By Nicholas Hebb
Occasionally I get asked in FlowBreeze can create Nassi-Shneiderman diagrams. Short answer: No. However, you can make them using the built-in shapes and drawing tools in Excel. there are free tools available online, but if you already have Microsoft Excel, then hopefully this short tutorial will save you from having to download and install some specialized software.
Nassi-Shneiderman diagrams (aka, NS diagrams or structograms), are used to outline structured programs. They are not very common in industry today but are sometimes used as a computer science teaching tool, often as an alternate to flowcharts. A simple example is shown below.
Sample Nassi-Shneiderman diagram
Any statement that is not
a branch or loop.
Binary branch statement,
such as an if statement
with a true/false choice.
such as a switch-case statement.
Default case is the short leg of the triangle.
Loop with precondition,
such as a for loop or a while loop
Loop with postcondition,
such as a do-while loop.
Adding a bunch of shapes to the diagram is fairly straightforward. (Well, except for the Branch triangle, which is a pain. We'll show why below.) Since the default shape style in Excel is dark blue with centered text, we will create some baseline shapes with plain styling that we place off to the side and then copy and paste to build the diagram.
Before adding shapes, the first step is to create a grid and then turn on Snap to Grid. These steps are covered in the How to Flowchart in Excel article, so we won't repeat them here.
Click the Insert tab, then click the Shapes dropdown and select either a Rectangle from the Basic Shapes group or a Process shape from the Flowchart group. Use you left mouse button to draw it to size on the sheet. With the shape still selected, right-click on the shape and select Format Shape from the context menu. Follow these steps to set the styling:
There are several triangle shapes available in Excel, but the only one that is suitable is the Isoceles Triangle under the Basic Shapes group. It has an adjustment handle (covered below in Adjusting Branch Shapes) that lets you move the center vertex for making switch branches. There is one problem, though. It points upward, and if we rotate the shape, then any text will be upside down. So what we will do is to create a branch shape and use a borderless, transparent textbox on top of that.
Grouping the Triangle and Textbox:
Grouping shapes together lets you treat multiple shapes as a single object, which is how we will want to use our contrived branch block.
Once shapes are grouped you can select individual shapes within the group by first clicking on the group to select it, and then clicking again on the individual shape. Do this now to select the textbox. Right-click on the textbox and use the Format Shape dialog to change the Fill to None and the Line Color to No line. You now have a reusable branch element.
Now that we have created the base shapes, creating the diagram is just a matter of copying and pasting the baseline shapes. Of course, you will need to position and size the shapes as you build the diagram. You will also need to set the z-order* of the triangles and adjust their vertices so that the branch statements are properly positioned on top of the process blocks, which are both covered below in the Editing tips section.
* Z-order is the front to back positioning. When shapes overlap, it determines which shape is on top.
In Excel, shapes that can be altered display yellow "adjustment handles" at the adjustment points. You can click and drag these points to change the shape of the shape, so to speak.
When dealing with branches and loop tests, it is often necessary to change the z-order stacking of a shape by bringing it forward or sending it backward. The easiest way to do this is to right click on a shape and use Bring to Front or Send to Back from the context menu. But the following keyboard shortcuts are useful when a shape is hard to select with a mouse.
The multi-key shortcuts look odd compared to most keyboard shortcuts, but when you hold the Alt key, Excel hightlights them on the screen making the path becomes obvious. The letters map to characters in each word, even though they are not underlined like menu systems. For example, Send to Back is Page Layout > Arrange > Send Backward > Send to Back.
You can select shapes with your mouse and use the Tab key to toggle between selected shapes. To select multiple shapes, click the first shape and then hold the Shift key down as you click on the others. You can also use a special Select Objects cursor available on the Home tab under the Find & Select menu. You need to toggle this cursor off to resume normal mouse usage - via the menu or by double-clicking anywhere on the worksheet.
Clicking and dragging with the mouse is the most obvious way, but you can also use the keyboard arrow keys to move a shape around. With Snap to Grid on, shapes will snap to the next cell as you do this.
On the Home tab, use the standard horizontal and vertical text alignment buttons.
On the View tab, uncheck the Gridlines checkbox.
Starting with Excel 2007, copy operations place an image of the copied range onto the Windows clipboard. To save the diagram as an image, select the cells fully encompassing the diagram, copy, and then paste into an image editor (even MS Paint will suffice). Alternately, you can paste into Word. I recommend doing a Paste Special and choosing the Enhanced Metafile format. Metafiles are like vector graphics in that when you resize the image, the lines and text will still render nicely.
Lastly, you can save to PDF. Excel has some problems when saving shapes to PDF. I recommend that you read the How to Save Excel Flowcharts to PDF article