I wonder if Radio Shack breadboards are as good as the ones they sold back in the 70's & 80's? The ones from Amazon, E-bay, etc. are junk.
The ones I used in the 70s had gold plated contacts.
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edgemoron:
Not meaning to hijack but I wonder if Radio Shack breadboards are as good as the ones they sold back in the 70's & 80's? The ones from Amazon, E-bay, etc. are junk.
All the Radio Shacks near me have closed.
End of an era.
IMHO, this manufacturer has very good quality bread boards.
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I hate breadboards in general - I never use them. They are fiddly and unreliable, and they are very prone to intermittents, especially if you've been reckless with what you plug into them. Every time someone shows me something they built on one, they fumble around for at least 10 minutes finding the loose wire(s) while I stand there waiting them and cracking jokes at their expense.
I use prototyping board (like the stuff I sell - available in all sorts of sizes and styles!), solder everything (for reliability), and use 0.1" pin header w/dupont connectors for removable connections. I buy the dupont line without the connector housings on the end, and a collection of connector housings, and that way I can make whatever sort of cable I need, and can easily make adapters to connect commercial modules (like the ones you get on ebay with a cool part and supporting components on it), adjusting the pinout to what I want via the cable.
I tend to agree Doc, but when you get to 70 yrs and afflicted with hand tremors so your soldering iron tip goes around in 20mm orbits and you need to do a quick setup you'll appreciate breadboards. 
Hey @ larryd, I already have one of those I've never used, thanks for tip. K++.